This group node performs a Constructive Solid Geometry (CSG) rendering. The children of this group define the solids that are involved in the CSG rendering. It is important to know that all children of this group are assumed to be solid (i.e., having a closed surface). Otherwise the results may be incorrect.

An expression defines the operations between these solids. Possible operations are: intersection (indicated as "." in the expression), union ("+"), and subtraction ("-"). A solid is identified by a letter (or a group of letters) in the expression and corresponds to the child number as follows: 'A' corresponds to child 0, 'B' corresponds to child 1, "AA" corresponds to child 26, "AZ" corresponds to child 51, and so forth. The expression is not case sensitive. If you decide for example to use "AA", you must have previously created children 0-25 (A through Z).

A letter can be followed by a number to indicate the convexity of the solid. The convexity of a solid is defined as the maximum numbers of pairs of front and back surfaces. For instance, the convexity of a cube is 1 whereas the convexity of a torus is 2. The convexity of a solid is assumed to be 1 if no digit follows the corresponding letter in the expression. The maximum allowable convexity value is 128.

The more complex the expression, the slower the performance. The same holds true for the convexity of an object - greater convexity implies slower performance.

For best performance, the graphics accelerator should perform stencil buffer operations in hardware.

Example of CSG Tree:

In this example, "myShape1" is 'A', "Separator1" is B, and "Separator2" is C. A CSG expression could be: "A . B - C + A" and corresponds to the following CSG tree:

The CSG tree defined by expression is assumed to be in normal form (normalized). A CSG tree is in normal form when all intersection and subtraction operators have a left subtree that contains no union operator and a right subtree that is simply a shape. For example, the tree shown above is in normal form.

A CSG tree can be converted to normal form by repeatedly applying the following set of production rules to the tree and then its subtrees:

A - ( B + C )	=>	( A - B ) - C
A . ( B + C )	=>	( A . B ) + ( A . C )
A - ( B . C )	=>	( A - B ) + ( A - C )
A . ( B . C )	=>	( A . B ) . C
A - ( B - C )	=>	( A - B ) + ( A . C )
A . ( B - C )	=>	( A . B ) - C
( A - B ) . C	=>	( A . C ) - B
( A + B ) - C	=>	( A - C ) + ( B - C )
( A + B ) . C	=>	( A . C ) + ( B . C )

Because there are two Buffer Region extensions, the value of this environment variable can be:

KTX_BUFFER_REGION: Uses the GL_KTX_buffer_region extension.

ARB_BUFFER_REGION: Uses the WGL_ARB_buffer_region extension.

FILE FORMAT/DEFAULT

CSGGroup {

expression	""
isShapesBefore	false
isActive	true
boundingBoxPruningEnabled	false

}

Reference