I don't know what that means. Do you mean that you might have a, say, 10 megapixel image but you want to blur only a 200x200 +/- 4? In other words, blur an area that's somewhere between 39,996 and 40,004 pixels? How would you specify where that small area is located inside your 10 megapixel image? Here's some code snippets that may be helpful
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Mask by some binary image.
maskedRed = redChannel .* mask;
maskedGreen = greenChannel .* mask;
maskedBlue = blueChannel .* mask;
% Recombine separate channels into a single RGB image.
rgbImage = cat(3, redChannel, greenChannel, blueChannel);
maskedRgbImage = cat(3, maskedRed, maskedGreen, maskedBlue);
% Blur an image (average in a window)
blurredImage = conv2(redChannel, ones(windowWidth)/windowWidth^2, 'same');
Note: this is not a working script - it's just some snippets. See what you can get with those code statements I gave you. Write back with your program if you need more hints.
I'm not sure what it means to change a pixel into the average of the surrounding pixels plus or minus 4 color values. The pixel is either the average value or it's some other value, like the average+4 or the average-4, but you have to specify because the pixel has to have one value, not a range of values.
