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Portable Any Map
A super-class extension for the PBM, PGM, and PPM formats.
PNM (Portable Any Map) is not a distinct file format itself but rather a collective name (and file extension) used to handle files that could be any of the specialized Netpbm formats: PBM (Bitmap), PGM (Graymap), or PPM (Pixmap). A file ending in `.pnm` allows a program to implement a single reader that automatically detects the internal format ('P1' through 'P6') and handles it accordingly.
A `.pnm` file is simply a PBM, PGM, or PPM file renamed. The header magic number (e.g., `P6`) determines the actual content. This abstraction simplifies command-line pipelines, allowing users to pipe `image.pnm` without worrying if the image is black-and-white or full color.
Part of the Netpbm project philosophy to create a unified library for handling raster graphics. It simplifies naming conventions for software that can output any of the subtypes.
PNG (Portable Network Graphics)
The web's standard for lossless images with transparency, designed as a patent-free replacement for GIF.
PNG (Portable Network Graphics) emerged in 1996 as a direct response to the patent issues surrounding the GIF format's LZW compression algorithm. Developed by an informal working group and later standardized by the W3C, PNG was engineered from the ground up to be completely patent-free while offering superior technical capabilities. Unlike JPEG, which sacrifices image data for smaller files, PNG preserves every pixel exactly as captured or created. This lossless nature makes PNG the definitive choice for images where precision matters—screenshots, digital artwork, logos, and any graphic with text or sharp edges. The format's support for full alpha transparency (256 levels of opacity per pixel) revolutionized web design, enabling smooth drop shadows, gradient fades, and complex overlays that were impossible with GIF's binary transparency. Today, PNG is universally supported across all browsers, operating systems, and image editing software. While newer formats like WebP offer better compression, PNG remains the standard for lossless web graphics due to its unmatched compatibility and reliability.
PNG uses DEFLATE compression, the same algorithm powering ZIP files and gzip. This two-stage process first applies filtering to exploit the correlation between adjacent pixels, then compresses the filtered data using LZ77 followed by Huffman coding. The result is lossless compression that typically achieves 10-30% size reduction compared to raw pixel data, with some images compressing significantly more. The format supports multiple color types: grayscale (1-16 bits), indexed color with up to 256 palette entries, truecolor RGB (24 or 48 bits), and each with optional alpha channels. PNG's chunk-based architecture allows for extensibility—the file consists of a signature followed by typed chunks containing image data, metadata, and optional features like gamma correction and color profiles. PNG offers two interlacing modes: no interlacing (smaller file size) or Adam7 interlacing, which progressively renders the image in seven passes. While interlacing increases file size by approximately 10%, it provides a better user experience on slow connections by showing a low-resolution preview almost immediately.
PNG development began in January 1995 when Unisys announced it would enforce patents on GIF's LZW compression. Within weeks, an informal group on comp.graphics formed to create a replacement. Thomas Boutell published the first PNG specification draft in March 1995, and after extensive community input, PNG 1.0 became an official W3C Recommendation on October 1, 1996. The format underwent one major revision: PNG 1.2 in 1999 added the iCCP chunk for ICC color profiles and the sRGB chunk for standard color space indication. PNG became an ISO/IEC standard (15948) in 2003, cementing its position as a core web technology. The related APNG (Animated PNG) extension emerged in 2004 but remains unofficial, though it's now supported by all major browsers.
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