Visiblity: Culling and Clipping

1. Visiblity: Culling and Clipping Computer GraphicsCOMP 770 (236)Spring 2009 January 21 & 26: 2009

2. Today’s topics • Lines and planes • Culling • View frustum culling • Back-face culling • Occlusion culling • Hierarchical culling • Clipping

3. What are culling and clipping? • Culling • Throws away entire objects and primitives that cannot possibly be visible • An important rendering optimization (esp. for large models) • Clipping • “Clips off” the visible portion of a primitive • Simplifies rasterization • Used to create “cut-away” views of a model

4. Power plant model 13 M triangles 1.7 M triangles - gutted version show here with no internal pipes Culling example

5. Culling example Full model1.7 Mtris View frustum culling1.4 Mtris Occulsion culling89 Ktris

6.  l  p = 0 Lines and planes • Implicit equation for line (plane): • If is normalized then d gives the distance of the line (plane) from the origin along • Transforming points and lines with transformation M: points: lines: d (0,0)

7.  l  p = 0 Lines and planes • Lines (planes) partition 2D (3D)space: • positive and negative half spaces • The union of negative half-spacesdefines a convex region d (0,0)

8. Testing objects for containment + + + + - + - - - + + + - - - Outside Straddling Inside

9. r r Indeterminate Inside Conservative testing • Use cheap, conservative bounds for trivial cases • Can use more accurate, more expensive tests for ambiguous cases if needed r Outside

10. 1 -1 1 -1 View frustum culling • Test objects against planes defining view frustum • How do you compute them? • Other planes can be computed similarly column 1- column 3

11. Hierachical culling • Culling needs to be cheap! • Bounding volume hierarchies accelerate culling by trivially rejecting/accepting entire sub-trees at a time • Simple algorithm: while( node is indeterminate ) recurse on children not visited Indeterminate visited Inside Indeterminate Inside Inside Indeterminate Outside

12. Back-face culling • Special case of occlusion - convex self-occlusion • for closed objects (has well-defined inside and outside) some parts of the surface must be blocked by other parts of the surface • Specifically, the backside of the object is not visible

13. Manifold • Back-face culling can be applied to any orientable two-manifold. • Orientable two-manifolds have the following properties. • All points on the surface are locally like a plane. No holes, cracks, or self-intersections. • Boundary partitions 3D space into interior and exterior regions • In our case, manifolds will be composite objects made of many primitives, generally triangles. • Back-face culling eliminates a subset of these primitives. • Assumes that you are outside of all objects.

14. Face plane test • Compute the plane for the face: • Cull if eye point in the negative half-space

15. Winding order test • Back-faces have a clockwise vertex ordering when viewed from outside. • Typically used by graphics hardware during triangle setup

16. Back-face culling in OpenGL • Can cull front faces or back faces • Back-face can sometimes double performance if (cull): glFrontFace(GL_CCW) # define winding order glEnable(GL_CULL_FACE) # enable Culling glCullFace(GL_BACK) # which faces to cull else: glDisable(GL_CULL_FACE) front-face culling

17. Occlusion culling with occlusion queries • Render objects visible in previous frame (occlusion representation)

18. Occlusion culling with occlusion queries • Turn off color and depth writes. • Render object bounding boxes with occlusion queries. • An occlusion query returns the number of visible pixels newly visible

19. Occlusion culling with occlusion queries • Re-enable color writes • Render newly visible objects

20. Clipping a line segment against a line • First check endpoints against the plane. If they are on the same side, no clipping is needed • Interpolate to get new point • Vertex attributes interpolated the same way

21. Traverse edges Keep edges that are entirely inside Create new point when we exit Throw away edges entirely outside Create new point and new edge when we enter Clipping a polygon against a line

22. Clipping against a convex region • Sutherland-Hodgman • Just clip against one edge ata time

23. Homogeneous clipping • NDC simplify view frustum clipping • Clip after applying projection matrix, but before the divide by w • clip coordinates • Easy in/out test and interpolation. Other coordinates treated exactly the same way Add extra coordinateso we can use our line equation w x -1 1

24. Culling and clipping in therendering pipeline Culling done by application Eye coordinates. View frustum clipping andback-face culling can be done here View frustum clipping on clip coordinates.Back-face culling done in setup phaseof rasterization.