Understanding PNG in Photoshop

Is this finally the Year of PNG? I’ve been writing about the PNG (often pronounced ping) format seemingly forever (actually, only since 1995 when the PNG standard was frozen). Every year, support for the PNG format grows, but its actual usage remains pitifully small. The chief roadblock has been a lack of browsers that support PNG fully, but that roadblock has largely vanished. (The vast majority of browsers in use today, including Netscape 4.x and later versions, as well as Internet Explorer 5.x and later, support at least some PNG features.) However, only later versions of Internet Explorer and Netscape Navigator support PNG natively; other versions require a plug-in, which can be irritating to viewers of your Web page.

In the Mac realm, support is good. Although Web developers are a cautious lot and want the largest group of people possible to be able to view their efforts, the main reason today that so few people use the PNG format is that so few other people use it. That’s a pity, because the PNG format is a potential GIF-killer. (It can give JPEG a run for the money, too, but it’s not specifically designed to reproduce photographic-quality images.) The features built into the PNG format (the letters of which are sometimes said to represent PNG’s Not GIF) counter most of the objections to GIF. Because of that, the advantages greatly outweigh the disadvantages. So, I focus on the advantages to the PNG format only, as listed here:
- It’s free. Unisys holds the patent on the compression algorithm used in GIF and charges a royalty to any software vendor who wants to support GIF. The PNG specification is a potential advantage because software developers can incorporate it into their products without paying for the privilege.
- It offers better compression. The PNG format produces files that are up to 20 percent smaller than the equivalent GIF, but they sometimes can be larger. In fact, one disadvantage is that they can sometimes be larger than the equivalent JPEG file.
- It isn’t limited to 8-bit, 256-color images. In fact, the PNG format isn’t limited to 24-bit, 16.7 million color images. It can support a selected range of bit depths from 1 to 64 bits, for zillions and zillions of colors. The bottom line is that the PNG format supports Grayscale, Indexed Color, and RGB modes.
- It uses a smarter interlacing scheme. The PNG format provide images that fade in faster and better than interlaced GIFs.
- It offers four different types of transparency. The PNG format allows 64 to 254 (not 256) different levels of transparency, so even full-color images can float over a background.

The PNG format also offers some technical advantages that aren’t readily visible to the user, such as the ability to adjust for variations in the display of images by certain devices (called gamma correction) and an error-correction routine that patches images that have lost a bit or two in transmission to your computer. The PNG format does not support animation. A related file format, MNG does, but it’s used even less widely than PNG.