[Mesa-dev] [PATCH 0/5] Add ARB_derivative_control support

[Ilia Mirkin]

On Thu, Aug 14, 2014 at 1:30 PM, Ian Romanick <idr at freedesktop.org> wrote:
> On 08/13/2014 11:58 PM, Matt Turner wrote:
>> On Wed, Aug 13, 2014 at 9:52 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>>> I left all the variants as separate operations in the glsl ir. However for
>>> gallium I only added the fine version, as it seems like DDX can do pretty much
>>> whatever it wants. I was on the fence about adding coarse versions as well and
>>> then using the FragmentShaderDerivative hint to select one or the other in the
>>> glsl -> tgsi conversion.
>>>
>>> In the case of nv50/nvc0, doing the fine version is pretty much the only
>>> (easy) way of doing derivatives. I haven't traced the blob to see how it
>>> handles things yet. In any case, on nv50/nvc0 all this is completely moot, at
>>> least for now. Curious about what the situation with other hardware is.
>>
>> i965 already implements coarse and fine derivatives, selectable by the
>> derivatives hint, coarse default.
>
> I don't think that's the same thing.  The "fine" derivatives in i965
> definitely do not meet this requirement:
>
>     "...second-order fine derivatives, e.g., dFdxFine(dFdxFine(x))
>     will properly reflect the difference between the independent
>     fine derivatives computed within the 2x2 square."
>
> As it is now, dFdxFine(dFdxFine(x*x*x))) will always be zero in the i965
> driver.  Two pixels on the same line will have different dFdy, but the
> dFdx will be the same.  Right?

I sent a question about this to the list earlier (with no response
other than my own), but I believe that to be a typo in the spec. Look
at Issue 2, which explicitly talks about dFdxFine(dFdyFine(...)).
There's no way to get second-order derivatives in a single variable
with only 2 points, so it would want a larger block.

>
> Is there a piglit test for that specific part?  (I haven't looked at the
> piglit list at all.)

There's a piglit test for dFdxFine(dFdyFine()).

>
>> The calculation of the derivative itself isn't faster for coarse
>> derivatives, but it was discovered that if all of the samples of a
>> sample_d are from the same LOD, it's a bunch faster on Haswell at
>> least. See commit 848c0e72. And with coarse derivatives they are.
>>
>> Maybe other hardware has similar optimizations?
>>
>>> Also, the extension spec claims to require GLSL 4.00, which seems a little
>>> extreme. Instead I restrict it to core contexts. Let me know if I should
>>> change this.
>>
>> Making it core-only doesn't help, nor does it satisfy the GLSL >= 4.0
>> requirement in the spec. I'm not sure if we have a way to arbitrarily
>> limit an extension to being exposed under certain GLSL versions... ?
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>