Clipping Circles

1
Clipping Circles

• Test against circles extent (bounding box)
• Test against each quadrants extent
• Optionally test octants
• Then analytically solve for arc and edge
  intersections
• Or
• If the circle is not large, do an extent test
• Use scissoring
• Clip each filled scan line

2
Clipping Ellipses

• Test against circles extent (bounding box)
• Test against each quadrants extent
• Then analytically solve for arc and edge
  intersections
• Or
• If the ellipse is not large, do an extent test
• Use scissoring
• Clip each filled scan line

3
Clipping Polygons

• Sutherland-Hodgman polygon clipping
• Uses a divide and conquer approach
• Can clip a concave or convex polygon against a
  convex clipping polygon
• Run several times, once for each edge

Clipping against an infinite edge
Vertices of clipped polygon
List of vertices
4
Clipping Polygons

• Vertex list v1, v2, v3, .vn
• Iterate from vn to v1 back to vn
• Return either 0, 1, or 2 vertices for each
  segment

5
Clipping Polygons

• Post processing maybe needed to remove
  connections between polygons on clipping region
  edges

6
Clipping Characters

• Characters can be defined as a series of curves
• Scalable, but complex to draw
• Bitmaps
• Not easily scalable, must save bitmaps at
  multiple resolutions
• Can be copied from a font cache by a copypixel
  operation

7
Clipping Characters

• Analytical clipping of characters would be too
  complex
• Characters make up a small number of pixels
• For both bitmap and stroked fonts
• use extent tests against string
• Use extent tests against each character
• Use scissoring
• For bitmap fonts, copypixel operations may give
  scissoring automatically