Re: [Mesa3d-dev] GLchan and texture images

[Brian P. <br...@va...>]

Gareth Hughes wrote:
> 
> 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 :-)

gcc may be doing what I intend but it probably would be safer to put
a cast in there for the sake of other compilers.

Good catch.

-Brian