[HOU-1969] OSL dPdu, dPdv partial derivatives always seem to be 0,0,0?

[malcolmh]

When running the following OSL, I would expect dPdu and dPdv to be valid?

--snip--
vector worldTangent = normalize(dPdu);
vector worldBitangent = normalize(dPdv);
vector worldNormal = normalize(N);
//
printf("worldTangent: %f\n", worldTangent);
printf("worldBitangent: %f\n", worldBitangent);
printf("worldNormal: %f\n", worldNormal);
--snip--

I'm only getting 0,0,0 while N is correctly being passed

--snip--
worldTangent: 0.000000 0.000000 0.000000
worldBitangent: 0.000000 0.000000 0.000000
worldNormal: -0.027439 -0.272305 0.961820
worldTangent: 0.000000 0.000000 0.000000
worldBitangent: 0.000000 0.000000 0.000000
worldNormal: 0.929598 -0.367538 0.027624
worldTangent: 0.000000 0.000000 0.000000
worldBitangent: 0.000000 0.000000 0.000000
worldNormal: -0.084531 0.070302 0.993938
--snip--

This makes it impossible to do any anisotropy type effects.

It would be good to check Dx(), Dy(), Dz() to see if these are working as expected.

[chakarov]

Hello malcolmh , I've logged the issue in our system and some of the devs will take a look at this problem.

[malcolmh]

Great Georgy, it would be good to check this shader https://docs.chaosgroup.com/display/.../Fabric+shader works as expected as it uses dPdu and dPdv

For Dx() and Dy() it would be good to check that these work with the other globals (like P and N) so it's possible to compute the curvature of the surface regardless of the parameterisation of the underlying surface. eg (http://madebyevan.com/shaders/curvature)