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Incorrect UV Space Interpolation?
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Description

Example image attached shows the issue.
To work around that weirdness(bug?) you have to subdivide it a few times before projecting.
(which is a huge pita and makes editing the mesh after projection a huge undertaking).
Maya doesn't suffer from this for example.

Andrew Price (blenderguru) noticed that behaviour with v.2.66 https://www.youtube.com/watch?v=B9r9tKX5olY&t=1321 so i guess this issue is there since a long time.

BA thread: http://blenderartists.org/forum/showthread.php?380498-Blender-for-Mattepainting-Projection-Mapping

Event Timeline

manuel grad (manuelgrad) raised the priority of this task from to 90.
manuel grad (manuelgrad) updated the task description. (Show Details)
manuel grad (manuelgrad) edited a custom field.
Bastien Montagne (mont29) lowered the priority of this task from 90 to 30.Sep 4 2015, 5:09 PM

Please follow our submission template and guidelines and make a complete, valid bug report, with required info, precise description of the issue, precise steps to reproduce it, small and simple .blend and/or other files to do so if needed, etc.

References to forums, third party apps etc. are not much useful, and certainly do not make a valid bug report.

I attached two files but it seems they`re gone and i do not have permission to edit the bug report ... but i`ll try again:

This issue exists since ever on every system i tested. But i attached a system-info.txt and a .blend.

Steps to reproduce:
Enter Edit mode for the object
Hit U
Select Project from View
texture gets distorted along triangle edge

I for sure are not the first one to notice this:

another thread: http://blenderartists.org/forum/showthread.php?372030-Project-from-View-distorts-texture-along-triangle-edge
blenderguru tutorial: http://www.youtube.com/watch?v=B9r9tKX5olY&t=1321
my thread: http://blenderartists.org/forum/showthread.php?380498-Blender-for-Mattepainting-Projection-Mapping

Same issue with the UV-Project modifier and a camera.
Workaround is to subdivide the shit out of the mesh.
But this seriously can`t be the ideal solution. What if i decide to edit the mesh some time after i projected something onto it?
Should i remove hundreds of edge loops and try again?
I really don`t want to use maya just because of this ... but this is a showstopper.

Bastien Montagne (mont29) raised the priority of this task from 30 to Normal.Sep 4 2015, 11:00 PM

Ok… so, this happening when projection from perspective view… The effect is basically the same thing as if you make a flat simple UV unwrapping of a mere square, and move only one points of that UVMap - one of the two points that are not common to both triangles in the tessellated square.

Imho deformation cannot be avoided here (at least, I cannot imagine how - unless you keep track of the projection matrix and apply some kind of correction to UV interpolation later, but this would go way beyond the scope of those tools imho).

Note that:

  • Ortho-projected unwrapping does obviously not suffer from this issue, since it always keep parallel edges parallel.
  • By sub-dividing, you kind of create 'intermediate reference points' in the UVMap that keep track of the deformation due to perspective projection, and allow to reduce its effects.

So, for me, no bug here at all, this is just limitation of UVMapping (maybe PTex could fix that? don’t know), @Sergey Sharybin (sergey), @Campbell Barton (campbellbarton), thoughts?

This is supported with Blender-Internal, there is an option for UV projection interpolation, heres a file from this report, working as expected.

See: Material -> Options -> UV Project


@Bastien Montagne (mont29), as you say it does need the projection-matrix.
https://developer.blender.org/diffusion/B/browse/master/source/blender/render/intern/source/shadeinput.c;ac51e2f326965816e23f7948e2e3fbdfb07cb66c$1123

This seems more like a Cycles TODO, @Sergey Sharybin (sergey) ?