Some more Image Processing: Otsu’s Method, Hough Transform and Motion-based Segmentation with Python

May 28, 2017July 10, 2018 / Sandipan Dey / 1 Comment

Some of the following problems appeared in the lectures and the exercises in the coursera course Image Processing (by NorthWestern University). Some of the following descriptions of the problems are taken from the exercise’s description.

1. Otsu’s method for automatic thresholding to get binary images

We need to find a thershold to binarize an image, by separating the background from the foreground. Using Otsu’s method we can automatically find the global optimal threshold, by maximizing the between-class variance. The following figure shows the outline for the technique.

The following figures show the thresholds computed and the output binary images obtained by using Otsu’s method on a few gray-scale low-contrast images:

The following figures show the output binary images obtained by using Otsu’s method on a few gray-scale high-contrast images obtained using histogram equalization:

The following figures show the distribution of the between-class variance for different images and the threshold chosen by Otsu’s method:

2. Hough Transform to find lines in images

The Hough transform can be used to find lines in an image by taking votes from each of the pixels for all possible orientations of lines, for different values of the parameters in a straight line’s equation. The following figure describes the method.

The following figures show a couple of binary images and the votes obtained by the image pixels in the parametric space of any straight line and how thresholding on the votes we can find the most prominent line(s) in the image (markd by green). The parameter values marked with arrow(s) represent the most prominent line(s) in the image.

The experiment on the following image is inspired by the youtube video How Hough Transform works (by Thales Sehn Korting) which can be found here. Again, by thresholding on the votes by the pixels for the differrent values of the straight line parameters, we can find the most prominent line(s) in the image (markd by green). The parameter values marked with arrow(s) represent the most prominent line(s) in the image.

The next figure shows all the steps in how the Hough transform can be used on any (gray-level) image to find lines (edges). As expected, the more voting threshold is increased, the lesser lines / edges are detected (since the more prominent ones will be detected by the Hough transform, they are colored red).

3. Motion based Segmentation using Accumulative Difference Image

In this problem we basically want to separate the moving objects in consecutive frames of a video from the non-moving objects. The following figures show the problem statement that appeared in an assignment in the same course, along with the theory to be used:

The following animation shows the motion of a moving rectangle.

motion

The next figure shows how the motion-based segmentation using ADI-based techniques can be applied to separate out the moving rectangle from the static background.

Again, the next animations show how the moving objects can be segmented from the non-moving ones from the consecutive frames of a video.

First the frames from the video are binarized using Ostu’s method, as shown below.

Then absolute ADI is applied to separate out the moving objects (here the students), as shown below:

The next video is from some past cricket match with Sachin Tendulkar batting (taken from youtube) and the following one is the motion-segmented video with ADI:

The next video is captured at a live performance of a dance-drama written by Tagore and the following one is the motion-segmented video with ADI:

kalmrigaya_adi

The markdown file can be found here.

Some Image Processing, Information and Coding Theory with Python

May 27, 2017June 9, 2017 / Sandipan Dey / 1 Comment

Some of the following problems appeared in the exercises in the coursera course Image Processing (by Northwestern University). The following descriptions of the problems are taken directly from the assignment’s description.

1. Some Information and Coding Theory

Computing the Entropy of an Image

The next figure shows the problem statement. Although it was originally implemented in MATLAB, in this article a python implementation is going to be described.

Histogram Equalization and Entropy

Histogram equalization is a well-known image transformation technique to imrpove the contrast of an image. The following figure shows the theory for the technique: each pixel is transformed by the CDF of the image and as can be shown, the output image is expected to follow an uniform distribution (and thereby with the highest entropy) over the pixel intensities (as proved), considering the continuous pixel density. But since the pixel values are discrete (integers), the result obtained is going to be near-uniform.

The following figures show a few images and the corresponding equalized images and how the PMF and CDF changes.

Image Data Compression with Huffman and Lempel Ziv (LZ78) Coding

Now let’s implement the following couple of compression techniques:

Huffman Coding
Lempel-Ziv Coding (LZ78)

and compress a few images and their histogram equalized versions and compare the entropies.

The following figure shows the theory and the algorithms to be implemented for these two source coding techniques:

Let’s now implement the Huffman Coding algorithm to compress the data for a few gray-scale images.

The following figures show how the tree is getting constructed with the Huffman Coding algorithm (the starting, final and a few intermediate steps) for the following low-contrast image beans. Here we have alphabets from the set {0,1,…,255}, only the pixels present in the image are used. It takes 44 steps to construct the tree.

beans

htree_40 htree_41 htree_42 htree_43 htree_44

The following table shows the Huffman Codes for different pixel values for the above low-contrast image beans.

index	pixel	code
15	108	0
3	115	1
20	109	10
1	112	11
32	107	100
44	110	101
18	96	110
11	95	111
21	120	1000
35	91	1001
6	116	1010
24	123	1011
2	98	1100
13	100	1101
17	94	1111
12	119	10000
4	114	10001
42	103	10011
16	106	10100
37	132	10101
41	130	10111
5	117	11000
26	125	11010
40	99	11011
38	118	11100
0	113	11101
22	121	11111
39	131	101011
27	127	101111
31	102	110011
25	124	110101
23	122	110111
33	104	111011
34	105	111111
29	129	1001111
30	93	1010101
7	137	1010111
14	255	1101011
28	128	1101111
36	133	11010111
10	134	11101011
9	135	101010111
43	90	1001010111
8	136	10001010111
19	89	100001010111

Let’s now repeat the Huffman-tree construction for the following histogram-equalized image beans. The goal is:

First construct the tree for the equalized image.
Use the tree to encode the image data and then compare the compression ratio with the one obtained using the same algorithm with the low-contrast image.
Find if the histogram equalization increases / reduces the compression ratio or equivalently the entropy of the image.

The following figures show how the tree is getting constructed with the Huffman Coding algorithm (the starting, final and a few intermediate steps) for the image beans. Here we have alphabets from the set {0,1,…,255}, only the pixels present in the image are used. It takes 40 steps to construct the tree, also as can be seen from the following figures the tree constructed is structurally different from the one constructed on the low-contract version of the same image.

