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I tried to UV unwrap this octahedron, by slicing one of the sides.

I was able to lay it out on the texture like this, but with severe distortions.

enter image description here

Can someone help me UV map this without distortions?

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Since they are all triangles, couldn't you just stack half the triangles on one half of the texture and the stack the other half on the rest of the texture? I know for sure you can get the triangles seperated on your uv-mapping tool. –  Sidar Oct 10 '12 at 3:19
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You UV wrapped it the right way, is your goal to have the model less stretched out and more relaxed? –  Zehelvion Oct 10 '12 at 4:18
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2 Answers 2

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The way you could have this texture look more relaxed and less stretched out is by taking the two extreme corners, bottom left and top right and pulling them towards one another until each of them sits approximately 3x3 from squares from it's current position. If you want to keep using all of the texture image, you would need to distort the texture in the opposite direction to make it appear even on the model itself.

There is another way to accomplish a more relaxed look. instead of texturing the object from the side, texture it from the top view, so the sharp points in the top and bottom are in the center.

How I'd go about stretching the texture:

  1. Get a high-res version of the texture. Stretching is a 'lossy(not lossless)' transformation.
  2. Create a square plain.
  3. UV map the texture you wish to stretch to that plain with the same UV map shape as the ones used on the original object.
  4. If you wish to adjust it some more, you may also stretch the square itself a little bit.
  5. Render the square with the texture and use the resulting image as your new texture.
  6. Because this transformation is lossy, you may want to re-size the texture to 50%.
  7. Test the new texture on your model and add adjustments if necessary.
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How do you distort the texture with the reverse distortion? I've seen this tool called uvmapper before, maybe I'll try that. –  bobobobo Oct 10 '12 at 15:42
    
@bobobobo I added some detail on how you can distort the texture. –  Zehelvion Oct 12 '12 at 15:39
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For simple shapes, you can usually find sensible unwrapping approaches by looking to papercraft (those guys are experts at unfolding shapes. Albeit backwards.) For an octohedron, you might consider the model here:

http://psmay.com/wp-content/uploads/2012/01/logihedron.pdf

This sort of approach doesn't entirely cover your texture the way that your unwrapping did, but it has the advantage that your texture isn't distorted across any face, and you get uniform texel density per unit of space on your object (whereas with your distorted mapping, you get areas of high density on some faces, and low density on others).

It's all tradeoffs, really.

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This is a very good and insightful answer +1 –  Zehelvion Oct 10 '12 at 5:36
    
Sadly this approach will create a lot of seams and will make it much harder to make the texture appear seamless (especially with a pebble texture like the OP wants). But there's always a tradeoff between no. of seams and distortion. –  bummzack Oct 10 '12 at 8:32
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In the real industry, this is the approach that is normally used; unwrap a complex shape into flat, uniform triangles, and then manually paint the texture to hide the seams. The tradeoff between seams and distortion really only exists if you're not creating your own textures. :) –  Trevor Powell Oct 10 '12 at 8:39
    
@TrevorPowell That's certainly not how it is in the real industry. Sure, textures are custom created for the model, but you almost never have flat uniform triangles! Just imagine the unwrapping of a head of a complex human like model. You'll have only a few seams there (maybe below the chin, and one at the backside of the head, maybe behind the ear and sometimes one at the hairline). To have a real flat projection on triangles, you would need much much more seams. The goal is to have as few and well hidden seams as possible with the least amount of distortion possible. –  bummzack Oct 10 '12 at 9:20
    
@bummzack I think you're splitting hairs. Compared against the degree of face-to-face distortion visible in the mapping illustrated in the original question, the typical methods of mapping heads do try to use relatively flat, uniform triangles. Not mathematically so, sure, but I think we're all smart enough to get the gist of the general idea. –  Trevor Powell Oct 10 '12 at 10:33
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