PNG (Portable Network Graphic) Image Format

Format name: Portable Network Graphic
File extension: .png

PNG (Portable Network Graphics) is an image format that was developed relatively recently for the Web and was meant to replace the outdated GIF format. It uses a lossless compression method named "deflate", which is similar to LZW. (The LZW algorithm was patented in 1995, which gave a stimulus for developing the P.N.G. format.) A compressed indexed P.N.G. file usually has a smaller file size than a GIF file containing the same image, and an RGB P.N.G. file is usually smaller than a corresponding TIFF file.

The P,N.G. format supports any color depth, up to 48 bits. It uses two-dimensional interlacing (not only by rows, but also by columns), which, just like that used in GIF, only slightly increases file size. Unlike GIF, where transparency is implemented on an on/off basis only, P.N.G. also supports semitransparent pixels (within the transparency range of 0 to 99 percent) by using an alpha channel with 256 shades of gray.

Portable Network Graphics files also store information on gamma correction. Gamma correction, or simply gamma, is a number used for encoding and decoding luminance values. If this number is read from the image file when displaying its image, the luminance levels can be automatically adjusted. Because of that, an image created on the Mac can look the same on the PC. Gamma correction helps implement the Web's general idea: Information should be displayed in the same manner regardless of the user's device.

The earliest versions of Microsoft Internet Explorer to support the P.N.G. format were v. 4.0b1 for Windows and v. 4.5 for Mac OS. Netscape implemented P.N.G. support in v. 4.0.4 for Windows and Mac OS. Nowadays, P.N.G. is supported by many web browsers, including Internet Explorer, Firefox, Google Chrome, Opera, and Safari, though older versions of these and other browsers may have some problems with rendering P.N.G. images correctly.