PNGPNG, an acronym for Portable Network Graphics, is a lossless image file format. It was designed as a superior replacement for the GIF format as the LZW data compression scheme used in GIF was patented by Unisys. PNG does not require a patent license.
Besides being a patent and license free format, PNG has several advantages over GIF - the main ones being alpha transparency, gamma correction, and two-dimensional interlacing. Alpha transparency allows what is called anti-aliasing, or making rounded and curved images that will look good on any background and not just white. This feature gets rid of the “ghosting” effects that appear in GIF images. Gamma correction ensures that the image appears clearly in any gamma-aware environment. In other image formats, images are displayed darker or lighter when there are gamma differences between platforms. The two-dimensional interlacing in PNG allows an image to be displayed both horizontally and vertically.
Another important feature of PNG is its compression algorithm. This non-patented lossless compression scheme uses a special method called deflate, which predicts the color of each pixel based on the colors of previous pixels, then filters the predicted color from the actual color. This filtering allows for easier compression of images, thus reducing a file size to up to 60%.
A still another advantage of PNG is that it supports three image types – palette based (2, 4, 16, or 256 color), true color (24-bit and 48-bit), and gray scale (2, 4, 16, 256 or 65,536 levels of gray). Since PNG is a lossless format, saving, opening and then resaving the file does not corrupt the color quality of an image.
The only limitation of PNG as compared to GIF is that it does not support animations.
A PNG (pronounced “ping”) file can be opened with a picture viewer or the Web browser on Windows and Mac OS, and with GIMP (The GNU Image Manipulation Program) on Linux.
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