The following table shows the Huffman Codes for different pixel values for the above high-contrast image beans.

index	pixel	code
13	119	0
11	250	1
5	133	10
0	150	11
8	113	100
32	166	101
36	60	110
40	30	111
25	208	1000
38	16	1001
21	181	1010
33	224	1011
9	67	1100
37	80	1101
27	244	1111
23	201	10000
17	174	10001
6	89	10011
34	106	10100
24	141	10101
28	246	10111
22	188	11000
10	235	11010
30	71	11011
2	195	11100
3	158	11101
1	214	11111
31	228	100001
15	84	101011
12	251	101111
39	18	110011
4	219	110111
14	93	111011
26	99	111111
19	253	1000001
7	238	1001111
18	21	1010111
29	241	1101111
35	248	1110011
20	0	10101111
16	255	110101111

The following figure shows the compression ratio for different images using Huffman and LZ78 codes, both on the low-contrast and high contrast images (obtained using histogram equalization). The following observations can be drawn from the comparative results shown in the following figures (here H represents Huffman and L represents LZ78):

The entropy of an image stays almost the same after histogram equalization
The compression ratio with Huffman / LZ78 also stays almost the same with an image with / without histogram equalization.
The Huffman codes achieves higher compression in some cases than LZ78.

The following shows the first few symbol/code pairs for the dictionary obtained with LZ78 algorithm, with the alphabet set as {0,1,..,255} for the low-contrast beans image (there are around 18k codes in the dictionary for this image):

index

symbol (pixels)

code


7081	0	0
10325	1	1
1144	2	10
7689	4	100
8286	8	1000
7401	16	10000
8695	32	100000
10664	64	1000000
5926	128	10000000
7047	128,128	1000000010000000
1608	128,128,128	100000001000000010000000
12399	128,128,128,128	10000000100000001000000010000000
3224	128,128,128,128,128	1000000010000000100000001000000010000000
3225	128,128,128,128,129	1000000010000000100000001000000010000001
3231	128,128,128,128,127	100000001000000010000000100000001111111
12398	128,128,128,129	10000000100000001000000010000001
12401	128,128,128,123	1000000010000000100000001111011
12404	128,128,128,125	1000000010000000100000001111101
12403	128,128,128,127	1000000010000000100000001111111
1609	128,128,129	100000001000000010000001
8780	128,128,129,128	10000000100000001000000110000000
7620	128,128,130	100000001000000010000010
2960	128,128,130,125	1000000010000000100000101111101
2961	128,128,130,127	1000000010000000100000101111111
8063	128,128,118	10000000100000001110110
1605	128,128,121	10000000100000001111001
6678	128,128,121,123	100000001000000011110011111011
1606	128,128,122	10000000100000001111010
1601	128,128,124	10000000100000001111100
1602	128,128,125	10000000100000001111101
…	…	…
4680	127,127,129	1111111111111110000001
2655	127,127,130	1111111111111110000010
1254	127,127,130,128	111111111111111000001010000000
7277	127,127,130,129	111111111111111000001010000001
9087	127,127,120	111111111111111111000
9088	127,127,122	111111111111111111010
595	127,127,122,121	1111111111111111110101111001
9089	127,127,123	111111111111111111011
9090	127,127,124	111111111111111111100
507	127,127,124,120	1111111111111111111001111000
508	127,127,124,125	1111111111111111111001111101
9091	127,127,125	111111111111111111101
9293	127,127,125,129	11111111111111111110110000001
2838	127,127,125,132	11111111111111111110110000100
1873	127,127,125,116	1111111111111111111011110100
9285	127,127,125,120	1111111111111111111011111000
9284	127,127,125,125	1111111111111111111011111101
10394	127,127,125,125,131	111111111111111111101111110110000011
4357	127,127,125,125,123	11111111111111111110111111011111011
4358	127,127,125,125,125	11111111111111111110111111011111101
6615	127,127,125,125,125,124	111111111111111111101111110111111011111100
9283	127,127,125,127	1111111111111111111011111111
9092	127,127,127	111111111111111111111
1003	127,127,127,129	11111111111111111111110000001
7031	127,127,127,130	11111111111111111111110000010
1008	127,127,127,121	1111111111111111111111111001
1009	127,127,127,122	1111111111111111111111111010
1005	127,127,127,125	1111111111111111111111111101
2536	127,127,127,125,125	11111111111111111111111111011111101
5859	127,127,127,127	1111111111111111111111111111

12454 rows × 2 columns

The next table shows the dictionary obtained for the high-contrast beans image using LZ78 algorithm:

index

symbol (pixels)

code


7003	0	0
11341	0,0	00
5300	0,0,16	0010000
5174	0,0,16,16	001000010000
10221	0,0,16,16,16	00100001000010000
10350	0,16	010000
9145	0,16,0	0100000
1748	0,16,0,0	01000000
12021	0,16,16	01000010000
7706	0,16,16,16	0100001000010000
6343	0,16,16,16,16	010000100001000010000
12355	0,16,16,16,16,18	01000010000100001000010010
6346	0,16,16,16,18	010000100001000010010
12019	0,16,18	01000010010
8496	0,16,18,18	0100001001010010
3459	0,67	01000011
8761	0,67,119	010000111110111
10345	0,18	010010
12295	0,18,80	0100101010000
5934	0,255	011111111
1356	0,255,214	01111111111010110
10272	1	1
1108	2	10
7611	4	100
8187	8	1000
7323	16	10000
9988	16,0	100000
8625	32	100000
10572	64	1000000
2806	16,0,0	1000000
…	…	…
7284	255,224	1111111111100000
10857	255,113	111111111110001
7286	255,228	1111111111100100
9053	255,228,228	111111111110010011100100
7734	255,235	1111111111101011
10853	255,119	111111111110111
1315	255,238	1111111111101110
7105	255,238,238	111111111110111011101110
10212	255,238,238,238	11111111111011101110111011101110
11699	255,30	1111111111110
6120	255,60	11111111111100
6846	255,241	1111111111110001
10205	255,241,241	111111111111000111110001
11894	255,241,241,251	11111111111100011111000111111011
9428	255,60,60	11111111111100111100
7768	255,60,60,60	11111111111100111100111100
111	255,60,60,60,67	111111111111001111001111001000011
5613	255,60,60,60,30	1111111111110011110011110011110
8713	255,60,60,60,30,30	111111111111001111001111001111011110
11729	255,60,60,60,30,30,30	11111111111100111100111100111101111011110
6848	255,244	1111111111110100
6850	255,246	1111111111110110
6845	255,248	1111111111111000
9401	255,248,248	111111111111100011111000
880	255,250	1111111111111010
881	255,251	1111111111111011
3725	255,251,251	111111111111101111111011
11316	255,251,251,235	11111111111110111111101111101011
879	255,253	1111111111111101
7215	255,253,253	111111111111110111111101

12394 rows × 2 columns

2. Spatial Domain Watermarking: Embedding images into images

The following figure describes the basics of spatial domain digital watermarking:

The next figures show the watermark image and the grayscale host image to be used for embedding the watermark image inside the host image.

