What would you think of my adding a "PNG88" subformat to the PNG writer, to indicate indexed PNG with full transparency (as opposed to PNG8 meaning indexed PNG with binary transparency)? I think it would reduce some confusion.
From my posting today on Stack Overflow:
The designation "PNG8" is a little ambigous. Usually it means 8-bit indexed PNG with optional binary transparency (like GIF), and that is what is well supported, even by old versions of Internet Explorer. This is what you get from ImageMagick or GraphicsMagick when you request PNG8 output.
Sometimes it means 8-bit indexed PNG with full transparency (conveyed by the PNG tRNS chunk), and that may not be quite as well supported, and wasn't supported by old IE, which is why we came up with PNG8 in the first place.
From discussion among the PNG developers on png-list, October 12, 2002:
"Adobe ImageReady and Photoshop's save-for-web option use the terms PNG8 and PNG24 so:
PNG8 means indexed-color with 8-bit sample depth. Binary transparency is supported, but not full-alpha tRNS-type.
PNG24 means 24-bit RGB or 32-bit RGBA, depending on whether transparency is enabled."
I don't know if that's still true of ImageReady or Photoshop.
I am thinking of using the term "PNG88" to refer to indexed PNG with 8-bit transparency. Such images can be created by "pngquant" but they aren't given a particular name by pngquant. It refers to them as 8-bit RGBA-palette PNGs.
