[Mesa-dev] [PATCH] lp_build_lod_selector: Disable brilinear folding on explicit lod.

[Jose Fonseca]

----- Original Message -----
> On Mon, Jun 25, 2012 at 11:40:08AM -0700, Jose Fonseca wrote:
> > My thoughts too.
> > 
> > Brilinear filtering provides a significant boost, and I don't see
> > why skip the optimization for explicit lod over implicit lods.
> 
> Warning: code misread :-)
> 
> Explicit lod does not need brilinear filtering because explicit lod
> is
> post log2.  Brilinear is only about a faster log2, nothing else.
> Explicit lod only needs the integer/fractional part separation.
> 
> The whole code is:
>    if (mip_filter == PIPE_TEX_MIPFILTER_LINEAR) {
>       if (!explicit_lod && !(gallivm_debug &
>       GALLIVM_DEBUG_NO_BRILINEAR)) {
>          lp_build_brilinear_lod(float_bld, lod, BRILINEAR_FACTOR,
>                                 out_lod_ipart, out_lod_fpart);
>       }
>       else {
>          lp_build_ifloor_fract(float_bld, lod, out_lod_ipart,
>          out_lod_fpart);
>       }
> 
>       lp_build_name(*out_lod_fpart, "lod_fpart");
>    }
>    else {
>       *out_lod_ipart = lp_build_iround(float_bld, lod);
>    }
> 
> and you're not going to tell me that lp_build_brilinear_lod is faster
> than lp_build_ifloor_fract (especially since it includes it ;-)

Indeed lp_build_brilinear_lod is not faster than lp_build_ifloor_fract, but brilinear is faster, not because log is a faster approximation, but because it increases the odds that fract part is zero, which means that we can sample from a single mip level, instead of lerping between two mip levels.

I think you have a good point here -- lp_build_brilinear_lod is a log2 approximation which is wrong here and that's a great catch, -- but I have a point too: lp_build_ifloor_fract will slow down texture sampling.

Just like log2 and brilinear log2, we need a variant of ifloor_fract, that increases the probability of fract part being zero, essentially by applying a stair case transformation like:

             ____/
            /
       ____/
      /
 ____/

Jose