3D transformation with world fixed UV transformations

[Manel_Goucha]

Hello,

I’m trying to work on a modeler for basic shapes (Cubes, Icospheres, Cones, etc…) in a way that only the mesh scruture is affected when transforming the object.

As such:

I have a cube object of1 unit in length (so 0.5, 0.5, 0.5 vetices) with a full image mapping for each side (0.0, 1.0 UV) and for any scaling performed onto it, I want to keep (adjust) the UVs to be the same relative to the world around, in a way that I before the transformation the texture had 1x1 on the right side of the cube, after scaling 2x on Y, the right side should have now 1x2 tiled.

So far I managed to scale the texture using the same transformation matrix as the model matrix and apply it to the texture coordinate on the vertex shader.

The issue here is that the texture only has 2 coordinates, but the model has 3, so I am only able to transform correctly 2 of the coordinates correctly (x and y = U and V).

I was trying to find for a way to correctly transform (Project?) the 3D axis onto the 2D map in order to keep it visually correct, so far these are my results:

Side View:

(here it works fine because coincidentally the Y axis in the world coordinates matches the V axis in the texture coordinates)

Top View:

(when scaling in the Y axis, the top view gets distorted because the V axis is getting wongly transformed along with the Y axis)

VERTEX

	gl_Position = modelViewProjMat * vert;
	v_TexCoord  = vec2(modelMat * vec4(texCoord, 1.0, 1.0));

and then I’m using the v_TexCoord normally to sample the texture.

Is there any way of projecting (or applying the same 3D element transformations onto the texture in order to keep the world coordinates fixed for the map? (without using 3D textures)

Regards,
Jakes

[GClements]

You can use 3D texture coordinates (which could be the object-space vertex coordinates) then project them onto a plane after transformation. The plane could be specified via vertex attributes, or you can calculate the plane of the face in a geometry shader or a fragment shader (in the latter case, using dFdx and dFdy).

The main problem is that whatever approach you use, it won’t always do what you want. In particular, while you can always calculate a transformation which gives the correct scale factor, the rotation and offset are somewhat arbitrary, particularly if you don’t limit yourself to cuboids.

[Manel_Goucha]

@GClements

it won’t always do what you want

So for better results it is better to just use separate objects by performing such operations individually not only yielding better control over contained transformations but also reducing the cost of performing calculations on the Fragment Shader.

@GClements

particularly if you don’t limit yourself to cuboids.

And in this case, any other non-regular shape would virtually work better.