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Step 1 - I can attach an HDRI to a skydome light and see a preview of the texture on the sphere in the 3d viewer.
Step 2 - I used an ALCombine node to add a ramp to the HDRI (in a render I could see the gradient did affect the HDRI) - but is there a way to see the combination of the HDRI and the gradient as a preview in the 3D viewer. The plan is to rotate/move the gradient and see the results live in the viewer
Yes, but not using Arnold nodes. Use native Maya nodes instead. There needs to be an opengl implementation of the node to have it previewed in the viewport.