I guess fast-by-default. I imagine that more apps care about
performance than care about the granularity of their derivatives.
After a bit more thought -- In HLSL shader model 5 there's both
ddx_coarse() and ddx_fine() which gives the shader author the choice
between roughly these options. In a *very* quick look I haven't found
anything equivalent -- but I might just be being blind.
CC'ing Ian -- any opinion? Is there any conformance issue here?
-- Chris
On Thu, Sep 12, 2013 at 8:41 PM, Chia-I Wu <olva...@gmail.com> wrote:
> On Thu, Sep 12, 2013 at 2:06 PM, Chris Forbes <chr...@ijw.co.nz> wrote:
>> Can we make this approximation conditional on an image-quality control
>> in driconf [or somewhere else]?
> Sure. What would be the default behavior?
>
>> On Thu, Sep 12, 2013 at 5:00 PM, Chia-I Wu <olva...@gmail.com> wrote:
>>> From: Chia-I Wu <o...@lunarg.com>
>>>
>>> Replicate the gradient of the top-left pixel to the other three pixels in
>>> the
>>> subspan, as how DDY is implemented. Before, different graidents were used
>>> for
>>> pixels in the top row and pixels in the bottom row.
>>>
>>> This change results in a less accurate approximation. However, it improves
>>> the performance of Xonotic with Ultra settings by 24.3879% +/- 0.832202% (at
>>> 95.0% confidence) on Haswell. No noticeable image quality difference
>>> observed.
>>>
>>> No piglit gpu.tests regressions.
>>>
>>> I failed to come up with an explanation for the performance difference. The
>>> change does not make a difference on Ivy Bridge either. If anyone has the
>>> insight, please kindly enlighten me. Performance differences may also be
>>> observed on other games that call textureGrad and dFdx.
>>>
>>> Signed-off-by: Chia-I Wu <o...@lunarg.com>
>>> ---
>>> src/mesa/drivers/dri/i965/brw_fs_emit.cpp | 17 +++++++++++++----
>>> 1 file changed, 13 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
>>> b/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
>>> index bfb3d33..c0d24a0 100644
>>> --- a/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
>>> +++ b/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
>>> @@ -564,16 +564,25 @@ fs_generator::generate_tex(fs_inst *inst, struct
>>> brw_reg dst, struct brw_reg src
>>> void
>>> fs_generator::generate_ddx(fs_inst *inst, struct brw_reg dst, struct
>>> brw_reg src)
>>> {
>>> + /* approximate with ((ss0.tr - ss0.tl)x4 (ss1.tr - ss1.tl)x4) on
>>> Haswell,
>>> + * which gives much better performance when the result is used with
>>> + * sample_d
>>> + */
>>> + unsigned vstride = (brw->is_haswell) ? BRW_VERTICAL_STRIDE_4 :
>>> + BRW_VERTICAL_STRIDE_2;
>>> + unsigned width = (brw->is_haswell) ? BRW_WIDTH_4 :
>>> + BRW_WIDTH_2;
>>> +
>>> struct brw_reg src0 = brw_reg(src.file, src.nr, 1,
>>> BRW_REGISTER_TYPE_F,
>>> - BRW_VERTICAL_STRIDE_2,
>>> - BRW_WIDTH_2,
>>> + vstride,
>>> + width,
>>> BRW_HORIZONTAL_STRIDE_0,
>>> BRW_SWIZZLE_XYZW, WRITEMASK_XYZW);
>>> struct brw_reg src1 = brw_reg(src.file, src.nr, 0,
>>> BRW_REGISTER_TYPE_F,
>>> - BRW_VERTICAL_STRIDE_2,
>>> - BRW_WIDTH_2,
>>> + vstride,
>>> + width,
>>> BRW_HORIZONTAL_STRIDE_0,
>>> BRW_SWIZZLE_XYZW, WRITEMASK_XYZW);
>>> brw_ADD(p, dst, src0, negate(src1));
>>> --
>>> 1.8.3.1
>>>
>>> _______________________________________________
>>> mesa-dev mailing list
>>> mesa-dev@lists.freedesktop.org
>>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
>
>
> --
> o...@lunarg.com
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