The next animations show the embedding of the most significant bit for each pixel of the watermark image inside the gray-scale host image at the upper-left corner at the bits 0,1,…,7 respectively, for each pixel in the host image. As expected, when embedded into a higher significant bit of the host image, the watermark becomes more prominent than when embedded into a lower significant bit.

The next figure shows how the distribution of the pixel intensities change when embedding into different bits.

Some Image and Video Processing: Motion Estimation with Block-Matching in Videos, Noisy and Motion-blurred Image Restoration with Inverse Filter in Python and OpenCV

May 20, 2017October 29, 2020 / Sandipan Dey / 2 Comments

The following problems appeared in the exercises in the coursera course Image Processing (by Northwestern University). The following descriptions of the problems are taken directly from the exercises’ descriptions.

1. Analysis of an Image quality after applying an nxn Low Pass Filter (LPF) for different n

The next figure shows the problem statement. Although it was originally implemented in MATLAB, in this article a python implementation is going to be described.

The following figure shows how the images get more and more blurred after the application of the nxn LPF as n increases.

The following figure shows how the quality of the transformed image decreases when compared to the original image, when an nxn LPF is applied and how the quality (measured in terms of PSNR) degrades as n (LPF kernel width) increases.

2. Changing Resolution of an Image with Down/Up-Sampling

The following figure describes the problem:

The following steps are needed to be followed:

Smooth the original image with a 3×3 LPF (box1) kernel.
Downsample (choose pixels corresponding to every odd rows and columns)
Upsample the image (double the width and height)
Use the kernel and use it for convolution with the upsampled image to obtain the final image.

Although in the original implementation MATLAB was used, but the following results come from a python implementation.

As we go on increasing the kernel size, the quality fo the final image obtained by down/up sampling the original image decreases as n increases, as shown in the following figure.

3. Motion Estimation in Videos using Block matching between consecutive video frames

The following figure describes the problem:

For example we are provided with the input image with known location of an object (face marked with a rectangle) as shown below.

Now we are provided with another image which is the next frame extracted from the same video but with the face unmarked) as shown below. The problem is that we have to locate the face in this next frame and mark it using simple block matching technique (and thereby estimate the motion).

As shown below, using just the simple block matching, we can mark the face in the very next frame.

Now let’s play with the following two videos. The first one is the video of some students working on a university corridor, as shown below (obtained from youtube), extract some consecutive frames, mark a face in one image and use that image to mark all thew faces om the remaining frames that are consecutive to each other, thereby mark the entire video and estimate the motion using the simple block matching technique only.

The following figure shows the frame with the face marked, now we shall use this image and block matching technique to estimate the motion of the student in the video, by marking his face in all the consecutive frames and reconstructing the video, as shown below..

The second video is the video of the Google CEO Mr. Pichai talking, as shown below (obtained from youtube), again extract some consecutive frames, mark his face in one image and use that image to mark all the faces in the remaining frames that are consecutive to each other, thereby mark the entire video and estimate the motion using the simple block matching technique only.

The following figure shows the frame with the face marked, now we shall use this image and block matching technique to estimate the motion of the Google CEO in the video, by marking his face in all the consecutive frames and reconstructing the video, as shown below.

The most interesting thing above is that we have not used any sophisticated image features such as HOG / SIFT, still it did a pretty descent job with simple block matching.

4. Using Median Filter to remove salt and paper noise from an image

The following figure describes the problem:

The following figure shows the original image, the noisy image and images obtained after applying the median filter of different sizes (nxn, for different values of n):

As can be seen from the following figure, the optimal median filter size is 5×5, which generates the highest quality output, when compared to the original image.

Using Inverse Filter to Restore noisy images with Motion Blurs

The following figure shows the description of the problem:

The following figure shows the theory behind the inverse filters in the (frequency) spectral domain.

The following are the steps for the restoring an image with motion blurs and noise-corruption using the Inverse Filter:

Generate restoration filter in the frequency domain (with Fast Fourier Transform) from frequency response of motion blur and using the threshold T.
Get the spectrum of blurred and noisy-corrupted image (the input to restoration).
Compute spectrum of restored image by convolving the restoration filter with the blurred noisy image in the frequency domain.
Genrate the restored image from its spectrum (with inverse Fourier Transform).

The following figures show the inverse filter is applied with different threshold value T (starting from 0.1 to 0.9 in that order) to restore the noisy / blurred image.

The following figure shows how the Restored PSNR and the ISNR varies with different values if the threshold T.

Some more Computational Photography: Creating Video Textures in Python and OpenCV

May 17, 2017May 18, 2017 / Sandipan Dey / 1 Comment

The following problem appeared as an assignment in the Coursera Course Computational Photography (by Georgia Tech, 2013). The following description of the problem is taken directly from the assignment’s description.

Introduction

In this article, we shall be applying our computational photography magics to video, with the purpose of creating video textures, or infinitely looping pieces of video. The input and output for the homework assignment is provided as a folder of images. The reasoning behind this, and suggestions for moving between different formats of video are given in the video Appendix in section 4.

Video Volume and SSD

Let’s get more familiar with the 4d coordinate system of a video volume (time x row x column x channel). To compute SSD, we need to find an image distance between every pair of frames in the video. SSD stands for sum of square distances, which as the name suggests, requires to take the pixelwise difference of the two frames, square it, and sum them all together.

