EE588 Project #1- Image Segmentation Solved

Matlab commands are specified in the text and in blue Courier font and python commands in parenthesis and in red Courier font; variables and filenames (or commands that are consistent between Matlab and python) are specified in black Courier font. In the following python syntax, I have used import matplotlib.pyplot as plt, import numpy as np, import os, import glob, import imageio, import skimage.color, import skimage.filters, and import skimage.measure, import skimage.morphology, and import skimage.feature; additionally, you probably want to use %matplotlib inline to include figures inline in your jupyter notebook. Additionally, for python users, I recommend using the image I/O functions in imageio as they have caused me the least heartburn of other options.

We will be using the Berkeley Segmentation Dataset (BSDS). Familiarize yourself with the overview of the dataset at https://www2.eecs.berkeley.edu/Research/Projects/CS/vision/bsds/. Of particular note is the original conference paper describing the dataset and the segmentation metrics that we will use in this project https://www2.eecs.berkeley.edu/Research/Projects/CS/vision/grouping/papers/mftm-iccv01.pdf (and martin2001_BSDS.pdf on canvas), the description of the segmentation (ground truth) file format at https:// www2.eecs.berkeley.edu/Research/Projects/CS/vision/bsds/seg-format.txt, and the links to download the images and the human segmentations (ground truths) at https://www2.eecs.berkeley.edu/Research/Projects/CS/vision/bsds/BSDS300-images.tgz and https://www2.eecs.berkeley.edu/Research/Projects/CS/vision/bsds/BSDS300-human.tgz, respectively. Download both the images and human segmentations and extract those files on your computer. You notice that the images are included in two basic directories BSDS300/images/train and BSDS300/images/test while the ground truths in BSDS300/human are separated by a numeric identifier associated with which human generated that segmentation. Dr. Boucheron has kindly done some leg work for you and sorted out one human generated segmentation for each dataset image so that you can have an easy correspondence. You can download this image sorted ground truth from canvas (BSDS_GT_image_sorted.tgz) and extract it from the same directory where you extracted the other .tgz files and it will extract to a directory BSDS300/ground_truth. Underneaththatdirectoryaregray/andcolor/forthehumansegmentations done on gray and color images, respectively. Underneath those directories are train/ and test/.

(a) Generate GT image masks.

• After carefully reading the description of the segmentation file format (https://www2.eecs.berkeley.edu/

  Research/Projects/CS/vision/bsds/seg-format.txt), write a function make_mask(seg_filename,mask_filename) that creates image masks of the ground truth. The function make_mask should write out the image masks to filename mask_filename to be read in later; as a suggestion, you may want to create folders masks/color/test, masks/color/train, masks/gray/test, and masks/gray/train directly underneath the directory where you extracted the BSDS and same masks with the same base filename ID as the original image. Note—you will want to save to an image format that does not compress, otherwise, you will introduce compression artifacts. I found .png to work well for me. The mask created should be the same size as the image (481×321 or 321×481) wherein each pixel is assigned a number corresponding to the “segment number” from the segmentation file. If you were to view your mask image using imagesc (plt.imshow), you should see regions with different intensities.

• Loop over the 200 training images and 100 test images and create masks for each. You can loop over filenames in a directory with the following code
  image_directory = “path_to_BSDS_images”; % with trailing / seg_directory = “path_to_BSDS_segs”; # with trailing / image_filenames = dir([image_directory,’*.jpg’]);

       seg_filenames = dir([seg_directory,’*.seg’]);
       for f=1:length(image_filenames)
  

           image_filename = image_filenames(f).name;
           seg_filename = seg_filenames(f).name;
  

end

(image_directory = ‘path_to_BSDS_images’ # with trailing / seg_directory = ‘path_to_BSDS_segs’ # with trailing / image_filenames = sorted(glob.glob(image_directory+’*.jpg’)) seg_filenames = sorted(glob.glob(seg_directory+’*.seg’)) for f,image_filename in enumerate(image_filenames):

seg_filename = seg_filenames[f])
at which point f will be a handy index and image_filename and seg_filename contain a string with the image filename or segmentation filename, respectively. Notes: 1) For Matlab, you will need to prepend the path to filename in question, e.g., [image_directory,image_filename] (for python, the filenames will have the paths included and can be removed with os.path.basename(image_filename) if desired). 2) The above code is not robust—it assumes the image and segmentation filenames are in the same order (so that the f-th image filename corresponds to the f-th segmentation filename); more robust code can be defined, but is out of the scope of this project.

