Allen Akin wrote:
>
> On Fri, Apr 12, 2002 at 07:01:08PM +0100, José Fonseca wrote:
> | ... Although doing (p*a+q*(1-a)) >> 8 can
> | introduce up to 1 LSB error and worst, it doesn't obey to the rule of
> | 255*255 = 255 as 255*255/256 = 254. I know that in Mesa's C blending code
> | this special case of a=255 is always checked, but in the MMX code it
> | isn't, and glean doesn't complain of that.
>
> If the expected value is 255 and the OpenGL implementation yields 254,
> that's only one LSB of error, so glean probably won't complain about it.
>
> We could make the test more stringent, but then some reasonable
> implementations (especially some hardware implementations) would fail.
> Also, maintaining enough accuracy to yield results correct to 1 LSB is
> already pretty challenging when color channels are deeper than 8 bits.
This brings up some interesting questions about blending that aren't
directly addressed in the OpenGL specification.
One might expect that the following identities be true for blending terms:
1.0 * 1.0 == 1.0
1.0 * x == x * 1.0 == x
0.0 * x == x * 0.0 == 0.0
So for 8-bit channels, in fixed point:
255 * 255 == 255
I can easily imagine cases in which applications would depend on these
identities being true (in blending and elsewhere). In fact, I have a
vague recollection of someone bringing up this issue a few years ago.
I'd like to see Mesa satisfy the 255*255=255 identity. Is it hard to
implement that in the MMX code? If it is, we could let it go for now
and see if anyone complains.
> | So how come the Mesa blending code in s_blend.c has coments such as "This
> | is pretty close, but Glean complains", "This is slower but satisfies
> | Glean", and "This satisfies Glean and should be reasonably fast"...?
>
> I don't know. It might be related to deep color channels, or possibly
> to glean tests other than the basic blending tests. Brian might
> remember.
It's been at least a year since I touched that code. As far as I can
remember the comments are correct. Though I don't remember if it was
an issue at 5/6/5 or 8/8/8 color depth, or both. I don't know what
else might have changed since then to cause different results with
Glean.
> Thanks for all your good work, by the way!
Yes!
-Brian
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