Computer Graphics

By :Abed Elrahman Al-Najjar ID : 120080075 Computer Graphics University of Palestine ITGD3107 Supervision:Assistant Professor Dr. Sana’a Wafa Al-Sayegh 2nd Semester 2008-2009

ITGD3107Computer Graphics Chapter 13 Visible-Surface Detection

Visible-Surface Detection Jehee Lee Seoul National University

Visible-Surface Detection Methods • Determine what is visible within a scene from a chosen viewing position • Two approaches • Object-space methods: Decide which object, as a whole, is visible • Image-space methods: The visibility is decided point-by-point • Most visible-surface algorithms use image-space methods • Sometimes, these methods are referred to as hidden-surface elimination

Approaches • Back-Face Removal • Depth Buffer • A-Buffer • Scanline • Depth Sorting • BSP Tree • Area Subdivision • Octree

Back-Face Removal (Culling) • Used to remove unseen polygons from convex, closed polyhedron • Does not completely solve hidden surface problem since one polyhedron may obscure another

Back-Face Removal (Culling) • Compute the equation of the plane for each polygon • A point (x,y,z) is behind a polygon surface if • Determine back-face • In projection coordinates, we need to consider only the z component of the normal vector N

Depth-Buffer (Z-Buffer) • Z-Buffer has memory corresponding to each pixel location • Usually, 16 to 20 bits/location.

Depth-Buffer (Z-Buffer) • Initialize • Each z-buffer location  Max z value • Each frame buffer location  background color • For each polygon: • Compute z(x,y), polygon depth at the pixel (x,y) • If z(x,y) < z-buffer value at pixel (x,y), then • z buffer(x,y)  z(x,y) • pixel(x,y)  color of polygon at (x,y)

Depth Calculation • Calculate the z-value on the plane • Incremental calculation

Depth-Buffer (Z-Buffer) • Advantages/Disadvantages • Lots of memory • Linear performance • Polygons may be processed in any order • Modifications needed to implement antialiasing, transparency, translucency effects • Commonly implemented in hardware  very fast

Depth-Buffer (Z-Buffer) Backface culling Z-buffer algorithm

Accumulation Buffer (A-Buffer) • An extension of the depth-buffer for dealing with anti-aliasing, area-averaging, transparency, and translucency • The depth-buffer method identifies only one visible surface at each pixel position • Cannot accumulate color values for more than one transparent and translucent surfaces • Even more memory intensive • Widely used for high quality rendering

Accumulation Buffer (A-Buffer) • Each position in the A-buffer has two fields • Depth field: Stores a depth value • Surface data field • RGB intensity components • Opacity parameter (percent of transparency) • Depth • Percent of area coverage • Surface identifier

Scan Line Method • Intersect each polygon with a particular scanline and solve hidden surface problem for just that scan line • Requires a depth buffer equal to only one scan line • Requires the entire scene data at the time of scan conversion • Maintain an active polygon and active edge list • Can implement antialiasing as part of the algorithm

Depth Sorting • We need a partial ordering (not a total ordering) of polygons • The ordering indicates which polygon obscures which polygon • Some polygons may not obscure each other • Simple cases

Depth Sorting • We make the following tests for each polygon that has a depth overlap with S • If any one of these tests is true, no reordering is necessary for S and the polygon being tested • Polygon S is completely behind the overlapping surface relative to the viewing position • The overlapping polygon is completely in front of S relative to the viewing position • The boundary-edge projections of the two polygons onto the view plane do not overlap

Depth Sorting • Example

Depth Sorting • Cyclically overlapping surfaces that alternately obscure one another • We can divide the surfaces to eliminate the cyclic overlaps

BSP Trees • Binary space partitioning is an efficient method for determining object visibility • Paint surfaces into the frame buffer from back to front • Particularly useful when the view reference point changes, but the objects are at fixed positions

BSP Tree Construction • Choose a polygon T and compute the equation of the plane it defines • Test all the vertices of all the other polygons to determine if they are in front of, behind, or in the same plane as T. • If the plane intersects a polygon, divide the polygon at the plane • Polygons are placed into a binary search three with T as the root • Call the procedure recursively on the left and right subtree

Area Subdivision • Image-space method taking advantage of area coherence in a scene • Recursively subdivide a square area into equal-sized quadrants if the area is too complex to analyze easily

Area Subdivision • Four possible relationships between polygon surfaces and a rectangular section of the viewing plane • Terminating criteria • Case 1: An area has no inside, overlapping, or surrounding surfaces (all surfaces are ourside the area) • Case 2: An area has only one inside, overlapping or surrounding surfaces • Case 3: An area has one surrounding surface that obscures all other surfaces within the area boundaries

Octrees • Visible-surface identification is accomplished by searching octree nodes in a front-to-back order

Computer Graphics