• Display the image 100075.jpg, the segmentation mask generated from the human color segmentation, and the segmentation mask generated from the human grayscale segmentation.

(b) Create GCE/LCE function.

• After carefully reading the definition of the global consistency error and local consistency error in

  martin2001_BSDS.pdf, write a function [GCE,LCE]=compute_GCE_LCE_loopy(Seg,GT) ([GCE,LCE]=compute_GCE_LCE_loopy(Seg,GT)) where GCE and LCE are the global consistency error and the local consistency error, respectively for the given segmentation Seg and ground truth GT. The way the equation is defined in martin2001_BSDS.pdf, you will loop over all pixels in the image. This will not be an efficient implementation, but is often the best place to start to make sure you have the correct logic. Vectorization can be done later.

• Using image 100075.jpg, read in the image I=double(imread(‘100075.jpg’)) (I=imageio.imread(‘100075.jpg’)). Convert the image I to grayscale using the equation I_gray=(0.2125*I(:,:,1)+0.7154*I(:,:,2)+0.0721*I(:,:,3))/255 (I_gray=skimage.color.rgb2gray(I)). Threshold the image I_gray at a value of 127/255 (127/255.), setting the lighter pixels to a value of 1. Display this thresholded image.
• You will note that this thresholding has resulted in many disconnected regions of white. We will treat each of the connected components in this thresholded image as a separate region (or “segment” in the terminology of the BSDS). Use the command bwlabel (skimage.measure.label) to assign a different integer number to each of the connected components in your thresholded image, and call this labeledimageSeg. Note—whileboththeGTmaskGTandyourlabeledimageSeghaveintegers assigned to different segments, there is not necessarily a correspondence (i.e., segment ‘3’ is not necessarily the same entity in your GT mask and segmentation). Display this labeled image Seg using imagesc (plt.imshow).
• Use your function compute_GCE_LCE_loopy to compute the GCE and LCE for your labeled image Seg and the GT mask from masks/gray/ and print out your values for GCE and LCE. You should getvaluesof(0.2864,0.1561)for(GCE,LCE),respectively. Mycompute_GCT_LCE_loopy (python) code took a few minutes to compute these values for the 100075.jpg image.
• You can avoid looping over all pixels and speed up the computation significantly. I have uploaded obfuscated code in compute_GCE_LCE.m (compute_GCE_LCE.py) which runs in a few seconds forthesame100075.jpgimage. Includecompute_GCE_LCE.minyourworkingdirectory (include compute_GCE_LCE.py in your working directory and import with from compute_GCE_LCE import compute_GCE_LCE) and call with the basic syntax [GCE,LCE]=compute_GCE_LCE(Seg,GT)(GCE,LCE=compute_GCE_LCE(Seg,GT)). You should get the same values (0.2864, 0.1561) for (GCE, LCE), respectively, using this code.
• I will give 10% extra credit if you write your own computationally efficient code to compute the GCE and LCE values without looping over each pixel. Please do not work on this at the expense of the rest of the project.

  (c) Evaluate Otsu thresholding.

• In this part, you will loop over the 100 test images in BSDS300/images/test and compute the

performance of the Otsu threshold for image segmentation. There are slight differences in implementation of the commands for converting images to grayscale using rgb2gray (skimage.color.rgb2gray) and in computing the Otsu threshold using graythresh (skimage.filters.threshold_otsu). In light of keeping you familiar with the most common functions in each language, the numerical results for Matlab versus python will be slightly different from here on out.

• As a check, using our now favorite image 100075.jpg, read in the image I=imread(‘100075.jpg’)(I=imageio.imread(‘100075.jpg’)). ConverttheimageI to grayscale using I_gray=rgb2gray(I) (I_gray=skimage.color.rgb2gray(I)). Calculate the Otsu threshold for this image T=graythresh(I_gray) (T=skimage.filters.threshold_otsu(I_gray)). Youshouldget0.4196(0.4133). Note that Matlab returns a threshold scaled to the range [0,1] even though your image I and I_gray are in the range [0,255]; this will be important when you apply this threshold to your image.
• Since there are no parameters in the Otsu method available to tune, we don’t need to use the BSDS300/ images/train images. You will compute the GCE and LCE value for each of the 100 images in BSDS/images/test, comparing to the masks in masks/gray/test and masks/color/test. You will thus create four length-100 vectors GCE_gray, LCE_gray, GCE_color, and LCE_color. Within your loop over the 100 images, your code will likely be structured something like the following pseudocode:
  • ReadineachimageI
  • ConverttheimageItograyscaleasdescribedabovecompute,yieldingI_gray
  • ComputetheOtsuthresholdT
  • ThresholdtheimageusingtheOtsuthreshold,yieldingI_thresh
  • AssigneachconnectedcomponentinI_threshtoasegmentusingbwlabel

