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**Constructive Solid Geometry**

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**Constructive Solid Geometry Methods**

Constructive Solid Geometry (CSG) performs solid modelling by generating a new object from two three dimensional objects using a set operation Valid set operations include Union Intersection Difference

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CSG Operations Primitives: Union: Intersection: Difference:

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**Constructive Solid Geometry Methods (cont…)**

Difference Intersection

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**Constructive Solid Geometry Methods (cont…)**

CSG usually starts with a small set of primitives such as blocks, pyramids, spheres and cones Two objects re initially created and combined using some set operation to create a new object This object can then be combined with another primitive to make another new object This process continues until modelling complete

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**Constructive Solid Geometry Methods (cont…)**

CSG models are usually represented as CSG trees CSG Object oper1 obj1 obj2 oper3 obj4 oper2 obj3

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**Constructive Solid Geometry Methods (cont…)**

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Ray-Casting Ray-casting is typically used to implement CSG operators when objects are described with boundary representations Ray casting is applied by determining the objects that are intersected by a set of parallel lines emanating from the xy plane along the z axis The xy plane is referred to as the firing plane

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Ray-Casting (cont…) Images taken from Hearn & Baker, “Computer Graphics with OpenGL” (2004)

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Ray-Casting (cont…) Surface intersections along each ray are calculated and these are sorted according to distance from the firing plane The surface limits for the composite object are then determined by the specified set operation

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Ray Casting Example Images taken from Hearn & Baker, “Computer Graphics with OpenGL” (2004)

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**CSG Data Structure typedef struct _csgtree } CSGTreePtr;**

Operator op; Quadric primitive; struct _csgtree *right; struct _csgtree *left; } CSGTreePtr; Each leaf node represents a primitive (usually a quadric primitive). ‘op’ can also be ‘leaf’ rather than a combination operator.

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Intersecting a Ray Pseudo code for ray-tree intersection

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Intersecting a Ray The result will be a sequence of intersection points along a ray, represented by the parametric values t0, t1, t2, …,tn where there are n intersections Each individual intersection with a solid will result (typically) in two values.

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Combine Function Shows how the result of the left and right solids combine together. Note that the set operations therefore occur at a 2D level.

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Examples www-2.cs.cmu.edu/afs/cs/misc/rayshade/ all_mach/omega/doc/Examples/jpg/csg.jpg ccvweb.csres.utexas.edu/ccv/projects/VisualEyes/ visualization/domainpara/algebraic2/parameterization/ gallery/Reconstruction/Csg/

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Summary Unified representation for quadric surfaces provides a simplicity and elegance for a wide class of shapes. CSG provides a simple methodology for combinations via set operations of solids CSG does not require quadrics, it can work other types of solid representation.

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