Web Design in a Nutshell, 3rd Edition

8-bit transparency

Indexed color PNGs can also contain variable levels of transparency (up to 256 levels); however, this information is not handled in a distinct alpha channel as for 24-bit images. Instead, each transparency level occupies a position in the index color table. So, if you have a red area that fades out using eight levels of transparency, that red would be present in eight slots in the color table, each with its own transparency setting. In other words, each slot in the color table can store RGBA information. So while it is alpha channel-like (because it has variable levels), it’s not true alpha-channel transparency.

Other than adding to the number of pixel colors in the color table, adding transparency to an 8-bit PNG does not significantly increase its file size, making it the preferable of the two methods for web use. Unfortunately, it faces the obstacle of poor tool and browser support. PNGs with palette transparency may display as binary (on/off) transparency by browsers that don’t support them.

Tip

Saving transparent PNGs in Fireworks and Photoshop is discussed in the upcoming "Creating PNG Files" section.