Liang–Barsky algorithm: Difference between revisions

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{{Short description|Line-clipping 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. So 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.

The algorithm uses the parametric form of a straight line:

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</math>

To compute the final [[line segment]]:

# A line parallel to a clipping window edge has <math>p_i = 0</math> for that boundary.

# If for that <math>i</math>, <math>q_i < 0</math>, then the line is completely outside and can be eliminated.

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<syntaxhighlight lang="c++">

// Liang—BarskyLiang–Barsky line-clipping algorithm

#include<iostream>

#include<graphics.h>

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// this function gives the maximum

float maxi(float arr[], int n) {

float m = 0;

for (int i = 0; i < n; ++i)

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float q4 = ymax - y1;

float posarrexitParams[5], negarrentryParams[5];

int posindexitIndex = 1, negindentryIndex = 1;

posarrexitParams[0] = 1;

negarrentryParams[0] = 0;

rectangle(xmin, ymin, xmax, ymax); // drawing the clipping window

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float r2 = q2 / p2;

if (p1 < 0) {

entryParams[entryIndex++] = r1;

negarr[negind++] = r1; // for negative p1, add it to negative array

posarrexitParams[posindexitIndex++] = r2; // and add p2 to positive array

} else {

negarrentryParams[negindentryIndex++] = r2;

posarrexitParams[posindexitIndex++] = r1;

}

}

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float r4 = q4 / p4;

if (p3 < 0) {

negarrentryParams[negindentryIndex++] = r3;

posarrexitParams[posindexitIndex++] = r4;

} else {

negarrentryParams[negindentryIndex++] = r4;

posarrexitParams[posindexitIndex++] = r3;

}

}

float xn1clippedX1, yn1clippedY1, xn2clippedX2, yn2clippedY2;

float rn1u1, rn2u2;

rn1u1 = maxi(negarrentryParams, negindentryIndex); // maximum of negativeentry arraypoints

rn2u2 = mini(posarrexitParams, posindexitIndex); // minimum of positiveexit arraypoints

if (rn1u1 > rn2u2) { // reject

outtextxy(80, 80, "Line is outside the clipping window!");

return;

}

xn1clippedX1 = x1 + p2(x2 - x1) * rn1u1;

yn1clippedY1 = y1 + p4(y2 - y1) * rn1u1; // computing new points

clippedX2 = x1 + (x2 - x1) * u2;

xn2clippedY2 = x1y1 + p2(y2 - y1) * rn2u2;

yn2 = y1 + p4 * rn2;

setcolor(CYAN);

line(clippedX1, clippedY1, clippedX2, clippedY2); // draw clipped segment

line(xn1, yn1, xn2, yn2); // the drawing the new line

setlinestyle(1, 1, 0);

line(x1, y1, clippedX1, clippedY1); // original start to clipped start

line(x1x2, y1y2, xn1clippedX2, yn1clippedY2); // original end to clipped end

line(x2, y2, xn2, yn2);

}

int main() {

cout << "\nLiang-barskyBarsky lineLine clippingClipping";

cout << "\nThe system window outlaylayout is: (0,0) at bottom left and (631, 467) at top right";

cout << "\nEnter the co-ordinatescoordinates of the window (wxminxmin, wyminymin, wxmaxxmax, wymaxymax): ";

float xmin, ymin, xmax, ymax;

cin >> xmin >> ymin >> xmax >> ymax;

cout << "\nEnter the end pointsendpoints of the line (x1, y1) and (x2, y2): ";

float x1, y1, x2, y2;

cin >> x1 >> y1 >> x2 >> y2;

int gd = DETECT, gm;

initgraph(&gd, &gm, ""); // using winbgim

// using the winbgim library for C++, initializing the graphics mode

initgraph(&gd, &gm, "");

liang_barsky_clipper(xmin, ymin, xmax, ymax, x1, y1, x2, y2);

getch();