*.pbm, *.pgm and *.ppm

.pbm, .pgm and .ppm are a series of file-formats with a similar logic to them. They are designed to save images in a way that the result can be read as an ASCII file, from back when email clients couldn’t read images reliably.

它们是年代久远的文件格式，只在非常特定的条件下使用，例如把图像嵌入到代码里面。

.pbm

One-bit and can only show strict black and white.

.pgm

Can show 255 values of gray (8bit).

.ppm

Can show 8bit rgb values.