Replacing .GIF file types with .PNG

Times change, and so do standards. What used to be the standard of internet web graphics now has to be replaced with a new file type. “PNGs uses the deflate compression algorithm typically with a 32KB sliding window. Deflate is an improved version of the Lempel-Ziv compression algorithm (LZ77) used in ZIP and GZIP files (3,4). Created by Phil Katz for version 2 of PKZip, deflate combines LZ77 with Huffman encoding and is 10% to 30% more efficient than LZW at lossless compression. Like gzip, some PNG compression tools have an optional “compression ratio” with values ranging from one to nine. Six is the default. Nine is almost always the best setting for maximum compression ratio.Not surprisingly, images saved as PNGs are typically 10% to 30% smaller than GIFs, although in rare cases they can be larger (5,6). In our tests, we found some images can compress 40% to 50% smaller or more (over 85% in one example) depending on the image. Typically images with large flat areas of color compress better than smooth-toned images with many color transition areas.

PNG Features

PNG has a number of features that make it appealing to the medical and graphics industries. Fireworks uses the PNG format to save its internal files, and PNG’s 16 bit grayscale capability makes it useful for accurate radiological imagery. PNG prefilters the image data using predictor functions, one of which is “Up” which looks for similarities in vertical patterns for full-color PNGs. Indexed color PNGs (8 bits or less) usually do not benefit from filtering, so use “none” when there is a choice. For true-color or grayscale images, use Adaptive.

“The place PNG is most commonly used for 24-bit RGB (even final versions) is rendered images such as ray-traced scenes (with minimal use of textures) or mathematical objects; they have artificially smooth color transitions that work well with PNG’s filters. Some fractals may be amenable to the same treatment, but many of the best ones have very noisy sections that compress quite poorly.” - Greg Roelofs (7)

For web use, the PNG8 (8-bit) is the form of PNGs that designers use to replace GIFs. PNGs can have an alpha value for each color in the palette, which effectively means the palette is RGBA, not RGB-with-one-exception like GIF. This lets you trade off color fidelity for transparency and still retain the size advantage of an 8-bit image versus a 32-bit one. PNGs can also work with one level of transparency, just like a GIF89a. The PNG compression algorithm looks for repeated horizontal patterns like GIF’s LZW compression scheme. Interlaced PNGs can also be recognized after about only 25% of the file has downloaded, as opposed to GIFs which require about 50% of the file to be downloaded before recognition (8). Although there are some rare exceptions, replacing your GIFs with PNGs will significantly reduce the size of your images.”

Read the full article through Website Optimization.com