Re: transparent textures

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Angus Dorbie (dorbie++at++multipass.asd.sgi.com)
Thu, 17 Jul 1997 06:20:07 -0700

On Jul 17, 8:28am, Nigel Jenkins wrote:
> Subject: Re: transparent textures
> Anita Kishore wrote:
> >
> > Anyway, the problem is that when this texture gets loaded, the neon light
> > around the sign appears completely banded with each band having a single
> > level of colour. The change in colour from the neon light colour to black
> > (completely transparent) is not smooth at all. What else should I set to
make
> > it smooth?
> >
>
> I've had a similar problem using our Onyx IR, and I found that in our
> case it seems to
> be caused by antialiasing. When antialiasing is on you get the baned
> transparency, and nice smooth lines on the rest of the model, but with
> the antialiasing off you get nice smooth transparency, but nasty jagged
> lines on the rest of the model!

Up until now you are exactly correct.

>
> What I think is happening (and I could well be completely wrong) is that
> the antialiasing somehow reduces the colour depth during its bit of
> processing, and so even though the pixel depth of the buffer you are
> rendering to is the same as it is with AA off, the colours coming out of
> the AA algorithm are at a much lower depth at so you get the bands.
>

Good guess but it's actually caused by multisample transparency instead
of blended. The reason AA affects this is that the same framebuffer
multisample mechanism is used for the full scene anti-aliasing as the
multi-sample transparency. For the same reason, fadeLOD capability
is only available when anti-aliasing. The alpha quantizing is due to
the limited number of multisamples, levels of transparency == # samples
because it's a mask. I think fade LOD gives more levels due to an
additional pixel level spatial dither, but alas not transparency.

You could also try a visual with more AA samples to improve transparency
quality. You should be aware that multi-sample transparency gives you more
transparent fill performance and provides order invariant transparency
rendering, so it may be a desirable thing.

Cheers,Angus.
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