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In Cycles, the Geometry node has a socket for Parametric. What would this input be useful for?

It looks to me to be identical to UV coordinates for each triangle where the vertices are (0,0),(0,1),(1,0) but with no means of indicating or influencing the orientation of the face. I can't find any meningful documentation that explains what it could be used for.

Can anyone plese explain what it can be used for and, if possible, provide examples of it in use.

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This output represents the "barycentric coordinates" in the rendered triangle. They can be used to interpolate any data defined at the triangle's vertices.

In practice, however, as The Cycles Input Encyclopedia explains:

It’s another one you probably won’t use very often, unless you’re trying to make an unusual wireframe effect, although we now have a specific node for that.

EDIT Here's an idea how you could use it: you can find the "center of mass" of the triangle - it is where all three coordinates have the same value. In practice you would need to work with an error threshold, and find a small triangle in the rendered triangle.

Another idea: Some software, such as Modo, have "Barycentric UV projection", you could probably emulate that, and use textures created in them.

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  • $\begingroup$ Thanks for the info - very interesting. So the Parametric can be used to blend between vertex properties - So if we could get, say, the Normal at each Vertex we could blend them across the face similar to Smooth Shading. However, we don't seem to be able to get those vertex data, nor determine or control which vertex is which. There must surely be a use for it other than an "unusual wireframe effect". $\endgroup$ – Rich Sedman Dec 22 '16 at 22:42
  • $\begingroup$ I came up with an idea, see the modified answer :) $\endgroup$ – lbalazscs Dec 22 '16 at 23:22
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Some more creative uses of parametric coordinates, creating center points. Ramp converters are used to map colours across a face, with two positive edges on either end, containing x1 and y1 values.

enter image description here

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Example using parametric co-ordinates

Example using parametric co-ordinates in case anyone else is wondering how it can be used.

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    $\begingroup$ Insufficient explanation, image alone is not enough. Is parametric only for color ramps? And what does it do? $\endgroup$ – Lukasz-40sth Jan 29 '19 at 8:52
  • $\begingroup$ This is an example to support the above answers. As to how it works, all I can say is it seems to output vector co-ordinates of each point per triangulated face as x1 y1 z0. When creating triangles the closing vertex of the first edge will be given an X value, second Y and the closing edge or last created edge, will hold no value (Z0). For example when you create a default circle with 'pin wheel' fill, this will create triangles in the order 'from circle to center and back' wheras filling it by extruding the circle and collapsing the new verts will make the center vert Z0. $\endgroup$ – PTCreative Feb 16 '19 at 10:35

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