|USING ALPHA CHANNELS IN IMAGES|
An Image Magick Tute by Flo Deems
|What is an Alpha Channel?|
Both the GIF and the PNG image formats contain separate color channels for red, green and blue - and may contain an alpha channel, also. Alpha controls the degree of opacity or transparency of these images. But the alpha channels behave differently in GIFs and PNGs. If you need transparency, make sure you output the image as either a GIF or a PNG, not a JPG.
Transparency is useful in making drop shadows, vignettes, overlays, and backgrounds. Alpha blending (anti-aliasing) prevents the stair-step edges so commonly seen in G
IGs with transparent backgrounds.
This type of alpha channel selects one of the 256 colors of a GIF to hold the transparency. We don't have any control over which color is selected by the GIF format.
In PNGs, the alpha channel is totally separate from any of the color channels.