Re: How are 565 textures generated?

Hi, Steve!

On Wed, Jan 25, 2006 at 11:22:23AM -0800, Stephen J Baker wrote:
| 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 ?

I believe the relevant parts of the spec (v2.0) are table 2.9 for the
conversion from fixed to float and section 2.4.19 for conversion from
float to fixed.

In this case the 8-bit values are mapped into the range [0.0, 1.0] by
dividing by 255.  Then the individual color channels are multiplied by
31 (R and B) or by 63 (G), and the results rounded to nearest.

I would bet that all the commercial implementations replace the
conceptual floating-point arithmetic with fast fixed-point arithmetic
that yields the same results.

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

Sounds like they're probably doing the right thing.

| 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.

The math is supposed to be done so that the highest value in any
fixed-point representation maps to the highest value in all other
fixed-point representations.  Some values less than the highest value in
one representation might also be rounded up to the highest value in
another representation, but there should never be any overflow.

| 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.

Nothing that I know of.

Allen
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