Implement Cohen-Sutherland Line Clipping Algorithm

Author: Moksedur Rahman Sohan

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

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+package com.thealgorithms.lineclipping;
+
+import com.thealgorithms.lineclipping.utils.Line;
+import com.thealgorithms.lineclipping.utils.Point;
+
+/**
+ * @author shikarisohan
+ * @since 10/4/24
+ * Cohen-Sutherland Line Clipping Algorithm
+ *
+ * This algorithm is used to clip a line segment to a rectangular window.
+ * It assigns a region code to each endpoint of the line segment, and
+ * then efficiently determines whether the line segment is fully inside,
+ * fully outside, or partially inside the window.
+ *
+ * Reference:
+ * https://en.wikipedia.org/wiki/Cohen%E2%80%93Sutherland_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 CohenSutherland {
+
+    // Region codes for the 9 regions
+    final int INSIDE = 0; // 0000
+    final int LEFT = 1;   // 0001
+    final int RIGHT = 2;  // 0010
+    final int BOTTOM = 4; // 0100
+    final int TOP = 8;    // 1000
+
+    // Define the clipping window
+    double xMin, yMin, xMax, yMax;
+
+    public CohenSutherland(double xMin, double yMin, double xMax, double yMax) {
+        this.xMin = xMin;
+        this.yMin = yMin;
+        this.xMax = xMax;
+        this.yMax = yMax;
+    }
+
+    // Compute the region code for a point (x, y)
+    private int computeCode(double x, double y) {
+        int code = INSIDE;
+
+        if (x < xMin) // to the left of rectangle
+            code |= LEFT;
+        else if (x > xMax) // to the right of rectangle
+            code |= RIGHT;
+        if (y < yMin) // below the rectangle
+            code |= BOTTOM;
+        else if (y > yMax) // above the rectangle
+            code |= TOP;
+
+        return code;
+    }
+
+    // Cohen-Sutherland algorithm to return the clipped line
+    public Line cohenSutherlandClip(Line line) {
+        double x1 = line.start.x, y1 = line.start.y;
+        double x2 = line.end.x, y2 = line.end.y;
+
+        int code1 = computeCode(x1, y1);
+        int code2 = computeCode(x2, y2);
+        boolean accept = false;
+
+        while (true) {
+            if ((code1 == 0) && (code2 == 0)) {
+                // Both points are inside the rectangle
+                accept = true;
+                break;
+            } else if ((code1 & code2) != 0) {
+                // Both points are outside the rectangle in the same region
+                break;
+            } else {
+                // Some segment of the line is inside the rectangle
+                double x = 0, y = 0;
+
+                // Pick an endpoint that is outside the rectangle
+                int codeOut = (code1 != 0) ? code1 : code2;
+
+                // Find the intersection point using the line equation
+                if ((codeOut & TOP) != 0) {
+                    // Point is above the rectangle
+                    x = x1 + (x2 - x1) * (yMax - y1) / (y2 - y1);
+                    y = yMax;
+                } else if ((codeOut & BOTTOM) != 0) {
+                    // Point is below the rectangle
+                    x = x1 + (x2 - x1) * (yMin - y1) / (y2 - y1);
+                    y = yMin;
+                } else if ((codeOut & RIGHT) != 0) {
+                    // Point is to the right of the rectangle
+                    y = y1 + (y2 - y1) * (xMax - x1) / (x2 - x1);
+                    x = xMax;
+                } else if ((codeOut & LEFT) != 0) {
+                    // Point is to the left of the rectangle
+                    y = y1 + (y2 - y1) * (xMin - x1) / (x2 - x1);
+                    x = xMin;
+                }
+
+                // Replace the point outside the rectangle with the intersection point
+                if (codeOut == code1) {
+                    x1 = x;
+                    y1 = y;
+                    code1 = computeCode(x1, y1);
+                } else {
+                    x2 = x;
+                    y2 = y;
+                    code2 = computeCode(x2, y2);
+                }
+            }
+        }
+
+        if (accept) {
+            return new Line(new Point(x1, y1), new Point(x2, y2));
+        } else {
+
+            return null; // The line is fully rejected
+        }
+    }
+}