Diff2

This function takes in a difference matrix created by ssd, and updates it with dynamic information. The intuition behind this is as follows when considering the transition cost from frame i to j, we should not only look at the frames themselves, but also consider the preceding and following frames. So, we have:

Or, in other words, we are going to take a weighting function, and sum across the ssd outputs of the preceding and following frames in order to update the transition costs with dynamic information. We are going to use binomial filter for this purpose. The following figure shows the basics of a binomial filter:

Find the biggest loop and synthesize loop

find the biggest loop

Now that we have the costs of transitioning from one frame to another, we should find a suitable loop for our video texture. Simply taking the smallest transition distance here might not be desirable what if the resulting loop is only 1 frame long? In order to state within the code that loop size matters, we will use a trick that is frequent in engineering disciplines. We are going to find the transition which is optimal under a metric: score(s, f) = α * (f − s) − diff2[f, s].

Note the two terms of the metric. The first is the difference between the final and starting frame of our loop. This term is large when the loop is large. The second term is the output of our diff2 function, which tells us the cost of transitioning from finish to start. Subtracting this term turns it into a ‘smoothness’ parameter. It is larger when the transition is less noticeable.

The last bit of wizardry is the alpha parameter. Because the size of the loop and the transition cost are likely to be in very different units, we introduce a new parameter to make them comparable. We can manipulate alpha to control the tradeoff between loop size and smoothness. Large alphas prefer large loop sizes, and small alphas prefer smoother transitions. The find biggest loop function has to compute this score for every choice of s and f, and return the s and f that correspond to the largest score.

synthesize loop

The finishing step is to take our video volume, and turn it back into a series of images, now cropping it to only contain the loop we found. Let’s implement this function does just that. It is pretty much the inverse of the video volume function implemented earlier, except for this time we are starting with a full video volume, and returning a list of only the image frames between start and finish (inclusive).

The following figure explains how we are going to find the video texture from an input candle video with 90 frames.

Input Video

Let’s use the following candle video as the input video and extract 100 frames from the video. As can be seen that there are some jumps from the end to the beginning of the video and our aim is to remove the jump and create a smooth video texture that can be played infinitely many times without any jump.

The following figure shows the 100×100 distance matrix (diff1) computed in between any two video frames:

The following figure shows the distance matrix (diff2) computed after taking into account the transition costs (weights) and distances of the adjacent frames in a window when computing the distance between any two frames.

Finally, the following figure shows the distance matrix (diff3) computed after taking into account the tradeoff in between the length of the video texture and the smoothness of transition between the start and the end frame (with the controlling parameter alpha=90).

Output Video Texture

The following video shows the output video texture produced (by choosing the start and end video frames i and j respectively that give the best score according to the above matrix), as can be seen the video now contains no jump and it’s pretty smooth.

The markdown file can be found here.

Some more Computational Photography: Merging and Blending Images using Gaussian and Laplacian Pyramids in Python

May 16, 2017January 29, 2018 / Sandipan Dey / 3 Comments

The following problem appeared as an assignment in the coursera course Computational Photography (by Georgia Tech, 2013). The following description of the problem is taken directly from the assignment’s description.

Introduction

In this article, we shall be putting together a pyramid blending pipeline that will allow us to blend any two images with a mask. The blending procedure takes the two images and the mask, and splits them into their red, green and blue channels. It then blends each channel separately.

Then a laplacian pyramid will be constructed for the two images. It will then scale the mask image to the range [0,1] and construct a gaussian pyramid for it. Finally, it will blend the two pyramids and collapse them down to the output image.

Pixel values of 255 in the mask image are scaled to the value 1 in the mask pyramid, and assign the strongest weight to the image labeled ‘white’ during blending. Pixel values of 0 in the mask image are scaled to the value 0 in the mask pyramid, and assign the strongest weight to the image labeled ‘black’ during blending.

The following figures describe the pyramid blending process with couple of sample images.

Reduce

This function takes an image and subsamples it down to a quarter of the size (dividing the height and width by two). Before we subsample the image, however, we need to first smooth out the image.

The following 5×5 generating kernel will be used for convolution, for the reduce and later for the expand function.

Expand

This function takes an image and supersamples it to four times the size (multiplying the height and width by two). After increasing the size, we have to interpolate the missing values by running over it with a smoothing filter.

Gaussian Pyramid

This function takes an image and builds a pyramid out of it. The first layer of this pyramid is the original image, and each subsequent layer of the pyramid is the reduced form of the previous layer. Within the code, these pyramids are represented as lists of arrays, so pyramid = [layer0, layer1, layer2, …]. The reduce function implemented in the previous part needs to be used in order to implement this function.

Laplacian Pyramid

This function takes a gaussian pyramid constructed by the previous function, and turns it into a laplacian pyramid. Like with gaussian pyramids, laplacian pyramids are represented as lists and the expand function implemented in the previous part needs to be used in order to implement this function.

Blend

In this part, the pipeline will be set up by implementing the actual blend function, and then implementing a collapse function that will allow us to disassemble our laplacian pyramid into an output image.

Blend function takes three pyramids:

white – a laplacian pyramid of an image
black – a laplacian pyramid of another image
mask – a gaussian pyramid of a mask image

It should perform an alphablend of the two laplacian pyramids according to the mask pyramid. So, we need to blend each pair of layers together using the mask of that layer as the weight. Pixels where the mask is equal to 1 should be taken from the white image, pixels where the mask is 0 should be taken from the black image. Pixels with value 0.5 in the mask should be an equal blend of the white and black images, etc.

Collapse

This function is given a laplacian pyramid, and is expected to ‘flatten’ it to an image. We need to take the top layer, expand it, and then add it to the next layer. This results in a pyramid of one level less than we started with. We continue this process until we are left with only a single layer.

Results (some images are taken from the same Computational Photography Course)

The following figures show a few pairs of input images, the masks, the blended output images, along with the gaussian and laplacian pyramids.

1. Input Images

Gaussian Pyramids

Laplacian Pyramids

Blended Laplacian Pyramids

Blended Output Image

2. Input Images

Gaussian Pyramids

Laplacian Pyramids

Blended Laplacian Pyramids

Blended Output Image

3. Input Images

Gaussian Pyramids

Laplacian Pyramids

Blended Laplacian Pyramids

Blended Output Image

4. Input Images

Gaussian Pyramids

Laplacian Pyramids

Blended Laplacian Pyramids

Blended Output Image

The next example shows how the pyramid pipeline can be used to edit an image background.