    (skimage.measure.label) yielding Seg

  • Readinthecorrespondingmasks/gray/masktoGT_gray
  • Readinthecorrespondingmasks/color/masktoGT_color
  • Callcompute_GCE_LCE(Seg,GT_gray)andstoretheoutputstotheGCE_grayand

    LCE_gray vectors

  • Callcompute_GCE_LCE(Seg,GT_color)andstoretheoutputstotheGCE_colorand

    LCE_color vectors

• Display the average and standard deviation of the GCE_gray, LCE_gray, GCE_color, and

  LCE_color. What do these numbers tell you about the performance of the Otsu algorithm compared

  to the human segmentations?

• Display the image and segmented image that had the worst segmentation performance (according to any

  of the four measures GCE_gray, LCE_gray, GCE_color, and LCE_color).

• Display the image and segmented image that had the best segmentation performance (again according to

  any of the four measures GCE_gray, LCE_gray, GCE_color, LCE_color). (d) Evaluate Canny edge detection.

• In this part, you will loop over the 100 test images in BSDS300/images/test and compute the performance of the Canny edge detection for image segmentation.
• We have an interesting problem in interpreting the Canny edge detector since it returns edges rather than regions. We have two choices: 1) we can try to wrangle the output of the Canny edge detector to result in closed contours which we can convert to edges or 2) we can wrangle the GT to output edges. We’ll take the approach of the latter. We will then use the GCE and LCE to compare those two edgy outputs. Code to compute the labeled boundaries of the ground truth masks:

       GT_b = zeros(size(GT));
       for s = 0:max(max(GT))
  

           current_b = (GT==s) – imerode((GT==s),strel('disk',1));
  

           GT_b = GT_b | current_b;
       end
  

       GT_b = bwlabel(GT_b);
  

  (GT_b = np.zeros(GT.shape,’uint8′) for s in range(0,GT.max()):

         current_b = np.asarray((GT==s),'uint8') -\
                     skimage.morphology.binary_erosion(GT==s,\

                     skimage.morphology.disk(1))
         GT_b = GT_b | current_b

     GT_b[0,:] = 0; GT_b[-1,:] = 0; GT_b[:,0] = 0; GT_b[:,-1] = 0

GT_b = skimage.measure.label(GT_b))

• We will use the default parameters (the upper and lower thresholds for the Canny edge detector) and thus

  don’t need to use the BSDS300/images/train images. You will compute the GCE and LCE value for each of the 100 images in BSDS/images/test, comparing to the masks in masks/gray/test and masks/color/test. You will again create four length-100 vectors GCE_gray, LCE_gray, GCE_color, and LCE_color. Within your loop over the 100 images, your code will likely be structured something like the following pseudocode:

  • ReadineachimageI
  • ConverttheimageItograyscaleasdescribedabovecompute,yieldingI_gray
  • ComputetheimageedgesusingI_canny=edge(I_gray,’canny’)

    (I_canny=skimage.feature.canny(I_gray))

  • AssigneachconnectedcomponentinI_cannytoasegmentusingbwlabel

    (skimage.measure.label) yielding Seg

  • Readinthecorrespondingmasks/gray/masktoGT_gray
  • Readinthecorrespondingmasks/color/masktoGT_color
  • Callcompute_GCE_LCE(Seg,GT_gray)andstoretheoutputstotheGCE_grayand

    LCE_gray vectors

  • Callcompute_GCE_LCE(Seg,GT_color)andstoretheoutputstotheGCE_colorand

    LCE_color vectors

• Display the average and standard deviation of the GCE_gray, LCE_gray, GCE_color, and

  LCE_color. What do these numbers tell you about the performance of the Canny algorithm compared

  to the human segmentations?

• Display the image and segmented image that had the worst segmentation performance (according to any

  of the four measures GCE_gray, LCE_gray, GCE_color, and LCE_color).

• Display the image and segmented image that had the best segmentation performance (again according to

  any of the four measures GCE_gray, LCE_gray, GCE_color, LCE_color).

