Форматы файлов изображений без потерь: PNG, MNG, APNG

Family of lossless compression image file formats

Portable Network Graphics (PNG, officially pronounced /p//ɪ//ŋ/ PING, colloquially pronounced ,/p//iː//ɛ//n/'/dʒ//iː/ PEEenJEE) is a raster graphics file format that supports lossless data compression. PNG was developed as an improved, non patented replacement for Graphics Interchange Format (GIF)—unofficially, the initials PNG stood for the recursive acronym "PNG's not GIF". PNG supports palette based images (with palettes of 24 bit RGB or 32 bit RGBA colors), grayscale images (with or without an alpha channel for transparency), and full color non palette based RGB or RGBA images. The PNG working group designed the format for transferring images on the Internet, not for professional quality print graphics; therefore, non RGB color spaces such as CMYK are not supported. A PNG file contains a single image in an extensible structure of chunks, encoding the basic pixels and other information such as textual comments and integrity checks documented in RFC 2083. PNG files have the ". png" file extension and the "image/png" MIME media type. PNG was published as an informational RFC 2083 in March 1997 and as an ISO/IEC 15948 standard in 2004. and also the png extension. Other suggestions later implemented included the deflate compression algorithm and 24 bit color support, the lack of the latter in GIF also motivating the team to create their file format. The group would become known as the PNG Development Group, and as the discussion rapidly expanded, it later used a mailing list associated with a CompuServe forum. In 2001, the developers of PNG published the Multiple image Network Graphics (MNG) format, with support for animation. MNG achieved moderate application support, but not enough among mainstream web browsers and no usage among web site designers or publishers. In 2008, certain Mozilla developers published the Animated Portable Network Graphics (APNG) format with similar goals. APNG is a format that is natively supported by Gecko and Presto based web browsers and is also commonly used for thumbnails on Sony's PlayStation Portable system (using the normal PNG file extension). In 2017, Chromium based browsers adopted APNG support. In January 2020, Microsoft Edge became Chromium based, thus inheriting support for APNG. With this all major browsers now support APNG.

PNG offers an optional 2 dimensional, 7 pass interlacing scheme—the Adam7 algorithm. This is more sophisticated than GIF's 1 dimensional, 4 pass scheme, and allows a clearer low resolution image to be visible earlier in the transfer, particularly if interpolation algorithms such as bicubic interpolation are used. However, the 7 pass scheme tends to reduce the data's compressibility more than simpler schemes.

The core PNG format does not support animation. MNG is an extension to PNG that does; it was designed by members of the PNG Group. MNG shares PNG's basic structure and chunks, but it is significantly more complex and has a different file signature, which automatically renders it incompatible with standard PNG decoders. This means that most web browsers and applications either never supported MNG or dropped support for it. The complexity of MNG led to the proposal of APNG by developers at the Mozilla Foundation. It is based on PNG, supports animation and is simpler than MNG. APNG offers fallback to single image display for PNG decoders that do not support APNG. Today, the APNG format is supported by all major web browsers. APNG is supported in Firefox 3.0 and up, Pale Moon (all versions), and Safari 8.0 and up. Chromium 59.0 added APNG support, followed by Google Chrome. Opera supported APNG in versions 10–12.1, but support lapsed in version 15 when it switched to the Blink rendering engine; support was re added in Opera 46 (inherited from Chromium 59). Microsoft Edge has supported APNG since version 79.0, when it switched to a Chromium based engine. The PNG Group decided in April 2007 not to embrace APNG. Several alternatives were under discussion, including ANG, aNIM/mPNG, "PNG in GIF" and its subset "RGBA in GIF". However, currently only APNG has widespread support. With the development of the Third Edition of the PNG Specification, now maintained by the PNG working group, APNG will finally be incorporated into the specification as an extension.