5. Input Images

Gaussian Pyramids

Laplacian Pyramids

Blended Laplacian Pyramids

Blended Output Image

As can be seen from the output image above, the pyramid blending pipleline did a good job in blending the two images.

6. Input Images

Gaussian Pyramids

Laplacian Pyramids

Blended Laplacian Pyramids

Blended Output Image

The markdown file can be found here.

Some Image Processing and Computational Photography: Convolution, Filtering and Edge Detection with Python

May 12, 2017January 29, 2018 / Sandipan Dey / 3 Comments

The following problems appeared as an assignment in the coursera course Computational Photography (by Georgia Institute of Technology). The following descriptions of the problems are taken directly from the assignment’s descriptions.

Introduction

In this article, we shall be playing around with images, filters, and convolution. We will begin by building a function that performs convolution. We will then experiment with constructing and applying a variety of filters. Finally, we will see one example of how these ideas can be used to create interesting effects in our photos, by finding and coloring edges in our images.

Filtering

In this section, let’s apply a few filters to some images. Filtering basically means replacing each pixel of an image by the linear combination of its neighbors. We need to understand the following concepts in this context:

Kernel (mask) for a filter: defines which neighbors to be considered and what weights are to be given.
Cross-Correleation vs. Convolution: determines how the kernel is going to be applied on the neighboring pixels to compute the linear combination.

Convolution

The following figure describes the basic concepts of cross-correlation and convolution. Basically convolution flips the kernel before applying it to an image.

Cross-Correlation vs. Convolution

The following figures show a custom kernel applied on an impulse response image changes the image both with cross-correlation and convolution.

The impulse response image is shown below:

impulse_response

The below figure shows a 3×3 custom kernel to be applied on the above impulse response image.

The following figure shows the output images after applying cross-correlation and convolution with the above kernel on the above impulse response image. As can be seen, convolution produces the desired output.

The following figures show the application of the same kernel on some grayscale images and the output images after convolution.

Now let’s apply the following 5×5 flat box2 kernel shown below.

The following figures show the application of the box kernel above on some grayscale images and the output images after convolution. on some images and notice how the output images are blurred.

The following figures show the application of the box kernels of different size on the grayscale image lena and the output images after convolution. As expected, the blur effect increases as the size of the box kernel increases.

Gaussian Filter

The following figure shows a 11×11 Gaussian Kernel generated by taking outer product of the densities of two 1D i.i.d. Gaussians with mean 0 and s.d. 3.

Here is how the impulse response image (enlarged) looks like after the application of the above Gaussian Filter.

The next figure shows the effect of Gaussian filtering / smoothing (blur) on some images.

The following figure shows the 11×11 Gaussian Kernels generated with 1D i.i.d. Gaussians with different bandwidths (s.d. values).

The following figures show the application of the Gaussian kernels of different bandwidth on the following grayscale image and the output images after filtering. As expected, the blur effect increases as the bandwidth of the Gaussian kernel increases.

Sharpen Filter

The following figure shows a 11×11 Sharpen Kernel generated by subtracting a gaussian kernel (with s.d. 3) from a scaled impulse response kernel with 2 at center.

The next figure shows the effect of Sharpen flitering on some images.

The following figure shows the 11×11 Sharpen Kernels with different bandwidths (s.d. values).

The following figures show the application of the Sharpen kernels of different bandwidth on the following grayscale image and the output images after filtering. As expected, the sharpen effect increases as the bandwidth of the Gaussian kernel increases.

Median filter

One last thing we shall do to get a feel for is nonlinear filtering. So far, we have been doing everything by multiplying the input image pixels by various coefficients and summing the results together. A median filter works in a very different way, by simply choosing a single value from the surrounding patch in the image.

The next figure shows the effect of Median flitering on some images. As expected, with a 11×11 mask, some of the images are getting quite blurred.

Drawing with edges

At a high level, we will be finding the intensity and orientation of the edges present in the image using convolutional filters, and then visualizing this information in an aesthetically pleasing way.

Image gradients

The first thing it does is to find the gradient of the image. A gradient is a fancy way of saying “rate of change”. The following figure shows the basic concepts about image gradients.

Edge Detection

In order to find the edges in our image, we are going to look for places where pixels are rapidly changing in intensity. The following figure shows the concept:

The Sobel Filter

There are a variety of ways for doing this, and one of the most standard is through the use of Sobel filters, which have the following kernels:

If we think about the x sobel filter being placed on a strong vertical edge:

im3

Considering the yellow location, the values on the right side of the kernel get mapped to brighter, and thus larger, values. The values on the left side of the kernel get mapped to darker pixels which are close to zero. The response in this position will be large and positive.

Compare this with the application of the kernel to a relatively flat area, like the blue location. The values on both sides are about equal, so we end up with a response that is close to zero. Thus, the x-direction sobel filter gives a strong response for vertical edges. Similarly, the y-direction sobel filter gives a strong response for horizontal edges.

The steps for edge detection:

Convert the image to grayscale.
Blur the image with a gaussian kernel. The purpose of this is to remove noise from the image, so that we find responses only to significant edges, instead of small local changes that might be caused by our sensor or other factors.
Apply the two sobel filters to the image.

Edge orientations and magnitudes

Now we have the rate at which the image is changing in the x and y directions, but it makes more sense to talk about images in terms of edges, and their orientations and intensities. As we will see, we can use some trigonometry to transform between these two representations.

First, let’s see how our sobel filters respond to edges at different orientations from the following figures:

The red arrow on the right shows the relative intensities of the response from the x and y sobel filter. We see that as we rotate the edge, the intensity of the response slowly shifts from the x to the y direction. We can also consider what would happen if the edge got less intense:

Let’s apply the sobel filters on the following butterfly image (taken from the slides of the same computational photography course).

The following figures show the result of applying the sobel filters on the above butterfly image. Notice that the angle of the edges show the direction (in red) for a few edge vectors for which the magnitude of the edge vectors were high (F is the image).

Again, let’s apply the sobel filters on the following tiger image (taken from the slides of the same computational photography course).

