Re: [Algorithms] Spurious DXT colors

[Andreas B. <and...@gm...>]

I just realized that just multiplying with 2 might not do the trick. I think
one has to take into consideration the way the compressed green channel is
decompressed from 6 to 8 bits on the graphics card. I can see three
different scenarios:

1. The 6 bits are just shifted by 2. This means that you will never get pure
white. In this case multiplying by 2 would be the correct approach.

2. Same as above but the last bits are padded with ones instead of zeros.
This means you will never get pure black. In this case you should multiply
by 2 and also add 1 / 255.

3. The bits are multiplied by 255 / 63 with correct rounding applied, so
that the entire range from 0 to 255 is covered. In this case you should
multiply by 63/31 to get our truncated green value to reach 255.

/A.B.

Hi,
>
> I vaguely remember reading somewhere about a method to remove the spurious
> greens and magentas that occur when decompressing a DXT (or any 565 format)
> texture. I think the idea was to multiply the green channel by 0.5 before
> compressing to remove a bit and then multiply it by 2 after decompressing.
>
> Has anyone tried this, and gotten it to work?
>
> /A.B.
>