POV-Ray : Documentation : UV Vectors
  POV-Ray 3.6 Documentation Online View Supported Objects   2.5.8 Triangle Texture Interpolation UV Vectors

With the keyword uv_vectors, the UV coordinates of the corners can be controlled for bicubic patches and standard triangle mesh.

For bicubic patches the UV coordinates can be specified for each of the four corners of the patch. This goes right before the control points.
The syntax is:

  uv_vectors <corner1>,<corner2>,<corner3>, <corner4>
with default
  uv_vectors <0,0>,<1,0>,<1,1>,<0,1>

For standard triangle meshes (not mesh2) you can specify the UV coordinates for each of the three vertices uv_vectors <uv1>,<uv2>,<uv3> inside each mesh triangle. This goes right after the coordinates (or coordinates & normals with smooth triangles) and right before the texture.

  mesh {
    triangle {
      <0,0,0>, <0.5,0,0>, <0.5,0.5,0>
      uv_vectors <0,0>, <1,0>, <1,1>
    triangle {
      <0,0,0>, <0.5,0.5,0>, <0,0.5,0>
      uv_vectors <0,0>, <1,1>, <0,1>
    texture {
      uv_mapping pigment {
        image_map {
          sys "SomeImage"
          map_type 0
          interpolate 0
  } Supported Objects   2.5.8 Triangle Texture Interpolation

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