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).

thanks much.

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