Liang–Barsky algorithm

In computer graphics, the Liang–Barsky algorithm (named after You-Dong Liang and Brian A. Barsky) is a line clipping algorithm. The Liang–Barsky algorithm uses the parametric equation of a line and inequalities describing the range of the clipping window to determine the intersections between the line and the clip window. With these intersections it knows which portion of the line should be drawn. This algorithm is significantly more efficient than Cohen–Sutherland.The idea of the Liang-Barsky clipping algorithm is to do as much testing as possible before computing line intersections.

Consider first the usual parametric form of a straight line:

x=x_{0}+u(x_{1}-x_{0})=x_{0}+t\Delta x\,\!

y=y_{0}+u(y_{1}-y_{0})=y_{0}+t\Delta y\,\!

A point is in the clip window, if

x_{\text{min}}\leq x_{0}+t\Delta x\leq x_{\text{max}}\,\!

and

y_{\text{min}}\leq y_{0}+t\Delta y\leq y_{\text{max}}\,\!

which can be expressed as the 4 inequalities

tp_{i}\leq q_{i},\quad i=1,2,3,4\,\!

where

p_{1}=-\Delta x,q_{1}=x_{0}-x_{{{\text{min}}}}\,\!

(left)

p_{2}=\Delta x,q_{2}=x_{{{\text{max}}}}-x_{0}\,\!

(right)

p_{3}=-\Delta y,q_{3}=y_{0}-y_{{\text{min}}}\,\!

(bottom)

p_{4}=\Delta y,q_{4}=y_{{\text{max}}}-y_{0}\,\!

(top)

To compute the final line segment:

A line parallel to a clipping window edge has $p_{i}=0$ for that boundary.
If for that $i$ , $q_{i}<0$ , the line is completely outside and can be eliminated.
When $p_{i}<0$ the line proceeds outside to inside the clip window and when $p_{i}>0$ , the line proceeds inside to outside.
For nonzero $p_{k}$ , $u={\frac {q_{i}}{p_{i}}}$ gives the intersection point.
For each line, calculate $u_{1}$ and $u_{2}$ . For $u_{1}$ , look at boundaries for which $p_{i}<0$ (i.e. outside to inside). Take $u_{1}$ to be the largest among $\left\{0,{\frac {q_{i}}{p_{i}}}\right\}$ . For $u_{2}$ , look at boundaries for which $p_{i}>0$ (i.e. inside to outside). Take $u_{2}$ to be the minimum of $\left\{1,{\frac {q_{i}}{p_{i}}}\right\}$ . If $u_{1}>u_{2}$ , the line is outside and therefore rejected.

References

Liang, Y.D., and Barsky, B., "A New Concept and Method for Line Clipping", ACM Transactions on Graphics, 3(1):1-22, January 1984.
Liang, Y.D., B.A., Barsky, and M. Slater, Some Improvements to a Parametric Line Clipping Algorithm, CSD-92-688, Computer Science Division, University of California, Berkeley, 1992.
James D. Foley. Computer graphics: principles and practice. Addison-Wesley Professional, 1996. p. 117.

External links

http://hinjang.com/articles/04.html#eight

This article is issued from Wikipedia - version of the 11/9/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.

Liang–Barsky algorithm

See also

References

External links