Libpng, Palette png with alpha or not?

Question

How to know if palette png is with alpha or not? I get information about the image png_get_IHDR

After that i look at color_type - PNG_COLOR_TYPE_PALETTE

But i can't find how to know if this png image has alpha channel or not.

Answer 1

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()

Answer 2

this may help:

if (color_type == PNG_COLOR_TYPE_RGBA || color_type == PNG_COLOR_TYPE_GA)
    *alphaFlag = true;
else
{
    png_bytep trans_alpha = NULL;
    int num_trans = 0;
    png_color_16p trans_color = NULL;

    png_get_tRNS(png_ptr, info_ptr, &trans_alpha, &num_trans, &trans_color);
    if (trans_alpha != NULL)
        *alphaFlag = true;
    else
        *alphaFlag = false;
}