1. CS 551 / 645: Introductory Computer Graphics
Clipping II: Clipping Polygons
David Luebke /26/2018

2. Administrivia
Assignment 2 due
David Luebke /26/2018

3. Recap: Clipping Lines Discard segments of lines outside viewport
Trivially accept lines with both endpoints inside all edges of the viewport
Trivially reject lines with both endpoints outside the same edge of the viewport
Otherwise, reduce to trivial cases by splitting into two segments
David Luebke /26/2018

4. Recap: Cohen-Sutherland Line Clipping
Divide viewplane into regions defined by viewport edges
Assign each region a 4-bit outcode:
1001
1000
1010
0001
0000
0010
0101
0100
0110
David Luebke /26/2018

5. Recap: Cohen-Sutherland Line Clipping
Set bits with simple tests
x > xmax y < ymin etc.
Assign an outcode to each vertex of line
If both outcodes = 0, trivial accept
bitwise AND vertex outcodes together
If result  0, trivial reject
David Luebke /26/2018

6. Recap: Cohen-Sutherland Line Clipping
If line cannot be trivially accepted or rejected, subdivide so that one or both segments can be discarded
Pick an edge that the line crosses
Intersect line with edge
Discard portion on wrong side of edge and assign outcode to new vertex
Apply trivial accept/reject tests and repeat if necessary
David Luebke /26/2018

7. Cohen-Sutherland Line Clipping
Outcode tests and line-edge intersects are quite fast (how fast?)
But some lines require multiple iterations (see F&vD figure 3.40)
Fundamentally more efficient algorithms:
Cyrus-Beck uses parametric lines
Liang-Barsky optimizes this for upright volumes
F&vD for more details
David Luebke /26/2018

8. Clipping Polygons
Clipping polygons is more complex than clipping the individual lines
Input: polygon
Output: polygon, or nothing
When can we trivially accept/reject a polygon as opposed to the line segments that make up the polygon?
David Luebke /26/2018

9. How many sides can a clipped triangle have?
Why Is Clipping Hard?
What happens to a triangle during clipping?
Possible outcomes:
triangletriangle
trianglequad
triangle5-gon
How many sides can a clipped triangle have?
David Luebke /26/2018

10. Why Is Clipping Hard?
A really tough case:
David Luebke /26/2018

11. Why Is Clipping Hard? A really tough case:
concave polygonmultiple polygons
David Luebke /26/2018

12. Sutherland-Hodgman Clipping
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
David Luebke /26/2018

13. Sutherland-Hodgman Clipping
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
After doing all planes, the polygon is fully clipped
David Luebke /26/2018

14. Sutherland-Hodgman Clipping
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
After doing all planes, the polygon is fully clipped
David Luebke /26/2018

15. Sutherland-Hodgman Clipping
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
After doing all planes, the polygon is fully clipped
David Luebke /26/2018

16. Sutherland-Hodgman Clipping
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
After doing all planes, the polygon is fully clipped
David Luebke /26/2018

17. Sutherland-Hodgman Clipping
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
After doing all planes, the polygon is fully clipped
David Luebke /26/2018

18. Sutherland-Hodgman Clipping
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
After doing all planes, the polygon is fully clipped
David Luebke /26/2018

19. Sutherland-Hodgman Clipping
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
After doing all planes, the polygon is fully clipped
David Luebke /26/2018

20. Sutherland-Hodgman Clipping
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
After doing all planes, the polygon is fully clipped
David Luebke /26/2018

21. Sutherland-Hodgman Clipping: The Algorithm
Basic idea:
Consider each edge of the viewport individually
Clip the polygon against the edge equation
After doing all planes, the polygon is fully clipped
Will this work for non-rectangular clip regions?
What would 3-D clipping involve?
David Luebke /26/2018

22. Sutherland-Hodgman Clipping
Input/output for algorithm:
Input: list of polygon vertices in order
Output: list of clipped poygon vertices consisting of old vertices (maybe) and new vertices (maybe)
Note: this is exactly what we expect from the clipping operation against each edge
David Luebke /26/2018

23. Sutherland-Hodgman Clipping
Sutherland-Hodgman basic routine:
Go around polygon one vertex at a time
Current vertex has position p
Previous vertex had position s, and it has been added to the output if appropriate
David Luebke /26/2018

24. Sutherland-Hodgman Clipping
Edge from s to p takes one of four cases:
(Purple line can be a line or a plane)
inside
outside s p
p output
inside
outside s p
i output
inside
outside s p
no output
inside
outside s p
i output p output
David Luebke /26/2018

25. Sutherland-Hodgman Clipping
Four cases:
s inside plane and p inside plane
Add p to output
Note: s has already been added
s inside plane and p outside plane
Find intersection point i
Add i to output
s outside plane and p outside plane
Add nothing
s outside plane and p inside plane
Add i to output, followed by p
David Luebke /26/2018

26. Point-to-Plane test
A very general test to determine if a point p is “inside” a plane P, defined by q and n:
(p - q) • n < 0: p inside P
(p - q) • n = 0: p on P
(p - q) • n > 0: p outside P P n p q q q n n p p P P
David Luebke /26/2018

27. Point-to-Plane Test Dot product is relatively expensive
3 multiplies
5 additions
1 comparison (to 0, in this case)
Think about how you might optimize or special-case this
David Luebke /26/2018

28. Finding Line-Plane Intersections
Use parametric definition of edge:
E(t) = s + t(p - s)
If t = 0 then E(t) = s
If t = 1 then E(t) = p
Otherwise, E(t) is part way from s to p
David Luebke /26/2018

29. Finding Line-Plane Intersections
Edge intersects plane P where E(t) is on P
q is a point on P
n is normal to P
(E(t) - q) • n = 0
t = [(q - s) • n] / [(p - s) • n]
The intersection point i = E(t) for this value of t
David Luebke /26/2018

30. Line-Plane Intersections
Note that the length of n doesn’t affect result:
t = [(q - s) • n] / [(p - s) • n]
Again, lots of opportunity for optimization
David Luebke /26/2018