Assignment 4: Stereo

Overview

In this assignment you will analyze several algorithms for performing stereo-based 3D surface reconstruction. You should read and understand the paper by Scharstein & Szeliski (particularly Sections 1-3) which describes the notion of disparity space along with several diffusion techniques for improving the quality of reconstructed surface. This entire project should be doable in MATLAB.

1. Disparity space (20 points)

In class, we discussed the notion of disparity space: the 3D volume of intensity differences parameterized over the pixels in one stereo image and the range of possible disparities. It is useful to think about stereo algorithms as performing some type of aggregation (or diffusion) across disparity space in order to form a final estimate of the depth at each pixel in the respective camera (e.g., summing the differences within small windows of pixels is the most common example). The first goal of this assignment is to visualize disparity space for some real scenes and evaluate the performance of three different strategies for estimating scene depth: Do the following:

• Download stereo data from here (courtesy Middlebury College, Michael Holroyd, and Jiajun Zhu). You will find three test scenes. First focus on the 'cones' and 'teddy' datasets. Note that you are given a left/right rectified stereo pair and ground truth disparity images. Disparities are encoded using a scale factor 4 for gray levels 1 .. 255, while gray level 0 means unknown disparity. Therefore, the encoded disparity range is 0.25-63.75 pixels. These disparities will be used to evaluate the performance of various reconstruction techniques so understand how they are organized.
• Select 4 segments, each 200 pixels, within the reference image at interesting regions in the scene (i.e., across depth boundaries, textured areas, texture-less areas, above non-Lambertian surfaces, etc.).
• For each line segment (indexed here by 'x'), compute the following 2D function of pixel intensity differences between the two images for different disparity values (note this is a 2D slice of the full 3D disparity space we discussed in class and that is described in [Scharstein & Szeliski 1996]):
  
  
• Show these images in your write-up along with an indication of the corresponding segments in the stereo images (you only need to show the reference image from each pair). Comment on how this visualization of disparity space would indicates at what pixels finding the *correct* disparity would be easy/difficult. Relate properties of the disparity space image to properties in the scene.

2. Stereo (50 points)

Your next task is to implement several stereo matching algorithms.

• Implement the standard "sum-of-squares" algorithm for a range of window sizes. You may wish to think of this as aggregating the squared intensity differences (which are stored in the function you previously computed) within N-size windows and selecting that disparity at each pixel with the lowest total sum. Run your code on the segments from the first part for different window sizes (1, 3, 5, 10, 20) and plot the final disparity for each along with ground truth. Note that because we are considering only one row of pixels at a time you will only need to aggregate intensity differences within Nx1 image windows. Include these plots in your write-up and comment on what you see. How well does the reconstruction algorithm perform in mainly flat textured areas?, texture-free areas?, near depth boundaries? around non-Lambertian surfaces? How does its performance depend on the size of the window considered?
• Implement the "membrane model" of [Scharstein & Szeliski 1996] (Section 3 of their paper). Because we are only considering 1D segments of pixels, the neighborhood (N_4 in the paper) will only include the two neighborhing pixels to the left and right of each pixel. Use the values of \lambda and \beta that are reported in the paper and perform n=10 iterations of the discrete update rule. For one segment, include 10 images showing the disparity space image after each application of the update rule (you should see the "diffusion" process at work).
• Set the disparity at each pixel to that with the lowest value after applying these updates and add this result to the plot from above (so you are comparing this with the standard "sum-of-squares" approaches with varying window size and ground truth). Does the diffusion process improve the results? Where? Why?
• Implement diffusion with local stopping criteria (Section 3.2 in [Scharstein & Szeliski 1996]). The first step is to compute the winner margin certainty measure at each pixel in each segment from part 1. Visulize this certainty by plotting it on top of its corresponding 2D disparity space image (use plot command and watch the scales of the axes). Comment about this measure of certainty: How well does it identifying pixels at which you would expect accurate matches? Next, apply the discrete update rules that follow this certainty measure as described in Section 3.2 of the paper. Show 10 images after applying each update for the same segment as above. Lastly, add the plot of the final depth estimate to the plot with the other results (and, of course, include this in your write-up). Does incorporating a notion of certainty improve the quality of the matches at each pixel? Your answer should refer to specific areas and relate them to properties of the scene.

2. Space-time Stereo (30 points)

In this part of the assignment you will implement and evaluate a space-time stereo algorithm.

Implement the following:

• Download data from the 'vase' scene here. This zip file contains a rectified image sequence captured with synchronized cameras and a digital projector displaying high-frequency intensity patterns. You do not have ground truth disparity information for this scene.
• Find two, 200-pixel wide segments as before that interesect "interesting" features in the data.
• Using only the first pairs of frames (L_00.png and R_00.png), compute the disparity space function as before. Show this in your write-up along with an indication of the originating image segments.
• Using windows of varying sizes (1, 5, 10), find the disparity at each pixel in each segment that has a minimum SSD score. Plot these depth estimates together. Discuss the quality of these estimates keeping in mind that you don't have ground truth (refer to features in the reference image).
• Compute a slightly modified version of the disparity space function as follows (note the sum is over frames in the sequence):
  
  
• Compute the certainty of each pixel in each segment using the winner margin metric as before. Next, compare this visualization of disparity space that considers windows in time and its certainty to those computed for only the first frame in the sequence. What do you notice? How has the certainty improved? Where is the certainty still small?
• Finally, assign the disparity at each pixel in each segment to that with the minimum squared difference across the entire sequence of frames (i.e., this corresponds to a space-time window with a 1x1 spatial extent and a temporal extent equal to the length of the sequence, 32 in our case). Compare this estimate with the estimates from above that consider only the first frame and use varying window sizes. Comment on the accuracy obtained with the space-time approach.
• Extra credit (5 points): estimate the disparity at every pixel in the reference image using whichever method worked best from above (you may want to implement this in C). Visualize this disparity as a grayscale image and generate a PLY file that contains the resulting 3D point cloud. The trimesh2 library has many helpful I/O routines and an interactive viewer for PLY files. Include the disparity image, screenshots from an interactive viewer, and the PLY file in your submission.

Submitting

This assignment is due Thurday, November 20, 2007 at 11:59 PM. Please see the general notes on submitting your assignments, as well as the late policy and the collaboration policy.

Please submit:

• Your write-up as an HTML file with embedded images and links to your code. Please include all of your beautiful visualizations of disparity space and pixel certainty along with answers to any questions posed in the assignment.