[MPlayer-dev-eng] Draw Alpha

[Ivan Kalvatchev]

Few days ago i sow coments about drawing subtitles for
DVD. I sow the equation that is used for computing, i
sow that some changes are made. I didn't mind it, but
later I realize that the formula is wrong. I couldn't
find the fix and today i chaked the source. This is
form osd_template.c:
 
if(srca[x])dstbase[x]=((dstbase[x]*srca[x])>>8)+src[x];
or in float:
  dst=src*alpha+osd;

The right formula is:
  dst=src*alpha+osd*(1.0-alpha); 

so when alpha=1 we get  dst=src;
and when alpha=0 we get dst=osd;

As you work with sharp colors 0,255 you may not notice
it, becouse of overflow (0xFF=-1), but in case you
have osd=127 you should have strange effects (not
tested:)

So with the right formula it is just a matter of
preferences does alpha==0 means transparent or solid.

  Best Regards
Ivan Kalvachev

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