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I recently asked the same question for Cycles, but now I'm curious about the pros/cons of using Blender Render instead - can a curve's tangent be used to define the color of its beveled/tessellated output mesh?

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While not used very often, Blender Internal has support for node based materials. Almost the same node setup can be re-created in BI by changing the emission for a material node and the one missing being the Tangent node. While not the same result, the Global output of the Geometry node gives a similar effect. If you wanted a matching result between the two you could use Object output from the Texture Coordinates in cycles to get the same result or with some extra math nodes you may be able to get the BI nodes to give the same result as the cycles tangent node.

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  • $\begingroup$ This is giving some unexpected results.. In the bottom-right of your example renders, it looks like the curve is orange on one side and red on the other; I'd expect a circle-intersection at any position along the curve to have a constant colour. $\endgroup$ – ajwood Feb 2 '16 at 14:49
  • $\begingroup$ That is more like a seam, it is working in object space with x controlling red amount, that leads to x=0 giving a red line as the object crosses the axis. Where tangent is constant to the object, the other inputs vary too much as the object moves or is edited. The result isn't as close as it first appears. $\endgroup$ – sambler Feb 2 '16 at 18:33
  • $\begingroup$ here's what it looks like in my test case. I can't figure out exactly where it's going wrong, but I notice a couple things: 1. the left bundle has very little color variability, despite running in a variety of directions, and 2. the vertical bar on the right hand bundle looks very fishy $\endgroup$ – ajwood Feb 2 '16 at 21:53
  • $\begingroup$ It's using object space that is wrong, if you move the curve in edit mode the red line will stay in line with the object origin. It follows the origin not the geometry. $\endgroup$ – sambler Feb 3 '16 at 3:52
  • $\begingroup$ Not sure I understand.. Can you suggest a different space to use? $\endgroup$ – ajwood Feb 3 '16 at 4:11

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