This article is about the relation between an image size in pixels and a photographic print size. First, it recommends the photographic print size for given digital image dimensions in pixels. Second, it finds the resulting pixels per inch value for a printed digital image with known linear and pixel dimensions.

This online calculator converts inches to pixels and vice versa given the resolution in pixels per inch

This online calculator adjusts the second dimension of a picture/video, given the new first dimension, keeping the original proportions

This online calculator inverts image colors and produces photo negatives.

This article describes how to write calculators for an image manipulation