[MPlayer-dev-eng] Direct3D OSD discussion

[Reimar Döffinger]

On Tue, Nov 25, 2008 at 06:08:04PM +0000, Jim Hauxwell wrote:
> > I don't think so. Also
> > +            if(srca[x]){
> > +                dst[x]=(~srca[x] << 8) | src[x];
> > +            }
> > Why ~srca[x]? I think it should be -srca[x] (a quicker way of writing
> > ~srca[x]+1).
> 
> 
> No, I wrote what I wrote,
> 
> -srca[i] gives the incorrect answer.  If you take the value 0x0 then
> this gets transformed to 0x0 and not 0xFF which is the correct result.

Why do you think that 0xFF would be the correct result?
Due to the if, for your code the result is 0, too, btw.
Also, with your code, the final alpha value will never be 0xFF actually, thus
the OSD is never completely opaque.

> This is a bug in vo_gl.c, not my code.
> 
> If you need a small test then I'll knock up the four or so lines to
> prove it.

Sure, prove it because I don't think it is a good way (without a
specification right and wrong is a bit difficult to say, the differences
are almost never visible).

> > Note that for blending that matches the other vos, one of your blend
> > settings should be D3DBLEND_ONE (the one that applies to the OSD
> > texture).
> 
> Again, this is not a bug in my patch.  The others vos (well gl anyway)
> uses a two pass blend, mine is a single pass, so this is the correct
> blending.

I can't see how single or two pass has any relevance (except that
vo_gl.c current implementation of two-pass can not do "correct"
alpha-blending due to discarding the alpha).
The point is that the OSD rendering formula for MPlayer (as seen in
osd_template.c) is:
            if(srca[x]) dstbase[x]=((dstbase[x]*srca[x])>>8)+src[x];
(note that this is the C code which is somewhat broken, the asm code
does the clamping to [0,255] before writing back, it isn't noticed
usually because currently src is 0 when when srca > 1)
Alpha-blending as you do it is _not_ the same thing, e.g. if you do
  for (int i = h * stride - 1; i >= 0; i--)
  {
    if (srca[i]) srca[i] = 0xff;
    if (src[i]) src[i] = 90;
  }
The effect will be different from anything you can get with
alpha-blending.
This is not important enough to justify a huge workaround, but I do not
see why it would be a good idea to make vo_direct3d's behave
different than all of the rest of MPlayer on purpose.

Greetings,
Reimar Döffinger