The following figures show the result of applying the sobel filters on the above tiger image. Notice that the angle of the edges show the direction (in red) for a few edge vectors for which the magnitude of the edge vectors were high.

Next, let’s apply the sobel filters on the following zebra image (taken from the slides of the same computational photography course).

The following figures show the result of applying the sobel filters on the above zebra image. Notice that the angle of the edges show the direction (in red) for a few edge vectors for which the magnitude of the edge vectors were high.

Finally, let’s apply the sobel filters on the following image of mine.

The following figures show the result of applying the sobel filters on my image. Notice that the angle of the edges show the direction (in red) for a few edge vectors for which the magnitude of the edge vectors were high.

The following figure shows the horizontal (H_x) and vertical (H_y) kernels for a few more filters such as Prewitt and Roberts along with Sobel.

The Prewitt Filter

The following figures show the result of applying the prewitt filters on the butterfly image. Notice that the angle of the edges show the direction (in red) for a few edge vectors for which the magnitude of the edge vectors were high (F is the image).

The Roberts Filter

The following figures show the result of applying the roberts filters on the butterfly image. Notice that the angle of the edges show the direction (in red) for a few edge vectors for which the magnitude of the edge vectors were high (F is the image).

The LOG Filter

The following figure shows the Gaussian Kernels with different bandwidths, a Sharpen Kernel, along with the LOG (Laplacian of Gaussian Kernel), a very useful kernel for edge detection in images and DOG kernel.

The following figures show the results of applying the LOG filter on the above images.

The DOG Filter

Difference of Gaussian (DOG) Kernel can also be used to find edges. The following figure shows the results of application of DOG (with sd 2, 5) on the above images:

The Canny Edge Detector

The following figure shows the Canny Edge Detection algorithm:

The following figures show the results of applying the Canny Edge Detection algorithm on the above images (with the intensity thresholds as 50 and 100 respectively).

Mapping to color

We now have the magnitude and orientation of the edge present at every pixel. However, this is still not really in a form that we can examine visually. The angle varies from 0 to 180, and we don’t know the scale of the magnitude. What we have to do is to figure out a way to transform this information to some sort of color values to place in each pixel.

The edge orientations can be pooled into four bins edges that are roughly horizontal, edges that are roughly vertical, and edges at 45 degrees to the left and to the right. Each of these bins are assigned a color (vertical is yellow, etc). Then the magnitude is used to dictate the intensity of the edge. For example a roughly vertical edge of moderate intensity would be set to a medium yellow color, or the value (0, 100, 100).

The following figures show some of the edges extracted from some of the images previously used as inputs and color-mapped using sobel filter:

The markdown file can be found here: https://github.com/sandipan/Blogs/blob/master/comp_photo.md

Some BioInformatics: Suffix Tree Construction and the Longest Repeated Substring Problem in Python

May 10, 2017May 12, 2017 / Sandipan Dey / Leave a comment

The following problem appeared as an assignment in the coursera course BioInformatics Algorithms II (by UCSD). The following description of the problem is taken from the course assignment, Rosalind (http://rosalind.info/) and Stepik (https://stepik.org/).

The following problems define a couple of data structures for fast pattern matching: a collection of (smaller) strings Patterns that we wish to match against a (larger) genome string Text.

Creating a Trie from Patterns

we would like to organize Patterns into a data structure to prevent multiple passes down Text and to reduce the runtime. To this end, we will consolidate Patterns into a directed tree called a trie (pronounced “try”), which is written Trie(Patterns) and has the following properties (example shown in the next figure).

The trie has a single node with indegree 0, called the root.
Each edge of Trie(Patterns) is labeled with a letter of the alphabet.
Edges leading out of a given node have distinct labels.
Every string in Patterns is spelled out by concatenating the letters along some path from
the root downward.
Every path from the root to a leaf, or node with outdegree 0, spells a string from Patterns.

Creating a Suffix Trie

A suffix trie, denoted SuffixTrie(Text), is the trie formed from all suffixes of Text (see figure below). A dollar sign (“$”) is appended to Text in order to mark the end of Text. We will also label each leaf of the resulting trie by the starting position of the suffix whose path through the trie ends at this leaf (using 0 based indexing). This way, when we arrive at a leaf, we will immediately know where this suffix came from in Text.

The following pseudocode constructs the suffix trie of a string Text by traversing the suffixes of Text from longest to shortest. Given a suffix, it attempts to spell the suffix downward by following edge labels as far as possible until it can go no further. At that point, it adds the rest of the suffix to the trie in the form of a path to a leaf, along with the position of each symbol in the suffix.

There are |Text| suffixes of Text, ranging in length from 1 to |Text| and having total length O(|Text|2). Thus, we need to reduce both the construction time and memory requirements of suffix tries to make them practical. We can reduce the number of edges in SuffixTrie(Text) by combining the edges on any nonbranching path into a single edge. We then label this edge with the concatenation of symbols on the consolidated edges, as shown in the figure below. The resulting data structure is called a suffix tree, written SuffixTree(Text).

Creating a Suffix Tree

Given a string s having length L, recall that its suffix tree T is defined by the following properties:

T is a rooted tree having exactly L+1 leaves.
Every edge of T is labeled with a substring of s$, where s$ is the string formed by adding a placeholder symbol $ to the end of s.
Every internal node of T other than the root has at least two children; i.e., it has degree at least 3.
The substring labels for the edges leading down from a node to its children must begin with different symbols.
By concatenating the substrings along edges, each path from the root to a leaf corresponds to a unique suffix of s.

The following figures show example Suffix Trie and Suffix Tree created from a Text and also the algorithm pseudocodes.

The following figures show how the suffix tree is constructed for the Text panamabananas$ as all the suffixes are inserted into the suffix tree starting from the largest to the smallest suffix.

Inserting suffix panamabananas$ to the empty tree

Inserting suffix anamabananas$ to the tree
tree1

Inserting suffix namabananas$ to the tree
tree2

Inserting suffix amabananas$ to the tree
tree3

Inserting suffix mabananas$ to the tree
tree4

Inserting suffix abananas$ to the tree

Inserting suffix bananas$ to the tree

Inserting suffix ananas$ to the tree

Inserting suffix nanas$ to the tree

Inserting suffix anas$ to the tree

Inserting suffix nas$ to the tree

Inserting suffix as$ to the tree

Inserting suffix s$ to the tree

tree12

Inserting suffix $ to the tree

The next figures show how the suffix tree is constructed for the genome Text ATAAATG$.

