Angus Dorbie (angus++at++division.demon.co.uk)
Mon, 9 Jan 1995 17:01:49 +0000
> We are using an RE2 and would like to render into the alpha buffer
> (to create masks for downstream video keying) without turning off
> antialiasing - is this possible?
>
> It seems that pfAntialias(PFAA_ON) configures things so that the
> alpha buffers are totally bypassed. I realize that with an RE2,
> antialiasing and transparency are obtained from multisampling.
> But for our application we do not want to use the alpha buffers
> for transparency, just for creating the masks.
Performer antialiasing uses multisampling on an RE2 but you are not limited to
multisample transparency when rendering alpha. You can still use good old
fashioned alpha blending.
Instead of:
pfTransparency(PFTR_MS_ALPHA);
experiment with:
pfTransparency(PFTR_BLEND_ALPHA);
pfTransparency(PFTR_HIGH_QUALITY);
I'm not sure what PFTR_ON defaults to on an RE2.
You can also apply these to the geostate attribute for transparency.
It sounds like you may only want to occlude the scene to some keying colour in
the background, if so, why not use the pre draw IRIS GL command:
RGBwritemask(0x00, 0x00, 0x00);
and post draw:
RGBwritemask(0xFF, 0xFF, 0xFF);
Instead of blending with alpha, I expect this will be faster and will fill the
zbuffer in the same manner as blending would.
WARNING (the real pitfall):
Around your mask the antialiasing will produce colours which are antialiased
with your keying colour so unsophisticated chroma keying could produce
artefacts around your titles unless your mask is aligned with the screen pixel
boundaries.
Regards,
-- ________________________________________________ Angus Dorbie Division Ltd, Software Engineer 19 Apex Court, Tel: (01454)615554 Woodlands, Fax: (01454)615532 Bristol BS12 4JT, angus++at++division.demon.co.uk UK ________________________________________________
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