对PPM或PGM的包头进行识别，进而实现图形的读取。并可将PPG转存成PGM-The first step towards image and video processing is reading images from file and write them to file. There exist different standards that store the information in different formats　before opening an image, knowledge of the standard is necessary. Two widely used image formats are PPM and PGM. The PGM format is a greyscale file format designed to be easy to manipulate. A PGM image represents a greyscale graphic image. For most purposes, a PGM image can just be thought of an array of integers. The name "PGM" is the acronym of "Portable Grey Map." The name "PPM" is the acronym for "Portable Pixel Map."