Re: [Mesa3d-dev] GLchan and texture images

[Gareth H. <ga...@va...>]

Brian Paul wrote:
> 
> OK, this sounds good.  I see why you were asking.

Cool :-)

> I think it's correct.  To convert a GLubyte color in [0,255] to a
> 16-bit GLchan value in [0,65535] you'd do:
> 
>   chan = b * 65535 / 255;
> 
> which is equivalent to:
> 
>   chan = (b << 8) | b;
> 
> Here's the proof:
> 
> #include <assert.h>
> int main(int argc, char *argv[])
> {
>    int b;
>    for (b = 0; b < 256; b++) {
>       int c1 = b * 65535 / 255;
>       int c2 = (b << 8) | b;
>       assert(c1 == c2);
>    }
> }
> 
> :)

I'm not disagreeing re: the math, I'm concerned about shifting GLubytes
up by 8.  Maybe it does automatic conversion to GLushort and thus you
won't overflow.  On testing this, it appears that this is the case.  Oh
well :-)

-- Gareth