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8/8/2019 Cg Unit 7 Full
http://slidepdf.com/reader/full/cg-unit-7-full 1/24
object-space methods and image-space methods.
An object-space method compam objects and parts of objects to each
other within the scene definition to determine which surfaces, as a whole, we
should label as visible.
In an image-space algorithm, visibility is decided point by
point at each pixel position on the projection plane.
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BACK-FACE DETECTION
A point (x , y,z) is "inside" a polygon surface
if Ax+By+Cz+D < 0
If V.N > 0 then this polygon is back faced
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BACK-FACE DETECTION
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DEPTH-BUFFER METHOD
(Z-Buffer)
With object descriptions converted to projection coordinates, each (x, y, z )
position on a polygon surface corresponds to the orthographic projection point
(x, y) on the view plane.
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DEPTH-BUFFER METHOD
Plane Value of Z
Back plane 0
Front plane Zmax
Types of Buffers
Depth Buffer
Refresh Buffer
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Algorithm
1. Initialize the depth buffer and refresh buffer so that for all buffer positions(x , y),
depth(x,y)=0 and refresh(x,y)=Ibground
2. For each position on each polygon surface, compare depth values to previously stored values in the depth buffer to determine visibility.
- Calculate the depth t for each (x , y) position on the polygon.
- If z > depth(x, y), then set
depth(x,y)=Z and refresh(x,y)=Isurface(x,y).
where Ibground is the value for the background intensity, and Isurface(x,y) isthe projected intensity value for the surface at pixel position (x,y).
After all surfaces have been processed, the depth buffer contains
depth values for the visible surfaces and the refresh buffer contains
the corresponding intensity values for those surfaces.
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Depth values for a surface position (x , y)
are calculated from the plane equation for
each surface:
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A-BUFFER METHOD
An extension of the ideas in the depth-buffer method is the A-buffer method.
it deals only with opaque surfaces and cannot accumulate intensity values
for more than one surface.
Each position in the A-buffer has two fields:
1.depth field - stores a positive or negative real number 2. intensity field - stores surface-intensity information or a pointer value.
Foreground
Transprent
surface
Backgroundopaque
surface
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Data for each surface in the linked list includes
RGB intensity components opacity parameter (percent of transparency)
depth
percent of area coverage
surface identifier
other surface-rendering parameters
pointer to next surface
A-BUFFER METHOD
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SC AN-LINE METHOD
It is an image-space technique.
It is an extended form of the scan-line fill alg.
Visible surface can be calculated by finding
the intersection between scan line and polygon surface.
Two tables are maintained ± Edge table
± Polygon table
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For positions along this scan line between edges AB and BC, onlythe flag for surface Sl is on.
Similarly, between edges EH and FG, only the flag for surface S2 ison.
For scan lines 2 and 3 the active edge list contains edges AD, EH ,BC , and FG.
But between edges EH and BC , the flags for both surfaces are on.
SC AN-LINE METHOD
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DEPTH-SORTING METHOD
Using both image-space and object-space
operations, the depth-sorting method
performs the following basic functions:
1. Surfaces are sorted in order of decreasing
depth.
2. Surfaces are scan converted in order,
starting with the surface of greatest depth.
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DEPTH-SORTING METHOD
Triangle Behind counter List of triangles in f r ont of it
1 2 5
2 0 4,3,1
3 2 1,5
4 1 3
5 2
6 0
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DEPTH-SORTING METHODTriangle Fr ont List Behind counter output New Behind
counter
1 5 2 1
2 4,3,1 0 -1
3 1,5 2 1
4 3 1 0
5 2
2
6 0 -1
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DEPTH-SORTING METHODTriangle Fr ont List Behind counter output New Behind
counter
1 5 1 1
2 4,3,1 -1 -1
3 1,5 1 0
4 3 0 -1
5
2 2
6 -1 -1
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DEPTH-SORTING METHODTriangle Fr ont List Behind counter output New Behind
counter
1 5 1 0
2 4,3,1 -1 -1
3 1,5 0 -1
4 3 -1 -1
5
2 1
6 -1 -1
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DEPTH-SORTING METHODTriangle Fr ont List Behind counter output New Behind
counter
1 5 0 -1
2 4,3,1 -1 -1
3 1,5 -1 -1
4 3 -1 -1
5
1 0
6 -1 -1
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DEPTH-SORTING METHODTriangle Fr ont List Behind counter output New Behind
counter
1 5 -1 -1
2 4,3,1 -1 -1
3 1,5 -1 -1
4 3 -1 -1
5 0 -1
6 -1 -1
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BSP-TREE METHOD
binary space-partitioning (BSP)
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ARE A-SUBDIVISION b1ETHOD
No further sub division is needed if
1. All surfaces are outside surface with
respect to the area.
2. Only one inside , overlapping, or surrounding surface is in the area.
3. A surrounding surface obscures all
other surfaces within the area
boundaries.
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Surr ounding sur f ace-One that
completely encloses the area.
Overlapping sur f ace-One that is partlyinside and partly outside the area.
Inside sur f ace-One that is completely
inside the area.
Outside sur f ace-One that is completely
outside the area.
ARE A-SUBDIVISION METHOD
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ARE A-SUBDIVISION METHOD
Surr ounding sur f ace
Overlapping sur f ace
Outside sur f ace
inside sur f ace
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OCTREE METHODS
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RAY-CASTING METHOD