Programming Project #4 (second part) 15-463: Computational Photography

FEATURE MATCHING for AUTOSTITCHING

(second part of a larger project)

Due Date: by 11:59pm, Friday, Nov 12

The goal of this project is to create a system for automatically stitching images into a mosaic. A secondary goal is to learn how to read and implement a research paper. The project will consist of the following steps:

1. Detecting corner features in an image (10 pts)
2. Extracting a Feature Descriptor for each feature point (10 pts)
3. Matching these feature descriptors between two images (20 pts)
4. Use a robust method (RANSAC) to compute a homography (30 pts)
5. Proceed as in Project 3 to produce a mosaic (30 pts; you may use the same images from part A, but show both manually and automatically stitched results side by side) [produce at least three mosaics]
6. Submit your results

Steps 1-3

For steps 1-3, we will follow the paper “Multi-Image Matching using Multi-Scale Oriented Patches” by Brown et al. but with several simplifications. Read the paper first and make sure you understand it. Then implement the algorithm:

• Start with Harris Interest Point Detector (Section 2). We won’t worry about muti-scale – just do a single scale. Also, don’t worry about sub-pixel accuracy. Re-implementing Harris is a thankless task – so you can use my sample code: harris.m . Include on your webpage a figure of the Harris corners overlaid on the image.
• Implement Adaptive Non-Maximal Suppression (Section 3). Include on your webpage a figure of the chosen corners overlaid on the image. The paper section is confusing; you may need to read it a few times. You may want to skip this step and come back to it; just choose a random set of corners instead in the meantime.

• Implement Feature Descriptor extraction (Section 4). Don’t worry about rotation-invariance – just extract axis-aligned 8x8 patches. Note that it’s extremely important to sample these patches from the larger 40x40 window to have a nice big blurred descriptor. Don’t forget to bias/gain-normalize the descriptors. Ignore the wavelet transform section.
• Implement Feature Matching (Section 5). That is, you will need to find pairs of features that look similar and are thus likely to be good matches. You may find dist2.m useful for fast distance computations. For thresholding, use the simpler approach due to Lowe of thresholding on the ratio between the first and the second nearest neighbors. Consult Figure 6b in the paper for picking the threshold. Ignore Section 6.

Step 4

For step 4, use 4-point RANSAC as described in class to compute a robust homography estimate.

Submit Your Results

You will need to submit all your code. Please include a README with your code describing where each of the steps was implemented. If you skipped a step, say so, to save your TA some time!

Bells & Whistles

• (10 points) Add multiscale processing for corner detection and feature description.
• (10 points) Add rotation invariance to the descriptors.
• (20 points) Implement panorama recognition. Given an unordered set of images, some of which might form panoramas, you need to automatically discover and stitch these panoramas together. Don’t worry about bundle adjustment, just see how far you can get with pair-wise homography estimation.