![]() Indexed color produces very small files when the image only needs a few colors. The maximum pixel size is 8 bits, so there can be up to 256 colors. Each pixel is an index into the color palette. The PNG format supports several color models: It lets the computer draw a low-resolution version of the image to start with, then add detail on subsequent passes until the user sees the highest quality image. This feature isn't used as often today as it was in the days of slow connections and processors. An interlaced PNG image reorders the data so it can be drawn in two or more passes while processing the data in sequence. It's hard to imagine a case where this wouldn't be enough. The image height and width can be as much as 2,147,483,648 pixels. They can even define new chunk types for their own use, and PNG readers that don't recognize the chunks will ignore them. File creators can define new metadata keys. The metadata can include the title, author, copyright, source, creation time, and more. The compression method, called "Deflate," is the same as a widely used ZIP compression method.Ī PNG file is made out of "chunks." They include the image data, information needed to draw it, and metadata describing the file. Photographic images and drawings with delicate color shadings won't benefit as much from PNG compression. It still compresses well when the image consists of solid chunks of color. This is different from formats like JPEG, which lose some low-order data bits for the sake of smaller file sizes. The original bits of each pixel can be recovered, bit for bit, by decompressing the file. "Lossless" means that the compression is fully reversible. Since then the format hasn't changed.Īll Web browsers that support graphics will display PNG files.Įxactly what is a PNG file? It's an image file using a lossless compressed image format. In 2003, the format became an ISO standard, with some minor revisions in 2004. The first version of the format definition came out in 1996. You can pronounce it "P-N-G" or "ping." Unlike GIF, which is limited to 256 colors, it supports full color and grayscale. They called it PNG, meaning Portable Network Graphics. TIFF was so loaded with features it had the same problem.Ī group of developers vented their frustration by creating a new, license-free format that was simple but powerful. JPEG didn't work well on the computers of the nineties. They weren't going to pay Unisys to post their graphics. But then Unisys started demanding license fees for the use of its patented compression algorithm. People could use it to create small image files, which was important when 2400 bps was considered a fast connection. Before everyone was using the Internet, GIF images were popular on services like CompuServe.
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