Polygon Filling Chemical Engineering Notes | EduRev

Chemical Engineering : Polygon Filling Chemical Engineering Notes | EduRev

 Page 1


2/3/2000
CS 4/57101 Lecture 6
1
Polygon Filling
? 2 parts to task
? which pixels to fill
? what to fill them with
? First consider filling unclipped primitives with solid color
? Which pixels to fill
? consider scan lines that intersect primitive
? fill in spans of pixels lying inside primitive
? exploit spatial coherence, span coherence, scan-line 
coherence, edge coherence
? Can write frame buffer one word at time rather than one 
bit
Page 2


2/3/2000
CS 4/57101 Lecture 6
1
Polygon Filling
? 2 parts to task
? which pixels to fill
? what to fill them with
? First consider filling unclipped primitives with solid color
? Which pixels to fill
? consider scan lines that intersect primitive
? fill in spans of pixels lying inside primitive
? exploit spatial coherence, span coherence, scan-line 
coherence, edge coherence
? Can write frame buffer one word at time rather than one 
bit
2/3/2000
CS 4/57101 Lecture 6
2
Line Segments Sharing Pixels
? Important not to write pixels twice
? could be set to background color if xor used
? could be double intensity (film writer)
? Is boundary part of an area defining primitive
? mathematically yes
? graphically more complicated
? Rule: left and bottom edges are part of primitive
? shared vertical edges belong to rightmost primitive
? Still problems
? bottom left point still drawn twice
? rectangles missing right and top edges !!
Page 3


2/3/2000
CS 4/57101 Lecture 6
1
Polygon Filling
? 2 parts to task
? which pixels to fill
? what to fill them with
? First consider filling unclipped primitives with solid color
? Which pixels to fill
? consider scan lines that intersect primitive
? fill in spans of pixels lying inside primitive
? exploit spatial coherence, span coherence, scan-line 
coherence, edge coherence
? Can write frame buffer one word at time rather than one 
bit
2/3/2000
CS 4/57101 Lecture 6
2
Line Segments Sharing Pixels
? Important not to write pixels twice
? could be set to background color if xor used
? could be double intensity (film writer)
? Is boundary part of an area defining primitive
? mathematically yes
? graphically more complicated
? Rule: left and bottom edges are part of primitive
? shared vertical edges belong to rightmost primitive
? Still problems
? bottom left point still drawn twice
? rectangles missing right and top edges !!
2/3/2000
CS 4/57101 Lecture 6
3
Polygon Types
? simple convex, simple concave, non-simple (self-
intersecting)
? want no holes, no intersections (line crossings)
? rectangles and triangles always simple convex
Page 4


2/3/2000
CS 4/57101 Lecture 6
1
Polygon Filling
? 2 parts to task
? which pixels to fill
? what to fill them with
? First consider filling unclipped primitives with solid color
? Which pixels to fill
? consider scan lines that intersect primitive
? fill in spans of pixels lying inside primitive
? exploit spatial coherence, span coherence, scan-line 
coherence, edge coherence
? Can write frame buffer one word at time rather than one 
bit
2/3/2000
CS 4/57101 Lecture 6
2
Line Segments Sharing Pixels
? Important not to write pixels twice
? could be set to background color if xor used
? could be double intensity (film writer)
? Is boundary part of an area defining primitive
? mathematically yes
? graphically more complicated
? Rule: left and bottom edges are part of primitive
? shared vertical edges belong to rightmost primitive
? Still problems
? bottom left point still drawn twice
? rectangles missing right and top edges !!
2/3/2000
CS 4/57101 Lecture 6
3
Polygon Types
? simple convex, simple concave, non-simple (self-
intersecting)
? want no holes, no intersections (line crossings)
? rectangles and triangles always simple convex
2/3/2000
CS 4/57101 Lecture 6
4
Convex, Concave, Degenerate
? Convex polygons are preferable to concave
? Polygon is convex if for any two points inside 
polygon, the line segment joining these two points is 
also inside. 
? Degeneracies
Page 5


2/3/2000
CS 4/57101 Lecture 6
1
Polygon Filling
? 2 parts to task
? which pixels to fill
? what to fill them with
? First consider filling unclipped primitives with solid color
? Which pixels to fill
? consider scan lines that intersect primitive
? fill in spans of pixels lying inside primitive
? exploit spatial coherence, span coherence, scan-line 
coherence, edge coherence
? Can write frame buffer one word at time rather than one 
bit
2/3/2000
CS 4/57101 Lecture 6
2
Line Segments Sharing Pixels
? Important not to write pixels twice
? could be set to background color if xor used
? could be double intensity (film writer)
? Is boundary part of an area defining primitive
? mathematically yes
? graphically more complicated
? Rule: left and bottom edges are part of primitive
? shared vertical edges belong to rightmost primitive
? Still problems
? bottom left point still drawn twice
? rectangles missing right and top edges !!
2/3/2000
CS 4/57101 Lecture 6
3
Polygon Types
? simple convex, simple concave, non-simple (self-
intersecting)
? want no holes, no intersections (line crossings)
? rectangles and triangles always simple convex
2/3/2000
CS 4/57101 Lecture 6
4
Convex, Concave, Degenerate
? Convex polygons are preferable to concave
? Polygon is convex if for any two points inside 
polygon, the line segment joining these two points is 
also inside. 
? Degeneracies
2/3/2000
CS 4/57101 Lecture 6
5
Polygon Representation
? Polygon Representation
? ordered list of vertices
? avoids redundant storage and computations
? associate other information with vertices
| colors, normals, textures
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