[Image-SIG] Re: How to tell whether/what is the alpha color in an indexed image file ?

[Fredrik Lundh]

Ray Pasco wrote:
> If a PNG image with an alpha layer is opened, the im.mode will be "RGBA".
> However, if an indexed PNG or GIF is opened with an alpha color set,
> the mode is simply "P" (no "A" as in "PA").
>
> 1) How can I get PIL to tell me if and what is the "alpha" color index ?

crop out the interesting pixel(s) and convert to RGBA, and use
getpixel on the resulting image.

> 2) When an RGBA image is converted to an indexed image, does PIL record
> the occurance in some manner that any pixels had alpha values set to less than 255 ?

RGBA->P is equivalent to RGBA->RGB->P, so the answer is no.

</F>