Ptex Modeling Requirements
Mari supports both quad and triangular face textures, which are exposed by the Ptex file format. All meshes use quad face textures, including mixed meshes; while wholly triangular meshes use triangular face textures. Quad textures are natively supported in graphics hardware. However, triangular textures are not supported in the current generation of graphics hardware. Mari’s canvas preview of triangular textures uses the system described for GL displays at http://ptex.us/tritex.html.
Though Mari supports both types of face textures, Ptex dictates that the dimensions of a triangular face must be equal. Due to this restriction to triangular textures, if they are applied to polygons with an aspect ratio significantly different to one, the texture appears blurred or smeared. When triangular textures are stored in packed format, GPU renderers can only display half of the texels, resulting in Mari filling in the other half of the texture with data that it is not representable in the display.
Note: Some renderers, such as RenderMan, read all texels and, thus, can render triangular texels.
Tip: For more information regarding packed format and triangular textures, please refer to http://ptex.us/tritex.html.
For low-frequency images, the even and odd texels are very similar in color, and the Ptex packing of triangular textures suggests that a simple mirror of texels can be used to fill the other half of the texture when exporting to Ptex. Mari implements this mirroring. For high-frequency images, however, this mirroring is incorrect and visual artifacts appear in software renderers using the exported Ptex file.
With the restrictions of triangular textures, we highly recommend not modeling with only triangular faces. Quad face textures do not have the same triangular face equality restrictions and are a preferable option for use in Mari.