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CS-184: Computer GraphicsLecture #10: Clipping and
Hidden Surfaces !!
Prof. James O’Brien University of California, Berkeley !!
V2013-F-10-1.0
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Today
• Clipping • Clipping to view volume • Clipping arbitrary polygons
• Hidden Surface Removal • Z-Buffer • BSP Trees • Others
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Clipping• Stuff outside view volume should not be drawn
• Too close: obscures view
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Clipping• Stuff outside view volume should not be drawn
• Too close: obscures view • Too far :
• Complexity • Z-buffer problems
• Too high/low/right/left: • Memory errors • Broken algorithms • Complexity
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Clipping Line to Line/Plane
a
bn̂
Line segment to be clipped
Line/plane that clips it r
x(t) = a+ t(b�a)
n̂ ·x� n̂ · r= 0
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Clipping Line to Line/Plane
a
bn̂
Line segment to be clipped
Line/plane that clips it
x(t) = a+ t(b�a)
n̂ ·x� f = 0
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Clipping Line to Line/Plane
a
bn̂
Line segment to be clipped
Line/plane that clips it
x(t) = a+ t(b�a)
n̂ ·x� f = 0
}
n̂ · (a+ t(b�a))� f = 0
n̂ ·a+ t(n̂ · (b�a))� f = 0
t =f � n̂ ·an̂ ·d
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Clipping Line to Line/Plane
a
bn̂
t =f � n̂ ·an̂ ·d
• Segment may be on one side
• Lines may be parallel
t 62 [0 . . .1]
n̂ ·d= 0
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Clipping Line to Line/Plane
a
bn̂
t =f � n̂ ·an̂ ·d
• Segment may be on one side
• Lines may be parallel
t 62 [0 . . .1]
n̂ ·d= 0|n̂ ·d| ε (Recall comments about numerical issues)
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Triangle Clip/Split
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a
b
c
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+
-a
b
c
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+
-a
b
c
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+
-
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Triangle Clip/Split
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a
b
c
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+
-a
b
c
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+
-a
b
c
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+
-
Double vertices if you want separation...
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Polygon Clip to Convex Domain
• Convex domain defined by collection of planes (or lines or hyper-planes)
• Planes have outward pointing normals • Clip against each plane in turn • Check for early/trivial rejection
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Polygon Clipping• Find the part of a polygon inside the clip window?
Before Clipping
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Polygon Clipping• Find the part of a polygon inside the clip window?
After Clipping
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• Clip to each window boundary one at a time
Sutherland-Hodgman Clipping 12
• Clip to each window boundary one at a time
Sutherland-Hodgman Clipping 13
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• Clip to each window boundary one at a time
Sutherland-Hodgman Clipping 14
Sutherland-Hodgman Clipping• Clip to each window boundary one at a time
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Sutherland-Hodgman Clipping• Clip to each window boundary one at a time
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Polygon Clip to Convex Domain
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Note double edges.
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Polygon Clip to Convex Domain
Inside Outside
s
p
Output p
Inside Outside
sp
Output i
Inside Outside
s
p
No output
Inside Outside
sp
Output i and p
i
i
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Polygon Clip to Convex Domain
• Sutherland-Hodgman algorithm • Basically edge walking
• Clipping done often... should be efficient • Liang-Barsky parametric space algorithm • See text for clipping in 4D homogenized coordinates
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General Polygon Clipping
A B A�B
B�A
A\B
A[B
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• Weiler Algorithm • Double edges
General Polygon Clipping 21
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Hidden Surface Removal• True 3D to 2D projection would put every thing
overlapping into the view plane. • We need to determine what’s in front and display only
that.
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Z-Buffers• Add extra depth channel to image • Write Z values when writing pixels • Test Z values before writing
Images from Okan Arikan
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Z-Buffers
• Benefits • Easy to implement • Works for most any geometric primitive • Parallel operation in hardware
• Limitations • Quantization and aliasing artifacts • Overfill • Transparency does not work well
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Z-Buffers• Transparency requires partial sorting:
Partiallytransparent
Opaque
Opaque 1st
2nd
3rdFront
Good
Partiallytransparent
Opaque
Opaque 2nd
3rd
1st
Not Good
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Z-BuffersRecall depth-value distortions. !
It’s a feature... More resolution near viewer Best use of limited precision
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A-Buffers
• Store sorted list of “fragments” at each pixel • Draw all opaque stuff first then transparent • Stuff behind full opacity gets ignored !
• Nice for antialiasing...
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Scan-line Algorithm• Assume polygons don’t intersect • Each time an edge is crossed determine who’s on top
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Painter’s Algorithm• Sort Polygons Front-to-Back
• Draw in order • Back-to-Front works also, but wasteful
• How to sort quickly? • Intersecting polygons? • Cycles?
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BSP-Trees
• Binary Space Partition Trees • Split space along planes • Allows fast queries of some spatial relations
• Draw Front-to-Back • Draw same-side polygons first • Draw root node polygon (if any) • Draw other-side polygons last
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