Scan Conversion

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Scan Conversion
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Scan Conversion

• Last step in the graphics pipeline
• Efficiency is a central issue
• Common primitives
• Lines (done last class)
• Polygons (fill polygons, today)
• Circles
• Hardware implementations preferable

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Filling PolygonsScan Line Algorithm
14
12
10
8
6
4
2
0
2
4
6
8
10
12
14
0
4
(No Transcript)
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Filling PolygonsSpecial Cases
Edge Table (ET) Entry
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Scan Line Polygon Fill Algorithm
N
N
14
N
D
12
N
EF
DE
F
10
-5/2
6/4
N
9
7
12
7
CD
N
8
N
12
13
0
E
6
C
FA
N
4
N
9
2
0
A
2
N
AB
BC
B
0
6/4
N
7
-5/2
3
7
5
2
4
6
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14
0
N
Scan Line
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AET

• AET at scanline y8
• AET at Scanline y 9

ET Record format  (Ymax, Xmin, 1/m) AET Records
format (Ymax, Xcurrent, 1/m)
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Scan Line Polygon FillThe Algorithm

• Set y to smallest y coordinate that has an entry
  in ET
• Initialize the active edge table (AET) to empty
• Repeat until the ET and AET are both empty
• Move from ET bucket at y to AET those edges whose
  ymin y. (entering edges)
• Remove from AET those edges for which y ymax.
• Sort AET on x
• Fill in desired pixels on scan line using
  even-odd parity from AET
• Increment y by one (next scan line)
• For each edge in the AET, update x by adding Dx/Dy

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What color to fill?
Recall Shading in OpenGL Smooth Shading
Requested by glShadeModel(GL_SMOOTH) Ligh
ting calculations will be done at each vertex
using the material properties, and the vectors v
and l calculated for that vertex. Bilinear
interpolation is used to determine color values
in the interior of the polygon.
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Bilinear Interpolation

• Cp ?
• Cq ?
• Ck ?
• Xa Ya, Xb, Yb, Xk, Yk
• Need to calculate Xp, Xq.
• Cp, Cq, Xp, Xq can all use incremental methods
• Rate of color change!!

C
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Using Incremental Methods

• Cx
• Cx1 Cx (Cp Cq)/(Xq-Xp)