Re: [Algorithms] Filtering

[Sylvain G. V. <vi...@ii...>]

>   On Fri, Oct 1, 2010 at 9:57 AM, Andreas Brinck
> <and...@gm...> wrote:
>     filtered value = lerp(r0 * C0 + b0 * C1 + g0 * C2, r1 * C0 + g1 * C1
> +
>     b1 * C2, t)  =
>                       = (r0 * C0 + b0 * C1 + g0 * C2) * (1 - t) + (r1 *
>     C0 + g1 * C1 + b1 * C2) * t =
>                       = (r0 * C0) * (1 - t) + (r1 * C0) * t + ... =
>                       = lerp(r0, r1, t) * C0 + ...
>
>     So in the world of non floating point numbers these two should be
>     equivalent right?

No they're not, due to rounding.

Take for example 2 texels:
R=1 G=0 B=0 and R=0 G=255 B=0

Lerping in the texfetch would spawn something like:
R=0 G=128 B=0
Definitely not what you're expecting! You'd want :
R=0 G=255 B=something

Using texfetch interpolation fails because it doesn't treat the 3 channels as one number; it won't always interpolate in the correct direction (interpreting 128 as either 0,5 or 1,5 depending on what's on the next channel) and it won't alter the other channel accordingly either.