Libpng, Palette png with alpha or not?

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PNG supports transparency in two (or three) quite different ways:

  1. Truecolor or grayscale images with a separated alpha channel (RGBA or GA)

  2. Transparency extra info in the (optional) tRNS chunk . Which has two different flavors:

    2a. For indexed images: the tRNS chunk specifies a transparency value (“alpha”) for one, several or all the palette indexes.

    2b. For truecolor or grayscale images: the tRNS chunk specifies a single color value (RGB or Gray) that should be considered as fully transparent.

If you are interested in case 2a, and if you are using libpng, you should look at the function png_get_tRNS()

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