Raster Images: Portable Network Graphic (PNG)
The PNG format is a tremendous improvement over GIF. It generally compresses better, allows more colors, and it allows much more flexibility in transparency. It does not support animation, though.
PNG's compression algorithm is called Deflate. It is an improved version of LZW, and is the same algorithm used for zip archives. Where GIF compression looks at horizontal patterns, PNG is capable of also looking at vertical patterns, so it can compress an image even further than GIF.
A GIF image allows only 256 colors, but PNG supports up to 48 bits of color (16 bits per channel) and 16 more bits of transparency information. That's over 280 billion colors--far more than you're ever likely to need. It also allows for an alpha channel, which is a grayscale overlay that determines the level of transparency the image has. The alpha channel allows parts of the image to be translucent, and the edges to fade away. It also makes it possible to anti-alias the edges of a non-square image, eliminating the jagged edges.
The two most common varieties of PNG are PNG-8, which is an 8-bit palleted image very similar to a GIF, and PNG-24, which is a little bit of a misnomer, as it is 8 bits per channel. 24 bits for color, and 8 more for alpha, which means it is often actually a 32-bit image. Adobe (formerly Macromedia) Fireworks uses PNG as its native format, and a Fireworks PNG has a few extra features beyond the scope of this article. Some applications can open a Fireworks PNG, but those extra features will usually not be present.
Like GIF, png is best used for illustrative images with flat colors and sharp lines. It can also be used for textured or photographic images when transparency is needed, although it does not compress as well. Because it is a lossless format, it is possible to use it for working images without fear that they will lose quality after multiple saves.
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