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Trouble with texture on distorted Poly

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Hey, Real head scratchier this on...


I have a single 4 point polygon, with a single UV tile covering the whole polygon. Simple.


When I move points to change the shape of my polygon (on only 1 axis) id expect the UV and texture to stretch with the polygon,  but no, it goes crazy!

Left is the polly and texture as expected. Right image i've scaled apart the bottom points expecting the texture to stretch scale at the bottom too, but no... Any ideas?



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It is quite difficult to explain what is going on here, but I'll have a go. This problem stems from the fact that you are trying to UV just 1 polygon and trying to project it incorrectly, which is kind of in opposition to what Cinema is expecting you to do. I haven't got time to go into the long, tedious and complicated reasons this is the case with a single poly, but it is the case, and to solve it you need more polys in that shape...


For example... here I have a similar setup to your own.




First frame: 1 poly. There is simply not enough information here for Cinema to know HOW it should stretch in between the 4 points. We might imagine it would make sense for it stretch from the center out, but nothing has told Cinema that, so it guesses arbitrarily.

Second frame: 10 vertical edge loops added to model - UV substantially improved, because now Cinema has been given much more information about what should happen between the borders of the shape.

3rd Frame: 20 horizontal edges added to model - UV result even better because now it knows what to do in both directions.


But, if we actually want a correct UV result, which in this case would be this...




...then you do only need the one poly, and just need to re-project it accordingly (I did frontal projection from top view here) once your model is correct...


Hope that makes sense - I appreciate it is not exactly intuitive !



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CBR, thanks so much, thats totally sorted it out, and (I think) helped me understand the logic behind the issue.


I did actually consider subdividing the polly, but usually we are all about using as few pollys as necessary, particularly in this instance as the actual thing is going to be cloned allot...


I've also noted your second example of re-setting the UV correctly, but this isn't actually what I want in this case. I'm using Xpresso to deform the cloned polys and want the texture to be streached as they deform, but thanks for explaining why thats 'wrong' and why cinema is struggling to understand what I want.



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To what Cerbera wrote, I will add an explanation that in 3D modeling on polygons, always internal, the basic polygon is a triangle. Regardless of whether you use quads or n-gons. Everything is converted to triangles. Therefore, although you are distorting the quadrangle, they are actually two triangles. When you look at the picture below, you will see that on the third object I made a division into triangles. Then you can clearly see where the texture distortion is from. Analyze yourself how the cutting edge of the quad cut into two triangles in relation to the texture. Then you will understand where this problem comes from.



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And one more thing. If you do not want to add too many polygons, then your problem (if they are only shifted to the bottom two bottom points) will partially solve the intersection of the quad into four triangles. Diagonally. As in the example below.


But of course, the more divisions, the better.



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