Pixar Photorealistic RenderMan - Artist tools (RAT) Tutorials

Testing NURBS surfaces conversion to Maya Subdivision surfaces, RAT Subdivision surfaces and trying to achieve similar results.
 

Softwares used: Alias Maya 7.0.1, Pixar RAT 6.5.2, Pixar Renderman server pro 12.5.2

Click here to download the Maya scene file.

Suppose you have modeled a NURBS head and want to convert it to RAT subdivision to have benefits of polygons when rigging and have a higher speed PRman rendering than using Maya native SubD surfaces. This is what we're looking for here.

1. We start with creating two simple NURBS planes and using soft modification tool to change their shape to organic form, then we attach the two surfaces, rebuild them uniformly with "keep CV's ON", delete the history, select the isoparam that was the two surfaces former boundary and detach the surfaces, rebuild both surfaces with "keep CV's ON" and again delete the history.
This operation is called socking and is usually used in modeling organic objects using NURBS.

Details:

1. Click on Create>NURBS primitives>Plane[]*
   Set patches number to 4 along both U and V sides.
   Scale the plane to 10.

2. Click on Edit>Duplicate[]
   Reset duplicate menu settings and click on Duplicate button

3. Go to component mode and select some vertices

4. Click Soft modification tool in left vertical toolbox and use it to form organic shapes on both surfaces.

5. bring the two surfaces close to each other so they can be attached

6. Click on Edit Nurbs>Attach surfaces[].
   Reset the tool and deselect "Keep original".
   Click the attach button. If they are not attached move their borders closer and try again.

7. Click on Edit Nurbs>Rebuild surfaces tool[].
   Reset the tool and click on "keep CV's" checkbox.
   Click on parameter range from 0 to #spans. Now click the rebuild tool button.
   Now press Q to quit the tool.

8. Click on Edit> Delete all by type> History.

9. Go to isoparam mode and select the two surfaces former boundary by clicking and dragging over it. Make sure no other isoparam is selected. Avoid clicking directly on isoparam since there is always a chance of creating a new isoparam by doing so.

10. Click on Edit Nurbs>Detach surfaces[].
    Reset the tool and click on detach.

11. Click on Edit Nurbs>Rebuild surfaces.

12. Click on Edit> Delete all by type> History.
     

*: [] means click on the square button which brings the option box of the tool.

If section 1 seemed to be complicated, please download the Maya scene file to see the result. It's a modeling procedure and is not directly related to our subject.

The result of socking method is two surfaces with G2 continuity (It means it has both position and tangent continuity).

2. Now that we have our NURBS surfaces, we try to convert it to Maya SubDivision surface to see if they look similar.

1. Click on Modify>Convert>NURBS to SubDiv []

2. Move the new surfaces to the left of your screen so that you can compare them to original NURBS surfaces.

This is what the Maya SubD surface looks like. Please not that although our original NURBS surfaces had continuous corners where they meet, Maya SubD surfaces have round corners.
Other than this, Original NURBS surfaces and Maya SubD surfaces look identical.

Even if you had stitched the two NURBS surfaces before converting them to Maya SubD surfaces, they would have round corners.

3.Now we try to convert our NURBS surfaces to polygons to use RAT SubD surfaces.

First try:

1. Select the two Nurbs surfaces.

2. Click on Modify>Convert>NURBS to Polygons[]
   Reset the tool.
   Set Type to Quads. Remember that a Quad face is made of two triangle face and renders two times faster than them in PRman.
   Set Tessellation Method to “General”.
   Set Number U and Number V to 5.
   Click the convert button.
   Now we must have something similar to Nurbs surface if isoparams were replaced by polygon edges.

3. Click on Edit> Delete all by type> History.

4. Go to Rendering menu set and Click on RenderMan>Pixar SubDivs>Mesh as Subdiv[]
   Click on all check boxes but the one that says "Expose mesh to Maya".
   Click apply.

We come up with this result:

ooops! It is not what we had in mind. The RAT SubD result is not the same as Original Nurbs Object. It's somehow softened. So a polygon with edges exactly the same as Nurbs isoparams does not give us a SubD surface identical to Original Nurbs surface.

Second try:

Lets try another options when converting from NURBS to Polygons.

1. the two Nurbs surfaces.

2. Click on Modify>Convert> NURBS to Polygons[]
   Reset the tool.
   Set Type to Quads.
   Set Tessellation Method to “Control points”.
   Click the convert button.

3. Click on Edit> Delete all by type> History.

4. Go to Rendering menu set and Click on RenderMan>Pixar SubDivs>Mesh as Subdiv

This is the result:

The new RAT SubD surfaces look identical to the original Nurbs surfaces. Please also note that RAT SubD surfaces corners are not round like Maya native SubD surfaces.

Third try:

Lets see what happens if we convert our Maya native SubD surfaces (Section 2 of this tutorial) to polygons and then to RAT SubD surfaces.

1. Select the two Maya SubD surfaces and duplicate them using current settings so you can compare new objects to Maya SubD surfaces.

2. Click on Modify>Convert>SubDivs to Polygons[]
   Reset the tool.
   Set tessellation method to “Vertices”
   We use "Level 0 vertices" option since RAT subDivs don’t use higher level information.
   Click the convert button.

3. Click on Edit> Delete all by type> History.

4. Click on RenderMan>Pixar SubDivs>Mesh as Subdiv.

This is the result:

It looks similar to the original NURBS surfaces and does not have round corners anymore.

4. Now lets see what has happened to UV's. Since we had set UV range from 0 to #number of spans when rebuilding the NURBS surfaces, the UV range of NURBS surfaces is 0 to 4.

To see what happens in rendering we make a simple Blinn shader (palette) in RAT Slim and assign a texture to it and set manifold type to ST. We use Slim background picture which can be found in PRman documents tutorial files as the texture.

Now put some lights there to render it!

This is the final result:

As you can see using ST manifolds, PRman ignores NURBS UV range which is 0 to 4. The texture completely covers the two original NURBS surfaces.
The Maya native SubD UV is totally weird and has no round corner as what could be seen in view port but the one which is converted from Maya SubD to polygons and then to RAT SubD has something like 0 to #number of isoparams UV range! but PRman only recognizes 0 to 1 UV range using ST manifold, doesn't it?
The one that is converted to Polygons using "4 poly per span in 3D" option has weird UV too.
The one that is converted to Polygons using "Control points" option has real 0 to #number of isoparams UV range since the texture is not tiled.
 

Now it is only a matter of using Polygon UV editing tools to correct these all!

Sepehr Dehpour
Feb 06 2006

Freelance Maya /RenderMan
Rich media designer

Copyright - 2006 - Sepehr Dehpour - All right reserved.

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