More Drawing Tools

1
More Drawing Tools

• ( Clipping)
• www.cis.syr.edu/mohan/gra5.ppt

2
Clipping

• To eliminate those parts of the object that lie
  outside the region being drawn
• OpenGL does this automatically
• Issues that arise in clipping algorithms occur in
  others also
• Object clipping is generally treated as polygon
  clipping, based on line clipping

3
Cohen-Sutherland Clipper

• A routine called clipSegment that performs
  clipping on a line segment and returns 0 iff the
  entire line segment lies outside the clipping
  window
• Main idea fast computations based on comparing
  with max/min x, y values of window

4
First step trivial accept/reject

• Compare endpoints to determine if they are in/out
  of the rectangle
• Generate a 4-bit codeword for each endpoint,
  whose elements are T(1) if P.xltW.l, P.ygtW.t,
  P.xgtW.r, P.yltW.y, respectively (is the point P to
  the left/above/right/below the window W?)

5
Trivial accept/reject cases

• If codewords for both endpoints are FFFF, then
  the line segment lies entirely inside the window,
  no work needs to be done.
• If both codewords have a T in the same position,
  both points are to the left/above/right/below the
  window, no work needs to be done.
• Otherwise, successively chop the given line
  segment using the window boundary lines,
  eliminating clipped out part of the segment.

6
Computing the intersection

• Given end points p1, p2, the intersection of the
  line segment with right window border has
  x-coordinate the same as W.r, and y-coordinate
  obtained by similarity of triangles (Fig.3.22)
• (p1.y-p2.y)/(p1.x-p2.x) (A.y-p2.y)/(W.r-p2.x)

7
Procedure clipSegment()

• Cf. Fig.3.23