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Portable Arbitrary Map
The 'universal' bitmap format of the Netpbm toolkit.
PAM (Portable Arbitrary Map) is the newest and most flexible member of the Netpbm format family (which includes PBM, PGM, and PPM). While earlier formats were strictly strictly black/white, grayscale, or RGB, PAM was designed to be generic. It can handle images with any number of channels, including transparency (alpha) channels, which the older formats lacked.
A PAM file starts with a text header defining width, height, maximum component value (depth), and 'TUPLTYPE' (describing what the channels mean, e.g., 'RGB_ALPHA' or 'CMYK'). The data follows as a raw binary stream of tuples. It unifies the parsing logic: instead of writing separate parsers for PBM/PGM/PPM, a program can just implement a PAM parser to handle everything.
Introduced with the Netpbm library updates to solve the 'Alpha Channel Problem'. The older formats (PPM) had no standard way to store transparency, leading to hacky non-standard extensions. PAM standardized this by allowing arbitrary tuple types.
JPEG Network Graphics
JPEG compression with PNG transparency.
JNG (JPEG Network Graphics) is a sub-format of the MNG animation format. It was designed to combine the high compression of JPEG with the alpha transparency features of PNG, something that standard JPEG lacks. It essentially wraps a JPEG stream and a mask channel in a chunk-based container.
A JNG file structure mimics PNG. It uses a 8-byte signature and chunks. The image data is split into a color chunk (JDAT) which contains JPEG data, and an alpha chunk (IDAT) which contains PNG-compressed alpha mask. This allows for 'lossy color, lossless alpha' images, which is very space-efficient for photographic sprites.
Created by the PNG Development Group around 2001 as part of the MNG (Multiple-image Network Graphics) project. It failed to gain traction as MNG was rejected by Mozilla and Chromium in favor of APNG and later WebP.
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