HOG enhancement to accept color images #1963

[Pradyumna Kumar N]

Hi all,


I implemented the HOG for color images based on the excerpt from the 
original paper "For colour images, we calculate separate gradients for each 
colour channel, and take the one with the largest norm as the pixel’s 
gradient vector"[Dalal and Triggs] and the CVPR presentation "For color 
image, pick the color channel with the highest gradient magnitude for each 
pixel"[Dalal and Triggs].


To allow this change, I refactored the code so that image gradients are 
computed in a separate method. If the image has more than 1 channels, this 
method is called on each channel. Gradient magnitude is calculated for each 
channel and for each pixel, the gradient vector with maximum magnitude is 
considered.


This is my first open source contribution and I implemented the change 
based on my understanding. I pushed my changes to Github. Please review my 
code and let me know if I implemented the functionality correctly. Please 
comment on coding style too.
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