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Normal Reconstruct Z - Maya Plugin Development Notes

Goal

Create a Maya plugin that takes the R and G channels of a normal map and reconstructs the B (Z) channel using the unit normal constraint: X^2 + Y^2 + Z^2 = 1

The Core Problem

When connecting a file texture's outColor to a utility node's color input in Maya, the compute() method receives (0, 0, 0) instead of the actual texture values. This is because Maya's dependency graph evaluation doesn't automatically sample textures at UV coordinates for utility nodes.

Approaches Tried

1. Simple Utility Node with Color Input (FAILED)

  • Created MPxNode with inputColor (float3) attribute
  • Connected file texture's outColor to inputColor
  • Result: Always received (0, 0, 0) - textures aren't sampled

2. Added UV Coordinates (FAILED)

  • Added uvCoord and uvFilterSize attributes like texture nodes have
  • Connected place2dTexture.outUV to node's uvCoord
  • Result: Still (0, 0, 0) - UV coords received but texture not sampled

3. MRenderUtil::sampleShadingNetwork (FAILED)

  • Used MRenderUtil::sampleShadingNetwork() in compute to explicitly sample connected texture
  • Result: Didn't work - this function needs proper render context

4. Texture Node with MImage (PARTIAL)

  • Changed to texture node that takes fileName string input
  • Used MImage::readFromFile() to load texture
  • Sampled pixels directly at UV coordinates
  • Result: Showed 0.5, 0, 0.5

5. MPxShadingNodeOverride for VP2 (WORKING)

  • Per forum post, need MPxShadingNodeOverride for VP2 viewport to work
  • Fragment receives inputColor and processes it
  • Fragment graph connects upstream texture fragment to our fragment automatically
  • Status: WORKING

Key Insights from Research

  1. Maya's DG evaluation doesn't sample textures - When you connect a file texture to a utility node, the utility node's compute() just gets (0,0,0) because there's no automatic texture sampling

  2. VP2 works differently - The fragment shader system composes fragments together, so texture sampling happens in the GPU shader pipeline

  3. Forum solution - User on Autodesk forums said they had to implement MPxShadingNodeOverride for it to work (not elegant but necessary)

  4. File node approach - Maya's built-in file node works because it samples textures itself using MImage - it doesn't receive color from another node


## References
- https://forums.autodesk.com/t5/maya-programming-forum/openmaya-plugin-use-texture-as-color-input/td-p/9392499
- https://help.autodesk.com/cloudhelp/2017/ENU/Maya-SDK/files/GUID-585F5656-4069-4D82-B9BB-3D1AB2D0DFE6.htm (Shading Node Overrides)
- Maya DevKit examples: fileTexture, checkerShader, brickShader