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When rendering hair or fur, it is often useful to pick up a color from a texture based on where the hair is attached to the emitter geometry.  For this, you will need a UV coordinate that is fixed per hair. In Houdini, you must specify a primitive UV, since each curve is a single primitive.  The Houdini fur setup will provide this on the guide curves as long as you have created UVs on the emitter geometry.

In Arnold 4, barycentric UVs are provided along each curve from [0,1] in each direction across the curve render primitive, and these will override any Houdini primitive provided. There is a work around where you can copy the curve UV primitives to a new 'uv2' primitive attribute, and then use this UV set in the image shader.

However, in Arnold 5 the The workflow is simplified so that when you specify UV coordinates on the curve per primitive, it will be passed to the hair's render primitives directly.  You will have the UVs from the emitter to use for shading by default.

Center

Image Modified

Color of splines driven by UV grid

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