first, I want to thank you for your long and clear response.
well, you're asking about what I'm trying to achieve.
what I want is to have somekind of transparent color which is written in
the PNG file.
I don't want to have an alpha channel, just that transparent color.
I'm don't know the details of the PNG file format, but I almost sure it
is possible, because IrfanView does it.
if you want, download that IrfanView, and choose PNG as the saving file
format, you'll find a checkbox called 'save transparent color'.
if you are curious why I do want this, well, when I load the image into
my program, I check the color of each pixel for equality with the
transparent color, and if they're not equal, I modify it, also having a
transparent color(as opposed to an alpha channel) speeds up drawing on
the library I'm using(which's SDL).
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