PNG

PNG. The Portable Network Graphics format is a raster-graphics file format that supports lossless data compression. It was created as an improved, non-patented replacement for the Graphics Interchange Format (GIF) and is the most widely used lossless image compression format on the Internet. The format supports palette-based images, grayscale images, and full-color non-palette-based RGB or RGBA images, and was designed for transferring images on the World Wide Web, not for professional-quality print graphics.

History

The development of PNG was initiated in early 1995 after Unisys announced it would enforce software patents on the Lempel–Ziv–Welch (LZW) compression method used in the GIF format. A discussion on the Usenet newsgroup comp.graphics led to the creation of a working group, with key contributors including Thomas Boutell, Tom Lane, and others from the World Wide Web Consortium. The first version of the specification was released on 1 October 1996 and later became an official W3C Recommendation and an ISO/IEC standard. The format's design was heavily influenced by the earlier TGA and SDL image formats, and it was explicitly designed to avoid the legal issues surrounding GIF.

Technical details

PNG uses a non-patented lossless data compression method called DEFLATE, which combines the LZ77 algorithm and Huffman coding. The format structures data in chunks, with critical chunks including the IHDR (image header), PLTE (palette), IDAT (image data), and IEND (image trailer). It supports up to 16-bit per channel for grayscale and 48-bit for truecolor, as well as an 8-bit alpha channel for transparency, allowing for smooth edges against any background. The format also includes built-in support for Gamma correction to ensure consistent display across different systems like Microsoft Windows and macOS.

Comparison with other formats

Compared to GIF, PNG offers superior compression, more colors (up to 16 million versus 256), and superior alpha transparency instead of binary transparency. Unlike the lossy JPEG format, PNG compression is lossless, making it ideal for images requiring precise reproduction, such as line art, text, or screenshots, though it typically results in larger file sizes for photographic content. The format also differs from TIFF in being simpler, more streamable, and having a single, well-defined compression method, whereas TIFF supports multiple, sometimes proprietary, compression schemes.

Applications and usage

PNG is extensively used on websites for logos, icons, and interface graphics where transparency and sharp edges are required. It is the default screenshot format for many operating systems, including macOS and various Linux distributions. The format is also widely used in open-source and proprietary software for storing texture maps and sprites in games, and in applications like Adobe Photoshop and GIMP for intermediate saves during editing due to its lossless nature. Specialized derivatives like APNG and MNG were created to support animation.

Software support

Virtually all modern web browsers, including Google Chrome, Mozilla Firefox, Safari, and Microsoft Edge, have native support for displaying PNG images. Major image editing and viewing software such as Adobe Illustrator, CorelDRAW, IrfanView, and XnView provide full support for the format. Libraries like libpng, the official reference library, and GD Graphics Library enable developers to implement PNG support in applications across platforms like Android and iOS. Even legacy systems and environments like OpenGL and DirectX can utilize PNGs through various middleware libraries.