Liang–Barsky algorithm

{{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:

:$x\; =\; x\_0\; +\; t\; (x\_1\; -\; x\_0)\; =\; x\_0\; +\; t\; \backslash Delta\; x,$

:$y\; =\; y\_0\; +\; t\; (y\_1\; -\; y\_0)\; =\; y\_0\; +\; t\; \backslash Delta\; y.$

A point is in the clip window, if

:$x\_\backslash text\{min\}\; \backslash le\; x\_0\; +\; t\; \backslash Delta\; x\; \backslash le\; x\_\backslash text\{max\}$

and

:$y\_\backslash text\{min\}\; \backslash le\; y\_0\; +\; t\; \backslash Delta\; y\; \backslash le\; y\_\backslash text\{max\},$

which can be expressed as the 4 inequalities

:$t\; p\_i\; \backslash le\; q\_i,\; \backslash quad\; i\; =\; 1,\; 2,\; 3,\; 4,$

where

:$$

\begin{align}

p_1 &= -\Delta x, & q_1 &= x_0 - x_\text{min}, & &\text{(left)} \\

p_2 &= \Delta x, & q_2 &= x_\text{max} - x_0, & &\text{(right)} \\

p_3 &= -\Delta y, & q_3 &= y_0 - y_\text{min}, & &\text{(bottom)} \\

p_4 &= \Delta y, & q_4 &= y_\text{max} - y_0. & &\text{(top)}

\end{align}

To compute the final line segment:

1. A line parallel to a clipping window edge has $p\_i\; =\; 0$ for that boundary.
2. If for that $i$, , then the line is completely outside and can be eliminated.
3. When , the line proceeds outside to inside the clip window, and when $p\_i\; >\; 0$, the line proceeds inside to outside.
4. For nonzero $p\_i$, $u\; =\; q\_i\; /\; p\_i$ gives $t$ for the intersection point of the line and the window edge (possibly projected).
5. The two actual intersections of the line with the window edges, if they exist, are described by $u\_1$ and $u\_2$, calculated as follows. For $u\_1$, look at boundaries for which (i.e. outside to inside). Take $u\_1$ to be the largest among $\backslash \{0,\; q\_i\; /\; p\_i\backslash \}$. For $u\_2$, look at boundaries for which $p\_i\; >\; 0$ (i.e. inside to outside). Take $u\_2$ to be the minimum of $\backslash \{1,\; q\_i\; /\; p\_i\backslash \}$.
6. If $u\_1\; >\; u\_2$, the line is entirely outside the clip window. If it is entirely inside it.

// Liang–Barsky line-clipping algorithm

1. include
2. include
3. include

using namespace std;

// this function gives the maximum

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

float m = 0;

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

if (m < arr[i])

m = arr[i];

return m;

}

// this function gives the minimum

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

float m = 1;

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

if (m > arr[i])

m = arr[i];

return m;

}

void liang_barsky_clipper(float xmin, float ymin, float xmax, float ymax,

float x1, float y1, float x2, float y2) {

// defining variables

float p1 = -(x2 - x1);

float p2 = -p1;

float p3 = -(y2 - y1);

float p4 = -p3;

float q1 = x1 - xmin;

float q2 = xmax - x1;

float q3 = y1 - ymin;

float q4 = ymax - y1;

float exitParams[5], entryParams[5];

int exitIndex = 1, entryIndex = 1;

exitParams[0] = 1;

entryParams[0] = 0;

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

if ((p1 == 0 && q1 < 0) || (p2 == 0 && q2 < 0) || (p3 == 0 && q3 < 0) || (p4 == 0 && q4 < 0)) {

outtextxy(80, 80, "Line is parallel to clipping window!");

return;

}

if (p1 != 0) {

float r1 = q1 / p1;

float r2 = q2 / p2;

if (p1 < 0) {

entryParams[entryIndex++] = r1;

exitParams[exitIndex++] = r2;

} else {

entryParams[entryIndex++] = r2;

exitParams[exitIndex++] = r1;

}

}

if (p3 != 0) {

float r3 = q3 / p3;

float r4 = q4 / p4;

if (p3 < 0) {

entryParams[entryIndex++] = r3;

exitParams[exitIndex++] = r4;

} else {

entryParams[entryIndex++] = r4;

exitParams[exitIndex++] = r3;

}

}

float clippedX1, clippedY1, clippedX2, clippedY2;

float u1, u2;

u1 = maxi(entryParams, entryIndex); // maximum of entry points

u2 = mini(exitParams, exitIndex); // minimum of exit points

if (u1 > u2) {

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

return;

}

clippedX1 = x1 + (x2 - x1) * u1;

clippedY1 = y1 + (y2 - y1) * u1;

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

clippedY2 = y1 + (y2 - y1) * u2;

setcolor(CYAN);

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

setlinestyle(1, 1, 0);

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

line(x2, y2, clippedX2, clippedY2); // original end to clipped end

}

int main() {

cout << "\nLiang-Barsky Line Clipping";

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

cout << "\nEnter the coordinates of the window (xmin, ymin, xmax, ymax): ";

float xmin, ymin, xmax, ymax;

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

cout << "\nEnter the endpoints 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

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

getch();

closegraph();

}