Lecture Computer graphics: Lecture 21 - Fasih ur Rehman

Computer Graphics
Lecture 21
Fasih ur Rehman


Last Class
•

Clipping Algorithms


Today’s Agenda
•

Clipping Algorithms


Cohen – Sutherland Line
Clipping


•

Cohen – Sutherland Line
Clipping
The view window
is divided into regions as
shown in the figure

•

Each region is assigned a 4-bit outcode


•

Cohen – Sutherland Line
Most significant bit (MSB)
called bit 1 indicates y-value of
Clipping
points are above ymax

•

Bit 2 indicates y-value of points are below ymin

•

Bit 3 indicates x-value of points are to the right of Xmax

•

Bit 4 indicates x-value of points are to the left of Xmin


•

Cohen – Sutherland Line
Clipping
For each line segment
–

Assign an outcode to each vertex

–

If both outcodes = 0, (by bit – wise OR
operation)
•

–

trivial accept (Line AB of the Figure)

Else
•

AND (bitwise) vertex outcodes

•

if result

0, trivial reject (e. g. EF)


•

Cohen – Sutherland Line
If line cannot beClipping
trivially accepted or

rejected, subdivide the line into the
segments either or both of which can be
rejected
–

Start with an edge of view window that the line
intersects

–

Find intersection of line with the edge

–

Reject the segment on wrong side of edge
(outside the view window) and assign new
outcode to new vertex

–

Apply trivial accept/reject tests


•

Cohen – Sutherland Line
If line can neither Clipping
be trivially accepted nor

rejected, subdivide the line so that o both or either

of the two segments can be discarded
•

Select an edge that the line crosses
–

Keep check order the same order each time


To find intersection point


Parametric Form of Line


Summary
•

Clipping


References
•

•

Fundamentals of Computer Graphics Third
Edition by Peter Shirley and Steve
Marschner
Interactive Computer Graphics, A Topdown Approach with OpenGL (Sixth

Edition) by Edward Angel.