Face.position_material and uvh.get_front_UVQ

fredo6

While working on the ThruPaint plugin, I encountered two issues with the API related to face texturing with FOUR UV specifications

1) face.position_material

When you pass a point set of 4 UV points, it seems that SU distorts the texture to match a perspective view, and does not really do a tapering of the texture image.

In the picture below, the UV specs for the 4 corners are (0, 0), (1, 0), (1, 1), (0, 1) for all three faces. This corresponds to a distorted texture which remains aligned with the 4 edges.

It works fine with a rectangular face (left) and with a sheared face (right)

When the face is tapered (middle case), it looks like SU is using a perspective distortion instead of tapering the texture along the edges. As a result, the texture patterns are not equally spread, here in the V axis. When you texture a full mesh, based on quads but with each face having its own shape, it results in non-alignment of pattens.

I did not find a way to change this behaviour with the method position_material and did not see any other API doing that.

2) uvh.get_front_UVQ

When you specify 4 points to position_material (say at the corners of a quad) and then you get the UV of these corners by get_front_UVQ, you get different values. The values which are different are usually 'strange'. This seems to corresponds to the perspective distortion that SU has applied.

I found out that you can retrieve the original UV, by dividing the U and V values by the 3rd coordinate (i.e. .z) of the UV point returned (so I guess the 'Q' value). This Q value is 1.0 when the texture is not distorted.


uvh = face.get_UVHelper(true, true, tw)
uv = uvh.get_front_UVQ(pt)
uv.x /= uv.z
uv.y /= uv.z
uv.z = 1.0

In conclusion, I am not sure there is any way to change the behavior of texture distortion of SU, but just in case someone has an idea, this would be really useful

Fredo

PS: I guess specialized UV mapping applications handle distortion properly.

TIG

Can't you add the final step uv.z=1.0 to normalize things again...

thomthom

This is two issues I encountered myself. In addition, There is not way to find out if a texture is projected or not. Nor set the Projected property. Nor is it possible to detect if a texture is Positioned or not.

fredo6

@tig said:

Can't you add the final step uv.z=1.0 to normalize things again...

You're perfectly right. Good remark.
I updated the post.

Stangely, Position.material ignores the Q value when applying the texture (you can set it to whatever value you wish with no effect).

Fredo

thomthom

Google claimed the Q value was not used - which in a way it isn't, when you apply a texture. But it sure is used when retrieving it. Really wish we could get some clarification to how the UV mapping system works.

Aerilius

I absolutely agree about the missing method to get/set the projected property.

Fredo,
as for the problem with tapered textures, I think this is a question of bilinear texture mapping versus projective (http://www.cs.cmu.edu/~ph/texfund/texfund.pdf p. 14 & p.17), but isn't projective mapping default in most other applications?
I can't think of any better solution than splitting the face into smaller quads.