• Briefly discuss the implications of comparing only edges for quantifying segmentation performance in

  this case as compared to using regions in the Otsu case. Do you think the GCE and LCE metrics are

  valid and/or accurate for this scenario? (e) Evaluate active contours.

• In this part, you will use the 200 training images in BSDS300/images/train to tune performance of the active contours without edges (ACWE) algorithm and the 100 test images in BSDS300/images/test to compute the performance.
• We need to decide how to specify the initial contour for the active contours algorithm. It is common practice to use a threshold to initialize the algorithm so as to put the initial contour relatively close to the final location. In this project, we will use the Otsu threshold to intialize the active contours algorithm. We also need to determine a reasonable number of iterations for the method to properly converge.
• Using our now favorite image 100075.jpg, read in the image I, convert to grayscale I_gray, and threshold at the Otsu threshold T, yielding I_init. Compute the ACWE segmentation using I_acwe=activecontour(I_gray,I_init,NumIts,’Chan- Vese’,’SmoothFactor’,0.25) (I_acwe=skimage.segmentation.chan_vese(I_gray,mu=smoothval,max_iter=Nu mIts,init_level_set=I_init)) for NumIts=1:250:1000 (NumIts=np.arange(1,1100,250)). This will give you an idea of how the contour is evolving over iterations. Display the resultant segmentation for each of these four segmentations.
• We will next look at the effect of the smoothing parameter ‘SmoothFactor’ (mu) on the performance of the resultant segmentations.
• You will loop over the range of smoothing parameters 0:0.1:1, (np.arange(0,0.055,0.005), compute the ACWE segmentation and GCE, LCE value for each of the 200 images in BSDS/images/

train, comparing to the masks in masks/gray/train and masks/color/train. You will create four 200×10 arrays GCE_gray_train, LCE_gray_train, GCE_color_train, and LCE_color_train to store the results for each image and each smoothing parameter value. Within your loop over the 200 images, your code will likely be structured something like the following pseudocode (note that you will also have an outer loop over the range of smoothing parameters):

◦ ReadineachimageI

• ConverttheimageItograyscaleasdescribedabovecompute,yieldingI_gray
• ComputethecontourinitializationI_init=I_gray>TbythresholdingwiththeOtsuthresholdT
• ComputetheACWEsegmentationusing

  I_acwe=activecontour(I_gray,I_init,NumIts,’Chan- Vese’,’SmoothFactor’,smoothval) (I_acwe=skimage.segmentation.chan_vese(I_gray,mu=smoothval,max_iter =NumIts,init_level_set=I_init))

• AssigneachconnectedcomponentinI_acwetoasegmentusingbwlabel (skimage.measure.label) yielding Seg
• Readinthecorrespondingmasks/gray/masktoGT_gray
• Readinthecorrespondingmasks/color/masktoGT_color
• Callcompute_GCE_LCE(Seg,GT_gray)andstoretheoutputstotheGCE_gray_trainand

  LCE_gray_train arrays

• Callcompute_GCE_LCE(Seg,GT_color)andstoretheoutputstotheGCE_color_train

  and LCE_color_train arrays

• Display the average and standard deviation of the GCE_gray_train, LCE_gray_train,

  GCE_color_train, and LCE_color_train for each of the smoothing parameters studied. What

  value for the smoothing parameter ‘SmoothFactor’ (mu) would you choose as the “best”? Why?

• For the value of ‘SmoothFactor’ (mu) that you determined to be the best, compute the segmentation

  for the 100 test images in BSDS/images/test and their corresponding GCE_gray, LCE_gray, GCE_color, and LCE_color metrics. Display the average and standard deviation of these four

  measures. What do these numbers tell you about the performance of the ACWE algorithm compared to

  the human segmentations?

• Display the image and segmented image that had the worst segmentation performance (according to any

  of the four measures GCE_gray, LCE_gray, GCE_color, and LCE_color).

• Display the image and segmented image that had the best segmentation performance (again according to

  any of the four measures GCE_gray, LCE_gray, GCE_color, LCE_color). (f) Summary.

• Provide a summary of what you learned about image segmentation and the process of quantifying image segmentation.
• Provide a summary of the highlights of the results obtained for the various image segmentations. What algorithms would you claim is “best”? “Worst”? Looking qualitatively at the best segmentations, do you feel they are doing a particularly good job?
• What do you think is the biggest issue affecting the segmentation performance? How might you address that issue?
• This summary should be lengthy enough to get your point across (probably 1-2 paragraphs), but not long for the sake of length. Quantity does not equal quality here…