The next figures show how the suffix tree is constructed for the genome TextTCTGAGCCCTACTGTCGAGAAATATGTATCTCGCCCCCGCAGCTT$. tree0 tree1 tree2 tree3 tree4 tree5 tree6 tree7 tree8 tree9 tree10 tree11 tree12 tree13 tree14 tree15 tree16 tree17 tree18 tree19 tree20 tree21 tree22 tree23 tree24 tree25 tree26 tree27 tree28 tree29 tree30 tree31 tree32 tree33 tree34 tree35 tree36 tree37 tree38 tree39 tree40 tree41 tree42 tree43 tree44 tree45

A more efficient implementation for suffix tree construction can be done using the Ukkonen’s suffix tree algorithm.

The Longest Repeated Substring Problem

im2

As we can see from the following figure, in order to find any repeated pattern inside the Text,

We need to start at the root from the suffix tree and traverse towards the internal nodes (concatenating all the edges from the root to any of the internal nodes represents a repeated pattern inside the text).
Choose the longest of all such texts.

In the next figure the paths colors red, blue and green each starting from the root and ending in some internal node of the suffix tree represent repeated patterns.

Analyzing the connectivity of a computer network undergoing a cyber-attack in Python

May 9, 2017 / Sandipan Dey / 3 Comments

The following problem appeared as a project in the coursera course Algorithmic Thinking (by RICE university), a part of Fundamentals of Computing specialization. The following description of the problem is taken directly from the project description.

Graph exploration (that is, “visiting” the nodes and edges of a graph) is a powerful and necessary tool to elucidate properties of graphs and quantify statistics on them. For example, by exploring a graph, we can compute its degree distribution, pairwise distances among nodes, its connected components, and centrality measures of its nodes and edges. Both DFS and BFS can be used to compute the connected components of a graph.

Now, the task is to analyze the connectivity of a computer network as it undergoes a cyber-attack. In particular, we shall simulate an attack on this network in which an increasing number of servers are disabled. In computational terms, we will model the network by an undirected graph and repeatedly delete nodes from this graph. We will then measure the resilience of the graph in terms of the size of the largest remaining connected component as a function of the number of nodes deleted.

Example graphs

Let’s compute the resilience of following 3 types of undirected graphs:

An example computer network – an undirected graph (with 1347 nodes and 3112 edges). The next figure shows a small part of the undirected network graph.
ER random graphs.
UPA graphs – the undirected version of the DPA graphs. Note that since the degree of the newly added node is no longer zero, its chances of being selected in subsequent trials increases.

Part of a computer network graph

To begin analysis, let’s examine the resilience of the computer network under an attack in which servers are chosen at random. Then let’s compare the resilience of the network to the resilience of ER and UPA graphs of similar size.

To begin, let’s determine the probability such that the ER graph computed using this edge probability has approximately the same number of edges as the computer network. Likewise, let’s compute an integer such that the number of edges in the UPA graph is close to the number of edges in the computer network. Remember that all three graphs being analyzed should have the same number of nodes and approximately the same number of edges.

Then for each of the three graphs (computer network, ER, UPA), let’s compute a random attack order and use this attack order to compute the resilience of the graph.

The following figure shows the algorithms to be used to find the largest component size in a graph.

The following figures show how BFS finds a component starting from a source node and visiting all the nodes reachable from the source.

Once the resilience for all three graphs are computed, let’s plot the results as three curves combined in a single plot. The horizontal axis for the plot is going to be the the number of nodes removed (ranging from zero to the number of nodes in the graph) while the vertical axis should be the size of the largest connected component in the graphs resulting from the node removal.

Consider removing a significant fraction of the nodes in each graph in random order . We will say that a graph is resilient under this type of attack if the size of its largest connected component is roughly (within ~25%) equal to the number of nodes remaining, at all times during the attack.

As can be seen from the above figure, both the ER and UPA graphs are resilient, but the network graph is not under this attack.

20% of nodes ~ 270 nodes. If these first 270 nodes are removed from the total 1347 nodes,

For network graph the initial size of the largest component = 1239, final size after removal of these nodes = 889 which is ~28% reduction in size > 25%

For ER graph the initial size of the largest component = 1337, final size after removal of these nodes = 1047 which is ~ 21.69% reduction in size < 25%

For UPA graph the initial size of the largest component = 1347, final size after removal of these nodes = 1077 which is ~ 20% reduction in size < 25%

In the next three problems, let’s consider attack orders in which the nodes being removed are chosen based on the structure of the graph. A simple rule for these targeted attacks is to always remove a node of maximum (highest) degree from the graph. The function targeted_order takes an undirected graph ugraph and iteratively does the following:

Computes a node of the maximum degree in ugraph . If multiple nodes have the maximum degree, it chooses any of them (arbitrarily).
Removes that node (and its incident edges) from ugraph.

Observe that targeted_order continuously updates ugraph and always computes a node of maximum degree with respect to this updated graph. The output of targeted_order is a sequence of nodes that can be used as input to compute_resilience.

As we examine the algorithm targeted_order, it can be seen that is not as efficient as possible. In particular, much work is being repeated during the location of nodes with the maximum degree. In this question, we will consider an alternative method (which we will refer to as fast_targeted_order) for computing the same targeted attack order. Here is a pseudo-code description of the method:

To continue our analysis of the computer network, we will examine its resilience under an attack in which servers are chosen based on their connectivity. We will again compare the resilience of the network to the resilience of ER and UPA graphs of similar size.

Using targeted_order (or fast_targeted_order), the task is to compute a targeted attack order for each of the three graphs (computer network, ER, UPA) from previous question. Then, for each of these three graphs, let’s compute the resilience of the graph using compute_resilience . Finally, let’s plot the computed resiliences as three curves (line plots) in a single plot.

Now, let’s consider removing a significant fraction of the nodes in each graph using targeted_order. Let’s examine the shape of the three curves from the previous plot in Question 4. Which of the three graphs are resilient under targeted attacks as the first 20% of their nodes are removed?