src/main/java/com/thealgorithms/lineclipping/utils/Line.java

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+package com.thealgorithms.lineclipping.utils;
+
+import java.util.Objects;
+
+/**
+ * @author moksedursohan
+ * @since 10/4/24
+ */
+public class Line {
+
+    public Point start, end;
+
+    public Line() {
+    }
+
+    public Line(Point start, Point end) {
+        this.start = start;
+        this.end = end;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (!(o instanceof Line line)) return false;
+        return Objects.equals(start, line.start) && Objects.equals(end, line.end);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(start, end);
+    }
+
+    @Override
+    public String toString() {
+        return "Line from " + start + " to " + end;
+    }
+}

src/main/java/com/thealgorithms/lineclipping/utils/Point.java

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+package com.thealgorithms.lineclipping.utils;
+
+import java.util.Objects;
+
+/**
+ * @author moksedursohan
+ * @since 10/4/24
+ */
+public class Point {
+
+    public double x, y;
+
+    public Point() {
+    }
+
+    public Point(double x, double y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (!(o instanceof Point point)) return false;
+        return Double.compare(x, point.x) == 0 && Double.compare(y, point.y) == 0;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(x, y);
+    }
+
+    @Override
+    public String toString() {
+        return "(" + x + ", " + y + ")";
+    }
+}

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

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+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 10/4/24
+ */
+class CohenSutherlandTest {
+
+    // Define the clipping window (1.0, 1.0) to (10.0, 10.0)
+    CohenSutherland cs = new CohenSutherland(1.0, 1.0, 10.0, 10.0);
+
+    @Test
+    void testLineCompletelyInside() {
+        // Line fully inside the clipping window
+        Line line = new Line(new Point(2.0, 2.0), new Point(8.0, 8.0));
+        Line clippedLine = cs.cohenSutherlandClip(line);
+
+        assertNotNull(clippedLine, "Line should not be null.");
+        assertEquals(line, clippedLine, "Line inside the window should remain unchanged.");
+    }
+
+    @Test
+    void testLineCompletelyOutside() {
+        // Line completely outside and above the clipping window
+        Line line = new Line(new Point(11.0, 12.0), new Point(15.0, 18.0));
+        Line clippedLine = cs.cohenSutherlandClip(line);
+
+        assertNull(clippedLine, "Line should be null because it's completely outside.");
+    }
+
+    @Test
+    void testLinePartiallyInside() {
+        // Line partially inside the clipping window
+        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 = cs.cohenSutherlandClip(line);
+
+        assertNotNull(clippedLine, "Line should not be null.");
+        assertEquals(expectedClippedLine, clippedLine, "Line should be clipped correctly.");
+    }
+
+    @Test
+    void testLineOnBoundary() {
+        // Line exactly on the boundary of the clipping window
+        Line line = new Line(new Point(1.0, 5.0), new Point(10.0, 5.0));
+        Line clippedLine = cs.cohenSutherlandClip(line);
+
+        assertNotNull(clippedLine, "Line should not be null.");
+        assertEquals(line, clippedLine, "Line on the boundary should remain unchanged.");
+    }
+
+    @Test
+    void testDiagonalLineThroughClippingWindow() {
+        // Diagonal line crossing from outside to outside through the window
+        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 = cs.cohenSutherlandClip(line);
+
+        assertNotNull(clippedLine, "Line should not be null.");
+        assertEquals(expectedClippedLine, clippedLine, "Diagonal line should be clipped correctly.");
+    }
+
+    @Test
+    void testVerticalLineClipping() {
+        // Vertical line crossing the top and bottom of the clipping window
+        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 = cs.cohenSutherlandClip(line);
+
+        assertNotNull(clippedLine, "Line should not be null.");
+        assertEquals(expectedClippedLine, clippedLine, "Vertical line should be clipped correctly.");
+    }
+}