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Exporting the Object from Mudbox
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We will base the render on the subdivision level 0 of the mesh, so we need to export that. Go to level 0, select the object and then go to File > Export selection... and choose an .obj file name. Then save the .obj file.
Extracting the PTex Texture in Mudbox
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The extract dialog should look like the picture below. Click on the Extract button to create the PTex texture.
Rendering with V-Ray
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First, import the level 0 version of the object that you exported to an .obj file earlier. Go to File > Import... and select the .obj file. Make sure that the Retriangulate Polygons option is disabled. Since the PTex textures are tied to the mesh topology, it is vital that the mesh is imported in 3ds Max exactly as it was in Mudbox.
Next, apply a VRayDisplacementMod modifier to the object, set the Type to Subdivision and turn on the Classic Catmull-Clark option. Set the Vector displacement mode to Object. Click on the Texmap button and select VRayPtex texture. Instance the texture to a slot in the Material Editor, click on the filename button and select the .ptx file that you exported earlier.
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At this point, the displacement modifier should look like this:
You may notice slight noise or moire effect in the displacement. To fix this, go back to the VRayPtex texture and set the filter type to Bilinear or Bicubic.
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Optimizing the Render
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Depending on the settings, rendering with Ptex vector displacement might be slow. You can speed it up a lot in exchange for some additional RAM if you turn go to the VRayDisplacementMod modifier and turn on the Static Geometry and Cache Normals options. The Static Geometry option will make V-Ray pre-tessellate the object at the start of the frame and treat it like regular geometry instead of render-time displacement. This allows you to use the Embree raycaster to further speed up the rendering. The Cache Normals option causes V-Ray to pre-calculate the surface normals, giving an additional speed boost although at the expense of slight errors in the normals due to interpolation.
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Deforming the Object
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Object-space displacement generally stores the displacement values relative to the original object mesh. If the object is deformed in some way, then the displacement values are no longer valid. However, since the PTex file can store information about the original object mesh, V-Ray is able to use the difference between that original mesh and the deformed one in order to recalculate the displacement values for the deformed state. V-Ray does this automatically; no extra effort is required from the user so long as the PTex file includes mesh information.
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