I recently made a console theme for my website. Photos left unmodified looked too clean and fancy to fit in with the theme. I played with the saturation and contrast functions of the CSS filter property, which gave okay results, but not enough. I thought a pixelated look fit in, and decided to see if it would be possible with pure CSS, something like pixel art or perhaps sixel on a low resolution terminal. This Stack Overflow answer led me to the solution of using an SVG filter through a url() value of the filter property.
filter posts
Apparently, callbacks to the WordPress function add_filter() don’t receive more than one argument to their callback unless you specify the fourth argument to the add call as a number greater than 1.