Project 1: Image Feature Extraction and Matching

This is Project 1 for UW CSE P576 Computer Vision.

Getting Started: To get started, download the source files here (Projects 1 and 2). To run the project locally you will need IPython/Jupyter installed, e.g., see instructions at The notebooks are written for Python 3.x. Launch Jupyter and open Project1.ipynb. Alternatively, you can import the standalone version of the notebook into Colaboratory and run it without installing anything. Use File->Upload Notebook in Colab and open the notebook in standalone/Project1s.ipynb.

This project: In this project you will build an image feature matcher, starting with simple convolution operations and working through interest point detection and descriptor extraction. Once you have a basic feature matcher working, try out some improvements and document your results. If you’re not already familiar with python/numpy, it is recommended to do an introduction such as:

What to turn in: Turn in your completed ipynb notebook as well as any source .py files that you modified. Clearly describe any enhancements or experiments you tried in your ipynb notebook. Put everything in a single flat zipfile and upload via the link in Canvas.

version 040820

In [ ]:
import numpy as np
import os.path
from time import time
import types
import matplotlib.pyplot as plt

import im_util
import interest_point

%matplotlib inline
# edit this line to change the figure size
plt.rcParams['figure.figsize'] = (16.0, 10.0)
# force auto-reload of import modules before running code 
%load_ext autoreload
%autoreload 2

Convolution and Image Filtering [25%]

Start by writing code to perform convolution in 1D. Open and edit the function convolve_1d. You should use only basic numpy array operations and loops. Don't worry about efficiency for now. You should see small errors compared to the reference numpy version.

Note that convolution and correlation are the same under a simple manipulation of the kernel (what is it?). For what kernels are convolution and correlation results identical?

In [ ]:
Test of convolve_1d
print('[ Test convolve_1d ]')
x = (np.random.rand(20)>0.8).astype(np.float32)
k = np.array([1,3,1])
y1 = im_util.convolve_1d(x, k)
y2 = np.convolve(x, k, 'same')
y3 = np.correlate(x, k, 'same')
print(' convolve error = ', np.sum((y1-y2)**2))
print(' correlate error = ', np.sum((y1-y3)**2))

We will now convolve a 2D image with a 1D kernel. Before you begin, get some image data by running in the data directory. Then complete the function convolve_rows in by convolving every row of the image by the kernel. Run the code below and check that the image output is sensible.

In [ ]:
Test of convolve_image

print('[ Test convolve_image ]')
im = im_util.image_open(image_filename)
k = np.array([1,2,3,4,5,6,5,4,3,2,1])
print(' convolve_rows')
im1 = im_util.convolve_rows(im, k)
print(' % .2f secs' % (t1-t0))
print(' scipy convolve')
im2 = im_util.convolve(im, np.expand_dims(k,0))
print(' % .2f secs' % (t1-t0))
print(' convolve_image error =', np.sum((im1-im2)**2))

# optionally plot images for debugging
#ax1,ax2=im_util.plot_two_images(im1_norm, im2_norm)          

You will probably find that the scipy convolve runs much faster than your version. To speed things up you can use this version (im_util.convolve) for all subsequent experiments. Note that this performs a general 2D convolution with a 2D kernel as input.

Now write code to perform Gaussian blurring. First implement the function gauss_kernel to compute a 1D Gaussian kernel. Then complete convolve_gaussian to perform a separable convolution with this kernel.

In [ ]:
Gaussian blurring test
print('[ Test convolve_gaussian ]')

print(' gauss kernel = ')

im1 = im_util.convolve_gaussian(im, sigma)
print(' % .2f secs' % (t1-t0))

ax1,ax2=im_util.plot_two_images(im, im1)

Now write code to compute horizontal and vertical gradients in the function compute_gradients. Use an explicit kernel that is convolved in each direction (i.e., do not use a built-in function such as numpy.gradient). Run the code below and check that the outputs look sensible.

