[MPlayer-dev-eng] Draw Alpha

[Ivan Kalvatchev]

--- Arpi <arpi at thot.banki.hu> wrote:
> Hi,
> 
> > 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); 
> 
> not really.
> in theory, the equeation is what you wrote above.
> but in mplayer, osd/font pixel colors shouldn't be
> multiplied.
> so, alpha value in mplayer means how decrease
> background (image)
> brightness and when it's done, it overlay with
> font/osd pixels by
> adding them. this is why osd pixel color + alpha
> pixel value <=255
> limitation. and it also speeds up rendering a lot.
> 
> read my mail about this topic before telling here
> bad things.
> we aren't in opengl world, we are in maximum-speed
> world now.
> and here 0 means no osd/font rendering, 1 mean
> rendering over
> black (full opacity) ... 255 means rendering over
> the image (full
> transparency).
> 
sorry for disturbance, i think that i get it. You meen
that as osd*(1.0-alpha);  doesn't depend on src it is
precalculated and osd is not simple gray image.



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