XPM

X11 already had XBM for monochrome bitmaps, stored as a C array you could #include straight into a program. XPM, written by Arnaud Le Hors at France's Groupe Bull around 1989, did the same for colour. A file is an array of strings: first the dimensions and number of colours, then a colour table mapping each character to a colour — by name from X's colour database, or None for transparent — and finally the image itself, row by row, readable in a text editor.

The libXpm library turned the file into a pixmap plus a mask. The format became the standard for icons in window managers such as FVWM and in early GTK applications. Parsing C-ish text turned out harder than it looked, though; libXpm was still having buffer overflows patched well into the 2020s.