Fundamentals of image convolutions

Author: Ravindra Sadaphule

Image convolution is powerful technique of modifying image by convolving a small 3x5, 5x5 matrix called kernel with image to product effects like emboss, outline, blur, sharpen. Convolving involves sliding a kernel over original image from left to right and top to bottom one row and column at a time, doing element wise multiplication and then summing the result. Depending on the values uses in the kernel, the outcome varies.

While human enjoys aesthetics of Pittsburg skyline on the left below, a machine sees black & white image just as array of pixel intensities on the right.

A convolution operation involves taking a small matrix like 3x3 shown below on the right, and slide it on original image matrix from left to right and top to beootom and at each iteration, perform element wise multiplication and then take a sum.