The JPEG (Joint Photographic Experts Group) format can produce a smaller file than PNG for photographic (and photo like) images, since JPEG uses a lossy encoding method specifically designed for photographic image data, which is typically dominated by soft, low contrast transitions, and an amount of noise or similar irregular structures. Using PNG instead of a high quality JPEG for such images would result in a large increase in filesize with negligible gain in quality. In comparison, when storing images that contain text, line art, or graphics – images with sharp transitions and large areas of solid color – the PNG format can compress image data more than JPEG can. Additionally, PNG is lossless, while JPEG produces visual artifacts around high contrast areas. (Such artifacts depend on the settings used in the JPG compression; they can be quite noticeable when a low quality [high compression] setting is used.) Where an image contains both sharp transitions and photographic parts, a choice must be made between the two effects. JPEG does not support transparency. JPEG's lossy compression also suffers from generation loss, where repeatedly decoding and re encoding an image to save it again causes a loss of information each time, degrading the image. Because PNG is lossless, it is suitable for storing images to be edited. While PNG is reasonably efficient when compressing photographic images, there are lossless compression formats designed specifically for photographic images, lossless WebP and Adobe DNG (digital negative) for example. However these formats are either not widely supported, or are proprietary. An image can be stored losslessly and converted to JPEG format only for distribution, so that there is no generation loss. While the PNG specification does not explicitly include a standard for embedding Exif image data from sources such as digital cameras, the preferred method for embedding EXIF data in a PNG is to use the non critical ancillary chunk label eXIf. Early web browsers did not support PNG images; JPEG and GIF were the main image formats. JPEG was commonly used when exporting images containing gradients for web pages, because of GIF's limited color depth. However, JPEG compression causes a gradient to blur slightly. A PNG format reproduces a gradient as accurately as possible for a given bit depth, while keeping the file size small. PNG became the optimal choice for small gradient images as web browser support for the format improved. No images at all are needed to display gradients in modern browsers, as gradients can be created using CSS.

JPEG LS is an image format by the Joint Photographic Experts Group, though far less widely known and supported than the other lossy JPEG format discussed above. It is directly comparable with PNG, and has a standard set of test images. On the Waterloo Repertoire ColorSet, a standard set of test images (unrelated to the JPEG LS conformance test set), JPEG LS generally performs better than PNG, by 10–15%, but on some images PNG performs substantially better, on the order of 50–75%. Thus, if both of these formats are options and file size is an important criterion, they should both be considered, depending on the image.

Tag Image File Format (TIFF) is a format that incorporates an extremely wide range of options. While this makes TIFF useful as a generic format for interchange between professional image editing applications, it makes adding support for it to applications a much bigger task and so it has little support in applications not concerned with image manipulation (such as web browsers). The high level of extensibility also means that most applications provide only a subset of possible features, potentially creating user confusion and compatibility issues. The most common general purpose, lossless compression algorithm used with TIFF is Lempel–Ziv–Welch (LZW). This compression technique, also used in GIF, was covered by patents until 2003. TIFF also supports the compression algorithm PNG uses (i. e. Compression Tag 000816 'Adobe style') with medium usage and support by applications. TIFF also offers special purpose lossless compression algorithms like CCITT Group IV, which can compress bilevel images (e. g., faxes or black and white text) better than PNG's compression algorithm. PNG supports non premultiplied alpha only It is published as free software under the terms of a permissive free software license. Therefore, it is usually found as an important system library in free operating systems.

The PNG format is widely supported by graphics programs, including Adobe Photoshop, Corel's Photo Paint and Paint Shop Pro, the GIMP, GraphicConverter, Helicon Filter, ImageMagick, Inkscape, IrfanView, Pixel image editor, Paint. NET and Xara Photo & Graphic Designer and many others (including online graphic design platforms such as Canva). Some programs bundled with popular operating systems which support PNG include Microsoft's Paint and Apple's Photos/iPhoto and Preview, with the GIMP also often being bundled with popular Linux distributions. Adobe Fireworks (formerly by Macromedia) uses PNG as its native file format, allowing other image editors and preview utilities to view the flattened image. However, Fireworks by default also stores metadata for layers, animation, vector data, text and effects. Such files should not be distributed directly. Fireworks can instead export the image as an optimized PNG without the extra metadata for use on web pages, etc.

