Presentation on Line clipping algorithm

1. Cohen-Sutherland Line
Clipping Algorithm
Presenting By: Aamir Sohail
Presenting to: Sir M.Arshad
Class: Computer Graphics
Department of Computer Science
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2. Introduction
When drawing a 2D line on screen, it might happen that
one or both of the endpoints are outside the screen while
a part of the line should still be visible.
In that case, an efficient algorithm is needed to find two
new endpoints that are on the edges on the screen, so
that the part of the line that's visible can now be drawn.
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3. Introduction cont…
This way, all those points of the line outside the screen
are clipped away and you don't need to waste any
execution time on them.
A good clipping algorithm is the Cohen-Sutherland line
clipping algorithm for this solution.
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4. Cohen Sutherland Clipping Algorithm
When drawing a 2D line, if one endpoint of the line is
outside the screen, and the other inside, you have to
clip the line so that only the part of it that's inside the
screen remains.
Even if both endpoints are outside the screen, it's still
possible that a part of the line should be visible.
The clipping algorithm needs to find new endpoints of
the lines, that are inside or on the edges of the screen.
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5. Cases
Here are a few cases, where the black rectangle
represents the screen, in red are the old endpoints,
and in blue the ones after clipping:
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6. Cont…
Case A: Both end-points are inside the screen, so no
clipping needed.
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7. Cont…
Case B: One end-point outside the screen, that one had to
be clipped.
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8. Cont…
Case C: both endpoint are outside the screen, and no part of
the line is visible, don't draw it at all.
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9. Cont…
Case D: both endpoint are outside the screen, and a part of
the line is visible, clip both endpoints and draw it.
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10. Cohen Sutherland Clipping Algorithm
Now we will learn what is Cohen Sutherland Clipping Algorithm
and how it works.
This algorithm clips a line to the clipping rectangle. It
concerns itself with performing the simple cases quickly.
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11. Cont…
In this algorithm it divides lines & edges into 2 cases.
1) Trivially Accept and
2) Trivially Reject.
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12. Conditions of Trivially Accept
Xmin ≤ X ≤ Xmax
Ymin ≤ Y ≤ Ymax
Lines fulfill this conditions then we will
mark those lines as trivially accept.
Ymax
Ymin
Xmin Xmax
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13. Conditions of Trivially Reject
 X0 < Xmin & X1 < Xmin or
Y0 < Ymin & Y1 < Ymin
X0 > Xmax & X1 > Xmax or
Y0 > Ymax & Y1 > Ymax
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14. Question Arrives
We must have a question now??
A
B
Then we will move forward for solve this ……..14Aamir Sohail

15. Cont…
The algorithm divides the 2D space in 9
regions:
This is also known as ABRL CODE
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16. ABRL Cont…
 The center region is the screen or Window Position (0000).
 If the region is above the screen, the first bit is 1.
 If the region is below the screen, the second bit is 1.
 If the region is to the right of the screen, the third bit is 1.
 If the region is to the left of the screen, the fourth bit is
1.
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17. A (0100) B (0010)
AND Operation
Then get the new point C (0000)
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18. C (0000) B (0010)
AND Operation
Then get the new point D (0000)
Then we have the final line after clipping is CD 18Aamir Sohail

19. Handling Similar Situations
If similar problems arrive then we have to clip
those according to mentioned method.
Some examples of similar situations
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20. I hope there will be no
question in your mind…
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21. References
https://en.wikipedia.org/wiki/Line_clipping
https://en.wikipedia.org/wiki/Cohen%E2%80%93Sutherland_algorithm
http://www.cc.gatech.edu/grads/h/Hao-wei.Hsieh/Haowei.Hsieh/mm.html
https://www.cs.helsinki.fi/group/goa/viewing/leikkaus/lineClip.html
Pptx files and some video lectures.
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