.PPM 文件扩展名

In the late 1980s, developer Jef Poskanzer created the PPM image format and added tools that could create and read PPM images to Pbmplus, his library of tools used to create and manage .PBM, .PGM, and PPM images. PPM files contain color images saved using an RGB color model. Typically, the images use a 24-bit color depth.

In most PPM files, each pixel's color is specified using a trio of ASCII numbers that set the pixel's red, green, and blue color values. Image rows are comprised of sets of RGB values equal to the image's width. These types of PPM files are referred to as plain PPM files and can be identified by the magic number P3 in their header.

"Raw" PPM files, on the other hand, specify each pixel's color using raw binary data, instead of ASCII-encoded numbers. If you were to open a "raw" PPM file in a text editor, its body would appear as gibberish instead of an understandable series of numbers. These files include the magic number P6 in their header.

While many programs support the PPM format, PPM images have quite large file sizes when compared with images saved in other formats, because they are not compressed. Thus, the PPM format is not used much.