PNG support first appeared in 1997, in Internet Explorer 4.0b1 (32 bit only for NT), and in Netscape 4.04. Despite calls by the Free Software Foundation and the World Wide Web Consortium (W3C), tools such as gif2png, and campaigns such as Burn All GIFs, PNG adoption on websites was fairly slow due to late and buggy support in Internet Explorer, particularly regarding transparency. PNG compatible browsers include: Apple Safari, Google Chrome, Mozilla Firefox, Opera, Camino, Internet Explorer, Microsoft Edge and many others. For the complete comparison, see Comparison of web browsers (Image format support). Especially versions of Internet Explorer (Windows) below 9.0 (released 2011) had numerous problems which prevented it from correctly rendering PNG images. 4.0 crashes on large PNG chunks. 4.0 does not include the functionality to view png files, but there is a registry fix. 5.01 prints palette images with black (or dark gray) backgrounds under Windows 98, sometimes with radically altered colors. 6.0 fails to display PNG images of 4097 or 4098 bytes in size. 6.0 cannot open a PNG file that contains one or more zero length IDAT chunks. This issue was first fixed in security update 947864 (MS08 024). For more information, see this article in the Microsoft Knowledge Base: 947864 MS08 024: Cumulative Security Update for Internet Explorer. 6.0 sometimes completely loses ability to display PNGs, but there are various fixes. 6.0 and below have broken alpha channel transparency support (will display the default background color instead). 7.0 and below cannot combine 8 bit alpha transparency AND element opacity (CSS – filter: Alpha (opacity=xx)) without filling partially transparent sections with black. 8.0 and below have inconsistent/broken gamma support. In 2006, Microsoft Windows support for PNG icons was introduced in Windows Vista. PNG icons are supported in AmigaOS 4, AROS, macOS, iOS and MorphOS as well. In addition, Android makes extensive use of PNGs.

PNG file size can vary significantly depending on how it is encoded and compressed; this is discussed and a number of tips are given in PNG: The Definitive Guide. compression With additional computation, DEFLATE compressors can produce smaller files. There is thus a filesize trade off between high color depth, maximal metadata (including color space information, together with information that does not affect display), interlacing, and speed of compression, which all yield large files, with lower color depth, fewer or no ancillary chunks, no interlacing, and tuned but computationally intensive filtering and compression. For different purposes, different trade offs are chosen: a maximal file may be best for archiving and editing, while a stripped down file may be best for use on a website, and similarly fast but poor compression is preferred when repeatedly editing and saving a file, while slow but high compression is preferred when a file is stable: when archiving or posting. Interlacing is a trade off: it dramatically speeds up early rendering of large files (improves latency), but may increase file size (decrease throughput) for little gain, particularly for small files.

Zopfli and the LZMA SDK provide DEFLATE implementations that can produce higher compression ratios than the zlib reference implementation at the cost of performance. AdvanceCOMP's advpng and advdef can use either of these libraries to re compress PNG files. Additionally, PNGOUT contains its own proprietary DEFLATE implementation. advpng doesn't have an option to apply filters and always uses filter 0 globally (leaving the image data unfiltered); therefore it should not be used where the image benefits significantly from filtering. By contrast, advdef from the same package doesn't deal with PNG structure and acts only as a re deflater, retaining any existing filter settings.

Since icons intended for Windows Vista and later versions may contain PNG subimages, the optimizations can be applied to them as well. At least one icon editor, Pixelformer, is able to perform a special optimization pass while saving ICO files, thereby reducing their sizes. FileOptimizer (mentioned above) can also handle ICO files. Icons for macOS may also contain PNG subimages, yet there isn't such tool available.