Implement Liang-Barsky Line Clipping Algorithm

Author: Moksedur Rahman Sohan

src/main/java/com/thealgorithms/lineclipping/LiangBarsky.java

@@ -0,0 +1,81 @@
+package com.thealgorithms.lineclipping;
+
+import com.thealgorithms.lineclipping.utils.Line;
+import com.thealgorithms.lineclipping.utils.Point;
+
+/**
+ * @author shikarisohan
+ * @since 10/5/24
+ *
+ *  * The Liang-Barsky line clipping algorithm is an efficient algorithm for
+ *  * line clipping against a rectangular window. It is based on the parametric
+ *  * equation of a line and checks the intersections of the line with the
+ *  * window boundaries. This algorithm calculates the intersection points,
+ *  * if any, and returns the clipped line that lies inside the window.
+ *  *
+ *  * Reference:
+ *  * https://en.wikipedia.org/wiki/Liang%E2%80%93Barsky_algorithm
+ *
+ * Clipping window boundaries are defined as (xMin, yMin) and (xMax, yMax).
+ * The algorithm computes the clipped line segment if it's partially or
+ * fully inside the clipping window.
+ */
+public class LiangBarsky {
+
+    // Define the clipping window
+    private double xMin, xMax, yMin, yMax;
+
+    public LiangBarsky(double xMin, double yMin, double xMax, double yMax) {
+        this.xMin = xMin;
+        this.yMin = yMin;
+        this.xMax = xMax;
+        this.yMax = yMax;
+    }
+
+    // Liang-Barsky algorithm to return the clipped line
+    public Line liangBarskyClip(Line line) {
+        double dx = line.end.x - line.start.x;
+        double dy = line.end.y - line.start.y;
+
+        double[] p = {-dx, dx, -dy, dy};
+        double[] q = {line.start.x - xMin, xMax - line.start.x, line.start.y - yMin, yMax - line.start.y};
+
+        double[] resultT = clipLine(p, q);
+
+        if (resultT == null) {
+            return null; // Line is outside the clipping window
+        }
+
+        return calculateClippedLine(line, resultT[0], resultT[1], dx, dy);
+    }
+
+    // clip the line by adjusting t0 and t1 for each edge
+    private double[] clipLine(double[] p, double[] q) {
+        double t0 = 0.0;
+        double t1 = 1.0;
+
+        for (int i = 0; i < 4; i++) {
+            double t = q[i] / p[i];
+            if (p[i] == 0 && q[i] < 0) {
+                return null; // Line is outside the boundary
+            } else if (p[i] < 0) {
+                if (t > t1) return null; // Line is outside
+                if (t > t0) t0 = t; // Update t0
+            } else if (p[i] > 0) {
+                if (t < t0) return null; // Line is outside
+                if (t < t1) t1 = t; // Update t1
+            }
+        }
+        return new double[]{t0, t1}; // Return valid t0 and t1
+    }
+
+    // calculate the clipped line based on t0 and t1
+    private Line calculateClippedLine(Line line, double t0, double t1, double dx, double dy) {
+        double clippedX1 = line.start.x + t0 * dx;
+        double clippedY1 = line.start.y + t0 * dy;
+        double clippedX2 = line.start.x + t1 * dx;
+        double clippedY2 = line.start.y + t1 * dy;
+
+        return new Line(new Point(clippedX1, clippedY1), new Point(clippedX2, clippedY2));
+    }
+}

src/test/java/com/thealgorithms/lineclipping/LiangBarskyTest.java

@@ -0,0 +1,64 @@
+package com.thealgorithms.lineclipping;
+
+import com.thealgorithms.lineclipping.utils.Line;
+import com.thealgorithms.lineclipping.utils.Point;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * @author shikarisohan
+ * @since 10/5/24
+ */
+class LiangBarskyTest {
+
+    LiangBarsky lb = new LiangBarsky(1.0, 1.0, 10.0, 10.0);
+
+    @Test
+    void testLineCompletelyInside() {
+        Line line = new Line(new Point(2.0, 2.0), new Point(8.0, 8.0));
+        Line clippedLine = lb.liangBarskyClip(line);
+
+        assertNotNull(clippedLine, "Line should not be null.");
+        assertEquals(line, clippedLine, "Line inside the window should remain unchanged.");
+    }
+
+    @Test
+    void testLineCompletelyOutside() {
+        Line line = new Line(new Point(12.0, 12.0), new Point(15.0, 18.0));
+        Line clippedLine = lb.liangBarskyClip(line);
+
+        assertNull(clippedLine, "Line should be null because it's completely outside.");
+    }
+
+    @Test
+    void testLinePartiallyInside() {
+        Line line = new Line(new Point(5.0, 5.0), new Point(12.0, 12.0));
+        Line expectedClippedLine = new Line(new Point(5.0, 5.0), new Point(10.0, 10.0)); // Clipped at (10, 10)
+        Line clippedLine = lb.liangBarskyClip(line);
+
+        assertNotNull(clippedLine, "Line should not be null.");
+        assertEquals(expectedClippedLine, clippedLine, "Line should be clipped correctly.");
+    }
+
+    @Test
+    void testDiagonalLineThroughClippingWindow() {
+        Line line = new Line(new Point(0.0, 0.0), new Point(12.0, 12.0));
+        Line expectedClippedLine = new Line(new Point(1.0, 1.0), new Point(10.0, 10.0)); // Clipped at both boundaries
+        Line clippedLine = lb.liangBarskyClip(line);
+
+        assertNotNull(clippedLine, "Line should not be null.");
+        assertEquals(expectedClippedLine, clippedLine, "Diagonal line should be clipped correctly.");
+    }
+
+    @Test
+    void testVerticalLineClipping() {
+        Line line = new Line(new Point(5.0, 0.0), new Point(5.0, 12.0));
+        Line expectedClippedLine = new Line(new Point(5.0, 1.0), new Point(5.0, 10.0)); // Clipped at yMin and yMax
+        Line clippedLine = lb.liangBarskyClip(line);
+
+        assertNotNull(clippedLine, "Line should not be null.");
+        assertEquals(expectedClippedLine, clippedLine, "Vertical line should be clipped correctly.");
+    }
+
+}