Re: Wierd problem when alpha blitting image with text in it

On Sat, 29 Jul 2006 23:02:35 -0700, "Dean Roddey"
<droddey@xxxxxxxxxxxxxxxx> wrote:
It's only if I alpha blit from the first to the second that the text pixels
get whacked, but everything else stays fine. At first I thought maybe it's
because the pixels are pre-blended but the drawn text pixels aren't (they
get an 0xFF I assume.) But the area of the image over which the text is
being drawn is fully opaque and would have 0xFF alpha anyway, so that
doesn't seem likely.

If the text (source) pixels have an alpha value of 0 but the RGB
values are not 0 (which is inconsistent with premultiplication), the
final pixels can overflow (which can end up looking "over-amped").
Could this be the case in your system? I haven't followed the entire
thread, but I just thought I'd throw this out as a potential cause.

I think the easiest way to debug this is to observe the actual R,G,B,A
values of the text pixels in a debugger (instead of guessing what they
might be) and then work them through the blending equation.

Chris
.

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