Describing rcnn_im_crop's mean subtraction from a region

region_mean_subtraction.m

# Crop the region out of the image with the bbox coordinates
# bbox = [x1 y1 x2 y2]
# size(im) = HEIGHT x WIDTH x NUM_CHANNELS

# window = im(y1:y2, x1:x2, all_channels)
# For every channel, crop out the height and the width
window = im(bbox(2):bbox(4), bbox(1):bbox(3), :);

# Warp the region
tmp = imresize(window, [crop_height crop_width], ...
    'bilinear', 'antialiasing', false);

# Subtract the portion of the mean that corresponds with the window
# See (0) for a walkthrough of the code
if ~isempty(image_mean)
  tmp = tmp - image_mean(pad_h+(1:crop_height), pad_w+(1:crop_width), :);
end
  
# (0)
1:crop_height % produces a vector of length == crop_height
1:crop_width  % produces a vector of length == crop_width

pad_h + (1:crop_height)
% assume pad_h = 10, crop_height = 3
1:crop_height = 1:3 = 1 2 3
pad_h + (1:crop_height) = [10+1 10+2 10+3] = [11 12 13]