ER graph is resilient, but the UPA and the network graphs are not resilient under this targeted attack.

20% of nodes ~ 270 nodes. If these first 270 nodes are removed from the total 1347 nodes,

For network graph the initial size of the largest component = 1239, final size after removal of these nodes = 3 which is ~99.76% reduction in size >> 25%

For ER graph the initial size of the largest component = 1333, final size after removal of these nodes = 990 which is ~ 25.73% reduction in size ~ 25%

For UPA graph the initial size of the largest component = 1347, final size after removal of these nodes = 19 which is ~ 98.59% reduction in size >> 25%

Some NLP: Spelling Correction with Noisy Channel Model in Python

May 6, 2017May 6, 2017 / Sandipan Dey / Leave a comment

The following problem appeared as an assignment in the coursera course Natural Languange Processing (by Stanford university). The following description of the problem is taken from the assignment description and the course slides.

The following figure shows the basic concepts of spelling correction using the noisy channel model. Given the misspelled word, the most probable correct word can be computed by

generating the possible correct words (from the vocabulary) with some edit-distance model.
estimating the prior and the likelihood probabilities using some language model.
computing the posterior probabilities and the MAP estimate using the Bayes Theorem.

The following figure shows how the candidate words (possible correct words) can be generated (mostly the words within edit distance 1) and then a suitable language model can be used to estimate (with MLE) the prior and the likelihood (insertion / deletion etc.) probabilities from the training corpus.

The following figure shows the real world spelling correction methods using the noisy channel model, we are going to use these techniques to solve the spelling-correction problem. Given a k-word sentence W=w1,w2,…,wk, we are going to apply the following steps to compute the most probable sentence with the possibly-corrected misspelled–words:

For each word w_i generate the candidate set (e.g., the words that are withing 1-edit distance).
Use a language model to compute the probability of a sequence P(W) and then find the most probable word-sequence W.

The following figure shows the different language models that are going to be used to compute the MLE (without / with smoothing) probabilities and also how they can be used to compute the probability of a sentence (i.e. a sequence of words).

Given a training corpus, the parameters for the language models are first estimated from the corpus.
Then these probabilities are used to compute the most probable (possibly spell-corrected) sequence of candidate words for each sentence in the dev corpus.
The accuracy of spell-correction on the dev corpus is computed using the ground truth for each language model and reported.

Results

The following results show the accuracy of spell-correction using the different language models on the dev corpus:

Uniform Language Model: correct: 31 total: 471 accuracy: 0.065817
Unigram Language Model: correct: 29 total: 471 accuracy: 0.061571
Laplace Unigram Language Model: correct: 52 total: 471 accuracy: 0.110403
Laplace Bigram Language Model: correct: 64 total: 471 accuracy: 0.135881
Stupid Backoff Language Model: correct: 85 total: 471 accuracy: 0.180467
Custom Language Model (Simple Linear Interpolation λ1=0.17,λ2=0.39,λ3=0.44): correct: 91 total: 471 accuracy: 0.193206

The following table shows some sentences from the dev corpus with misspelled word(s) along with the ground truths and the noisy channel model-corrected outputs with different language models.

The next figure shows the simple linear interpolation language model accuracy on dev corpus with different values of λ1 and λ2.

Implementing LASSO Regression with Coordinate Descent, Sub-Gradient of the L1 Penalty and Soft Thresholding in Python

May 4, 2017May 5, 2017 / Sandipan Dey / Leave a comment

This problem appeared as an assignment in the coursera course Machine Learning – Regression, part of Machine Learning specialization by the University of Washington. The following description of the problem is taken directly from the assignment.

The goal of this assignment is to implement an LASSO Solver using coordinate descent. The following are the sub-tasks:

First the features are needed to be normalized.
The coordinate descent for LASSO needs to be implemented (with the subgradient of the L1 penalty).
The effects of L1 penalty are going to be explored.

The following dataset (few rows and columns are shown in the below table) is from house sales in King County, the region where the city of Seattle, WA is located. This is going to be our input datatset for training the LASSO model, with the response variable as price.

Normalize features

In the house dataset, features vary wildly in their relative magnitude: sqft_living is very large overall compared to bedrooms, for instance. As a result, weight for sqft_living would be much smaller than weight for bedrooms. This is problematic because “small” weights are dropped first as l1_penalty goes up.

To give equal considerations for all features, we need to normalize features by dividing each feature by its 2-norm so that the transformed feature has norm 1.

Implementing Coordinate Descent with normalized features

The following theory is going to be used to implement the coordinate descent (for normalized feature_j, we shall have z_j=1):

Effect of L1 penalty

Let us consider a simple model with 2 features:

simple_features = ['sqft_living', 'bedrooms']

Single Coordinate Descent Step

Using the formula above, let’s implement coordinate descent that minimizes the cost function over a single feature i. Note that the intercept (weight 0) is not regularized. The function should accept feature matrix, output, current weights, l1 penalty, and index of feature to optimize over. The function should return new weight for feature i.

Cyclical coordinate descent

Now that we have a function that optimizes the cost function over a single coordinate, let us implement cyclical coordinate descent where we optimize coordinates 0, 1, …, (k-1) in order and repeat.

When do we know to stop? Each time we scan all the coordinates (features) once, we measure the change in weight for each coordinate. If no coordinate changes by more than a specified threshold, we stop.

For each iteration:

As we loop over features in order and perform coordinate descent, measure how much each coordinate changes.
After the loop, if the maximum change across all coordinates is falls below the tolerance, stop. Otherwise, go back to step 1.

Return weights

IMPORTANT: when computing a new weight for coordinate i, make sure to incorporate the new weights for coordinates 0, 1, …, i-1. One good way is to update the weights variable in-place.

The following figure shows the coefficient path for LASSO for different values of the L1-penalty using two simple features

im3

Evaluating LASSO fit with more features

Let us consider the following set of features.

all_features = ['bedrooms',
                'bathrooms',
                'sqft_living',
                'sqft_lot',
                'floors',
                'waterfront', 
                'view', 
                'condition', 
                'grade',
                'sqft_above',
                'sqft_basement',
                'yr_built',
                'yr_renovated']

The following figure shows the coefficient path for LASSO for different values of the L1-penalty using all the above features:

im4