In [ ]:
Gradient computation test
print('[ Test gradient computation ]')
img = np.mean(im,2,keepdims=True)
Ix,Iy = im_util.compute_gradients(img)

# copy greyvalue to RGB channels
Ix_out = im_util.grey_to_rgb(im_util.normalise_01(Ix))
Iy_out = im_util.grey_to_rgb(im_util.normalise_01(Iy))

im_util.plot_two_images(Ix_out, Iy_out)

Interest Point Extractor [25%]

Now you will use these convolution functions to implement a corner or interest point detector. Choose a well known detector, such as Harris or DoG, and implement the interest point strength function in corner_function of Run the code below to visualise your corner function output. Next detect corners as local maxima of this function by filling in find_local_maxima in the same file.

In [ ]:
Compute corner strength function
print('[ Compute corner strength ]')
ip_ex = interest_point.InterestPointExtractor()
ip_fun = ip_ex.corner_function(img)

# normalise for display

Find local maxima of corner strength
print('[ Find local maxima ]')
row, col = ip_ex.find_local_maxima(ip_fun)
ip = np.stack((row,col))

interest_point.draw_interest_points_ax(ip, ax2)

Descriptors and Matching [25%]

Now let's match our interest points. Start by extracting a very simple descriptor that is simply a patch of pixels around the interest point. To do this, fill in the function get_descriptors in The following code outputs a random set of normalised descriptor patches. Check that the output looks sensible. Once you have this working, try varying the sample spacing in your descriptor patch. "Sample spacing" here means distance in pixels in the base image between the pixels in your descriptor patch. What problem exists with sample spacings > 1 pixel? How can this be fixed?

In [ ]:
Extract descriptors
print('[ Extract descriptors ]')
descriptors=desc_ex.get_descriptors(img, ip)

We will now match descriptors between a pair of images. Run the following two code blocks to extract your interest points and extract and match descriptors. The second code block calls a function to perform nearest-neighbour matching of descriptors and filtering using a ratio test. Take a look at the code and check you understand how it works.

In [ ]:
Read a pair of input images and extract interest points

im1 = im_util.image_open(im_filename1)
im2 = im_util.image_open(im_filename2)

img1 = np.mean(im1, 2, keepdims=True)
img2 = np.mean(im2, 2, keepdims=True)

print('[ find interest points ]')
ip_ex = interest_point.InterestPointExtractor()
ip1 = ip_ex.find_interest_points(img1)
print(' found '+str(ip1.shape[1])+' in image 1')
ip2 = ip_ex.find_interest_points(img2)
print(' found '+str(ip2.shape[1])+' in image 2')
print(' % .2f secs ' % (t1-t0))

print('[ drawing interest points ]')
interest_point.draw_interest_points_ax(ip1, ax1)
interest_point.draw_interest_points_ax(ip2, ax2)
print(' % .2f secs ' % (t1-t0))
In [ ]:
Extract and match descriptors
print('[ extract descriptors ]')
desc_ex = interest_point.DescriptorExtractor()
desc1 = desc_ex.get_descriptors(img1, ip1)
desc2 = desc_ex.get_descriptors(img2, ip2)
print(' % .2f secs' % (t1-t0))

# Uncomment the following lines to use SIFT descriptors
# Note: you'll need to install cyvlfeat, e.g., conda install -c menpo cyvlfeat

#from cyvlfeat import sift 

print('[ match descriptors ]')
match_idx,ratio_pass=desc_ex.match_ratio_test(desc1, desc2)



print(' Number of interest points = '+str(N1))
print(' Number of matches passing ratio test = '+str(num_ratio_pass))

interest_point.draw_matches_ax(ip1r, ip2r, ax1, ax2)

The following code visualises matched descriptor patches. Can you distinguish the correct and incorrect matches? (reload to get another random sample).

In [ ]:
Plot descriptors for matched points

Testing and Improving Feature Matching [25%]

Try varying the ratio_threshold parameter in the descriptor matcher (DescriptorExtractor class params). What are good settings for this parameter? If everything is working, you should see a good set of correctly matched points (aim for about 100 or more). Experiment with your interest point and descriptor implementations to find which other parameters are important and try to get a good set of matches. Try out a new idea of your own to improve interest points or descriptors, and record your findings in the notebook below. Make sure to include sufficient figures/tables and explanations to demonstrate your base case and the effect of your modifications.

In [ ]:
### TODO experiments with your detector/descriptors