PGM

Portable GrayMap (PGM) is a grayscale image file format used to store per-pixel brightness. It is part of the Netpbm family, which also includes PBM (bitmap) and PPM (color). PGM emphasizes simplicity, portability, and ease of parsing, making it common in teaching, research, and basic image processing tasks.

A PGM file begins with a magic number: P2 for ASCII encoding or P5 for binary encoding. The header may contain comment lines starting with '#'. After the header, the image width, height, and the maximum gray value (maxval) are listed. Pixel data then follow: P2 stores values as ASCII decimal numbers; P5 stores binary samples. If maxval exceeds 255, binary samples use two bytes per sample; otherwise one byte per sample.

PGM records a single intensity value per pixel, with no color channels or alpha. It is uncompressed and lossless, facilitating straightforward parsing and conversion. Extensions are usually .pgm, though alternate names may appear.

Software support is broad; many image libraries and tools can read and write PGM files. It is often used for grayscale processing, computer vision demonstrations, and as a simple interchange format in pipelines.

Limitations include the lack of embedded metadata beyond width, height, and maxval, no color information, and limited efficiency for storage compared with compressed formats. Nevertheless, PGM remains a standard reference format within the Netpbm family.

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