Renderman

Constructive Solid Geometry with Renderman Booleans

There are three types of "Boolean" operations we can perform on the two objects:

• Union
• Difference
• Intersection

For modeling purposes, Union creates a single solid object by combining one or more simple objects. Difference creates an object by subtracting the area of one object from another object. Intersection creates a solid object in the common areas of two or more objects.

If we specify a difference operation to subtract the sphere area from the cube, here is the result:

Modeler Booleans

The Modeler tools for CSG (Punch and Boolean ) modify the geometry of the objects immediately. The resulting object can be viewed and worked with in Modeler, it works with all of the rendering engines, and can be exported.

There are a couple of disadvantages to the Modeler technique: the complexity of the model is increased, and the parametric surfaces are converted to polygon meshes. The model will be larger, and, in rare cases, there might be some loss of smoothness.

Renderman Booleans

The Renderman Boolean method does not show the result of the operation until the final rendering, and it only works with Renderman. But Renderman CSG has a couple of advantages: the parametric surfaces are preserved, and the CSG operation can be animated using the Renderman controls in Presenter.

(Netscape and Microsoft Internet Explorer users can view an animated GIF showing what happens when sphere passes through the cube with the Renderman Difference operation).

Modeler Steps

1. Create a new folder. For this demonstration, set the folder title to "csg 1".
2. Inside the "csg 1" folder, place a cube object.
3. Also inside "csg 1" folder, place a sphere that intersects the cube.
4. Verify that the objects and folder in the Groups window looks something like this, then save the model.

Presenter Steps

1. Apply the Plastic shader to the "csg 1" folder. Double-click the Plastic shader icon to set the shader attributes, then click the "Shader Esoterica" button. Click the "Solid Object" checkbox on and set the popup menu to "Difference".
   
2. Apply the Plastic shader to the cube object. Double-click the Plastic shader icon to set the shader attributes, then click the "Shader Esoterica" button. Click the "Solid Operation" checkbox on and set the popup menu to "Union".
3. Apply the Plastic shader to the sphere object. Double-click the Plastic shader icon to set the shader attributes, then click the "Shader Esoterica" button. Click the "Solid Object" checkbox on and set the popup menu to "Union".
4. Render the scene (using Renderman, of course). If everything works correctly, you should reproduce the CSG effect.

1. The main Boolean operation is specified in the folder enclosing the objects.
2. The first object in the folder is the main object, and subsequent objects in the folder will operate against that first object.

Typically, I set the Difference operation for the folder, then set the Union operation for each of the objects in the folder. This may seem redundant, but it is important for certain situations. In particular, the cube object is not a primitive in Renderman, so the Union operation is necessary to make it a solid object.