Many image effects are based on shifting the color of each pixel based on the color of the 8 pixels surrounding it, for example blurring is essentially averagin

Many image effects are based on shifting the color of each pixel based on the color of the 8 pixels surrounding it, for example blurring is essentially averaging the color value of the surrounding pixels. Experimenting with multiplying, dividing, combining, and inverting color values gives you effects such as emboss, drop shadow, glow, inverse, etc.