Scan Conversion

1. Scan Conversion

2. 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

3. Filling Polygons:Scan Line Algorithm 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 14

6. 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 14 Filling Polygons:Special Cases Edge Table (ET) Entry

7. 11 10 9 8 7 6 5 4 3 2 1 0 Scan Line Polygon Fill Algorithm N N 14 N D 12 N EF DE F 10 -5/2 6/4 N 9 7 7 12 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 -5/2 7 3 7 5 0 2 4 6 8 10 12 14 N Scan Line

8. AET • AET at scanline y=8 • AET at Scanline y =9 ET Record format : (Ymax, Xmin, 1/m) AET Records format: (Ymax, Xcurrent, 1/m)

9. Scan Line Polygon Fill:The 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

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

11. 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

12. Using Incremental Methods • Cx • Cx+1 = Cx + (Cp –Cq)/(Xq-Xp)