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On January 4, 1995, T. Bowtell proposed to create a free format in a number of Usenet conferences that would be no worse than GIF. Three weeks after the publication of the idea, four versions of the new format were developed. Initially, he had the name PBF (Portable Bitmap Format), and the current name was on January 23, 1995. As early as December of the same year, the PNG version 0.92 specification was reviewed by the W3C consortium, and with the release of October 1, 1996 version 1.0, PNG was recommended as a full-fledged network format.
The PNG format is designed to replace the outdated and simpler GIF format, as well as, to some extent, to replace the much more complex TIFF format. The PNG format is positioned primarily for use on the Internet and graphics editing.
PNG supports three main types of bitmap images:
Halftone image (with 16-bit color depth)
Color indexed image (8-bit palette for 24-bit color)
Full-color image (with 48-bit color depth)
PNG format stores graphic information in compressed form. Moreover, this compression is lossless, unlike, for example, JPEG with losses.
It has the following main advantages over GIF:
almost unlimited number of colors in the image (GIF uses at best 8-bit color);
optional alpha channel support;
the possibility of gamma correction;
two-dimensional interlaced scanning;
the possibility of expanding the format with user blocks (based on this, in particular, APNG).
The GIF format was developed by CompuServe in 1987 and was initially unavailable for free use. Until the end of 2004 of the patents on the LZW compression algorithm owned by Unisys and used in the GIF, its use in free software was difficult. At the moment, such difficulties have been removed. PNG, from the very beginning, uses the open, non-proprietary Deflate compression algorithm, whose free implementations are available on the Internet. The same algorithm is used by many data compression programs, including PKZIP and gzip (GNU zip).
PNG format has a higher compression ratio for files with a greater number of colors than GIF, but the difference is about 5-25%, which is not enough for absolute predominance of the format, since small 2-16-color files of GIF format compress with equal efficiency.
PNG is a good format for image editing, even for storing intermediate editing stages, since image restoration and re-storage is lossless. Also, unlike, for example, from TIFF, the PNG specification does not allow authors of implementations to choose which features they are going to realize. Therefore, any saved PNG image can be read in any other PNG-capable application.
Different implementations of the Deflate algorithm give different degrees of compression, so programs for compressing images with several settings were created in order to obtain the best compression — for example, fork pngcrush OptiPNG and advpng from AdvanceCOMP (uses 7-Zip).