How are 565 textures generated?

If I create a texture with an external format that is 8+8+8 bit RGB,
and an internal format of 5+6+5 RGB, how is the data converted by
glTexImage2D ?

Are the least significant bits dropped, rounded or something else?

It *looks* like (at least on nVidia 6800 on their latest Linux drivers)
they are doing something other than just truncating the LSbits.

I have two textures that don't match - one of which has been converted
to 5/6/5 in software (by just truncating bits) - the other has been
converted by the driver.  The path to this point is pretty convoluted
and it's *possible* that there is some other cause - but I'd like to
eliminate this cause if possible.

It seems that if the driver rounded the data then there would be a problem
with numbers like 0xFF being rounded up to 0x100 and overflowing. That's
what made me assume they'd be truncating it.

I also wonder what (if any) things are done to keep the extra bit of
green precision from generating magenta/green tints to otherwise grey
areas of the image.

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The second law of Frisbee throwing states: "Never precede any maneuver
by a comment more predictive than "Watch this!"...it turns out that
this also applies to writing Fragment Shaders.
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Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
L3Com/Link Simulation & Training (817)619-2466 (Fax)
Work: [email protected]           http://www.link.com
Home: [email protected]       http://www.sjbaker.org

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