PNG - Ideas to Integrated Circuits

Intro to PNG Features
Stefan Schneider's shadow-casting toucan displayed against different backgrounds
This transparency feature is far more important for the small web graphics that are typically used on web
pages, such as colored (circular) bullets and fancy text. Alpha blending allows one to use anti-aliasing-creating
the illusion of smooth curves on a grid of rectangular pixels by smoothly varying the pixels'
colors--to make rounded and curved images that look good against any background, not just against a
white background (for example). Thus the same image can be reused in many places without the
"ghosting" effect that occurs with GIFs.
Of course, effective replacements for GIF buttons and icons must be comparable in size as well, and that
mostly rules out truecolor RGBA images. But PNG supports alpha information with palette images as
well; it's just slightly harder to implement in a smart way. A PNG alpha-palette image is just that: an
image whose palette also has alpha information associated with it, not a palette image with a full alpha
mask. In other words, each pixel corresponds to an entry in the palette with red, green, blue and alpha
components. So if you want to have bright red pixels with four different levels of transparency, you must
use four separate palette entries to accommodate them. (All four entries will have identical RGB
components, but the alpha values will differ.) If you want all of your colors to have four levels of
transparency, you've effectively reduced your total number of available colors from 256 to 64. In
general, though, only some of the colors need more than one level of transparency, and recognizing
which ones is where things get tricky for the programmer. (If you don't want to trust your local
programmer, have a look at pngquant, which converts 32-bit RGBA PNGs into 8-bit RGBA-palette
images. If you are a programmer, also have a look at it; full source code is included.)
For a better explanation with some nice sample images, see the Anti-aliasing and Transparency chapter
of Chris Lilley's excellent WWW4 paper, Not Just Decoration: Quality Graphics for the Web.
Gamma Correction
http://libpng.org/pub/png/pngintro.html (5 of 9)10/23/06 12:44 PM