PTEX painting provides a new way for artists to paint texture maps - free from UVs. With PTEX painting, you apply paint to each base face of the mesh, creating high resolution detail without needing explicit UV coordinates.
While UVs provide a way to map your texture from a rectangular image onto a 3D object, PTEX lets you paint directly on the faces of the object itself, no UVs required. The paint information is associated with the base faces of the mesh.
Once your mesh is set up for PTEX painting, creating a new paint layer automatically creates a PTEX paint layer, which you can save and export as a PTEX file. (See Save or export paint layers.) When you save a PTEX file, mesh data is also saved by default, letting you easily inspect your PTEX textures in other applications.
Texel is an abbreviation of the term 'texture element'. A texel is like a pixel, but is not limited to your flat computer screen. You can think of a texel as a single point of color on your model.
When preparing a mesh for PTEX painting, you specify the texel resolution required for each base face of the mesh. The texel resolution you specify tells Mudbox how much detail per base face you require. You can assign different base-face resolutions to different areas of the mesh, letting you effectively 'budget' texels to where they are most needed.
