1. Visible-Surface Detection(identification)
&
Computer Graphics
CS2401
Visible-Surface Detection(identification) BY
N.SATHISH KUMAR AP
CSE

2. Visible-Surface Detection(identification)
CS2401- Computer Graphics
UNIT-2 LAST TOPIC
Visible-Surface Detection(identification)

3. Visible-Surface Detection

4. 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

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

6. 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

7. 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

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

9. 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)

10. Depth Calculation Calculate the z-value on the plane
Incremental calculation

11. 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

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

13. 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

14. 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

15. 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

16. 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

17. 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

18. Depth Sorting
Example

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

20. 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

21. 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

22. 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

23. 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

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