Visible Surface Detect

8/2/2019 Visible Surface Detect

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Visible-Surface Detection

Jehee Lee

Seoul National University


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Visible-Surface Detection Methods

Determine what is visible within a scene from achosen viewing position

Two approaches Object-spacemethods: Decide which object, as a

whole, is visible Image-spacemethods: 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


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Approaches

Back-Face Removal Depth Buffer

A-Buffer

Scanline Depth Sorting

BSP Tree

Area Subdivision Octree

Raycasting


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Back-Face Removal (Culling)

Used to remove unseen polygons from convex, closedpolyhedron

Does not completely solve hidden surface problem sinceone polyhedron may obscure another


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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 zcomponent of the normal vector N

0 DCzByAx

0NVview


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Depth-Buffer (Z-Buffer)

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


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Advantages/Disadvantages Lots of memory (not anymore)

Linear performance

Polygons may be processed in any order

Modifications needed to implement antialiasing,transparency, translucency effects

Commonly implemented in hardware very fast


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Backface culling Z-buffer algorithm


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Accumulation Buffer (A-Buffer)

An extension of the depth-buffer for dealing withanti-aliasing, area-averaging, transparency, andtranslucency

The depth-buffer method identifies only onevisible surface at each pixel position

Cannot accumulate color values for more than onetransparent and translucent surfaces

Even more memory intensive Widely used for high quality rendering


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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


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Scan Line Method

Intersect each polygon with a particular scanline andsolve 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


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Depth Sorting

Painters algorithm Draw polygons as an oil painters might do

Sort polygons by depth and draw them fromback to front

Depth sorting is NOT simple


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Depth Sorting

We need a partial ordering (not a total ordering)of polygons

The ordering indicates which polygon obscures whichpolygon

Some polygons may not obscure each other

Simple cases


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Depth Sorting

We make the following tests for each polygon that has adepth overlap with S

If any one of these tests is true, no reordering isnecessary for S and the polygon being tested

Polygon S is completely behind the overlapping surface relativeto the viewing position

The overlapping polygon is completely in front of S relative to theviewing position

The boundary-edge projections of the two polygons onto the

view plane do not overlap


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Depth Sorting

Example


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Depth Sorting

Cyclically overlapping surfaces that alternatelyobscure one another

We can divide the surfaces to eliminate thecyclic overlaps


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BSP Trees

Binary space partitioning is an efficient methodfor determining object visibility

Paint surfaces into the frame buffer from back tofront

Particularly useful when the view reference point

changes, but the objects are at fixed positions


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BSP Tree Construction

1. Choose a polygon T and compute the equation of the plane itdefines

2. Test all the vertices of all the other polygons to determine if theyare in front of, behind, or in the same plane as T.

3. If the plane intersects a polygon, divide the polygon at the plane

4. Polygons are placed into a binary search three with T as the root5. Call the procedure recursively on the left and right subtree


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Traversing BSP Trees

EYE 1

+X -X

C

B

A

D

E1

+ZF2

E2F1

EYE 2

A

C

F1 D

E2 F2

B

E1


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BSP Trees

BST tree construction requires a number ofpolygons to be fractured

It is possible for the number of triangles toincrease exponentially but, in practice, it is foundthat the increase may be as small as two fold

A heuristic to help minimize the number offractures is to enter the polygons into the tree inorder from largest to smallest


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Area Subdivision

Image-space method taking advantage of areacoherence in a scene

Recursively subdivide a square area into equal-sized quadrants if the area is too complex toanalyze easily


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Area Subdivision

Four possible relationships between polygon surfaces anda rectangular section of the viewing plane

Terminating criteria

Case 1: An area has no inside, overlapping, or surroundingsurfaces (all surfaces are ourside the area)

Case 2: An area has only one inside, overlapping or surroundingsurfaces

Case 3: An area has one surrounding surface that obscures allother surfaces within the area boundaries


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Octrees

Visible-surface identification is accomplished bysearching octree nodes in a front-to-back order


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Ray Casting

We consider the line of sight fromthe a pixel position through thescene

Useful for volume data

Ray casting is a special case ofray tracing that we will study later


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Ray Casting Examples

Documents

Visible Surface Detect