Add Angle.sinSnap and .cosSnap to avoid small errors, e.g. with buffer operations

modules/core/src/main/java/org/locationtech/jts/algorithm/Angle.java

@@ -313,12 +313,36 @@ public static double diff(double ang1, double ang2) {
     }
 
     if (delAngle > Math.PI) {
-      delAngle = (2 * Math.PI) - delAngle;
+      delAngle = PI_TIMES_2 - delAngle;
     }
 
     return delAngle;
   }
-
+
+  /**
+   * Computes sin of an angle, snapping near-zero values to zero.
+   *
+   * @param ang the input angle (in radians)
+   * @return the result of the trigonometric function
+   */
+  public static double sinSnap(double ang) {
+    double res = Math.sin(ang);
+    if (Math.abs(res) < 5e-16) return 0.0;
+    return res;
+  }
+
+  /**
+   * Computes cos of an angle, snapping near-zero values to zero.
+   *
+   * @param ang the input angle (in radians)
+   * @return the result of the trigonometric function
+   */
+  public static double cosSnap(double ang) {
+    double res = Math.cos(ang);
+    if (Math.abs(res) < 5e-16) return 0.0;
+    return res;
+  }
+
   /**
    * Projects a point by a given angle and distance.
    * 
@@ -328,8 +352,8 @@ public static double diff(double ang1, double ang2) {
    * @return the projected point
    */
   public static Coordinate project(Coordinate p, double angle, double dist) {
-    double x = p.getX() + dist * Math.cos(angle);
-    double y = p.getY() + dist * Math.sin(angle);
+    double x = p.getX() + dist * Angle.cosSnap(angle);
+    double y = p.getY() + dist * Angle.sinSnap(angle);
     return new Coordinate(x, y);
   }
 }

modules/core/src/main/java/org/locationtech/jts/operation/buffer/BufferParameters.java

@@ -11,6 +11,8 @@
  */
 package org.locationtech.jts.operation.buffer;
 
+import org.locationtech.jts.algorithm.Angle;
+
 /**
  * A value class containing the parameters which 
  * specify how a buffer should be constructed.
@@ -176,7 +178,7 @@ public void setQuadrantSegments(int quadSegs)
    */
   public static double bufferDistanceError(int quadSegs)
   {
-    double alpha = Math.PI / 2.0 / quadSegs;
+    double alpha = Angle.PI_OVER_2 / quadSegs;
     return 1 - Math.cos(alpha / 2.0);
   }
 

modules/core/src/main/java/org/locationtech/jts/operation/buffer/OffsetSegmentGenerator.java

@@ -114,7 +114,7 @@ public OffsetSegmentGenerator(PrecisionModel precisionModel,
 
     int quadSegs = bufParams.getQuadrantSegments();
     if (quadSegs < 1) quadSegs = 1;
-    filletAngleQuantum = Math.PI / 2.0 / quadSegs;
+    filletAngleQuantum = Angle.PI_OVER_2 / quadSegs;
 
     /**
      * Non-round joins cause issues with short closing segments, so don't use
@@ -428,7 +428,7 @@ public void addLineEndCap(Coordinate p0, Coordinate p1)
       case BufferParameters.CAP_ROUND:
         // add offset seg points with a fillet between them
         segList.addPt(offsetL.p1);
-        addDirectedFillet(p1, angle + Math.PI / 2, angle - Math.PI / 2, Orientation.CLOCKWISE, distance);
+        addDirectedFillet(p1, angle + Angle.PI_OVER_2, angle - Angle.PI_OVER_2, Orientation.CLOCKWISE, distance);
         segList.addPt(offsetR.p1);
         break;
       case BufferParameters.CAP_FLAT:
@@ -439,8 +439,8 @@ public void addLineEndCap(Coordinate p0, Coordinate p1)
       case BufferParameters.CAP_SQUARE:
         // add a square defined by extensions of the offset segment endpoints
         Coordinate squareCapSideOffset = new Coordinate();
-        squareCapSideOffset.x = Math.abs(distance) * Math.cos(angle);
-        squareCapSideOffset.y = Math.abs(distance) * Math.sin(angle);
+        squareCapSideOffset.x = Math.abs(distance) * Angle.cosSnap(angle);
+        squareCapSideOffset.y = Math.abs(distance) * Angle.sinSnap(angle);
 
         Coordinate squareCapLOffset = new Coordinate(
             offsetL.p1.x + squareCapSideOffset.x,
@@ -534,7 +534,7 @@ private void addLimitedMitreJoin(
     Coordinate bevelMidPt = project(cornerPt, -mitreLimitDistance, dirBisector);
 
     // direction of bevel segment (at right angle to corner bisector)
-    double dirBevel = Angle.normalize(dirBisector + Math.PI/2.0);
+    double dirBevel = Angle.normalize(dirBisector + Angle.PI_OVER_2);
 
     // compute the candidate bevel segment by projecting both sides of the midpoint
     Coordinate bevel0 = project(bevelMidPt, distance, dirBevel);
@@ -567,8 +567,8 @@ private void addLimitedMitreJoin(
    * @return the projected point
    */
   private static Coordinate project(Coordinate pt, double d, double dir) {
-    double x = pt.x + d * Math.cos(dir);
-    double y = pt.y + d * Math.sin(dir);
+    double x = pt.x + d * Angle.cosSnap(dir);
+    double y = pt.y + d * Angle.sinSnap(dir);
     return new Coordinate(x, y);
   }
 
@@ -607,10 +607,10 @@ private void addCornerFillet(Coordinate p, Coordinate p0, Coordinate p1, int dir
     double endAngle = Math.atan2(dy1, dx1);
 
     if (direction == Orientation.CLOCKWISE) {
-      if (startAngle <= endAngle) startAngle += 2.0 * Math.PI;
+      if (startAngle <= endAngle) startAngle += Angle.PI_TIMES_2;
     }
     else {    // direction == COUNTERCLOCKWISE
-      if (startAngle >= endAngle) startAngle -= 2.0 * Math.PI;
+      if (startAngle >= endAngle) startAngle -= Angle.PI_TIMES_2;
     }
     segList.addPt(p0);
     addDirectedFillet(p, startAngle, endAngle, direction, radius);
@@ -641,8 +641,8 @@ private void addDirectedFillet(Coordinate p, double startAngle, double endAngle,
     Coordinate pt = new Coordinate();
     for (int i = 0; i < nSegs; i++) {
       double angle = startAngle + directionFactor * i * angleInc;
-      pt.x = p.x + radius * Math.cos(angle);
-      pt.y = p.y + radius * Math.sin(angle);
+      pt.x = p.x + radius * Angle.cosSnap(angle);
+      pt.y = p.y + radius * Angle.sinSnap(angle);
       segList.addPt(pt);
     }
   }
@@ -655,7 +655,7 @@ public void createCircle(Coordinate p)
     // add start point
     Coordinate pt = new Coordinate(p.x + distance, p.y);
     segList.addPt(pt);
-    addDirectedFillet(p, 0.0, 2.0 * Math.PI, -1, distance);
+    addDirectedFillet(p, 0.0, Angle.PI_TIMES_2, -1, distance);
     segList.closeRing();
   }
 

modules/core/src/main/java/org/locationtech/jts/util/GeometricShapeFactory.java

@@ -11,6 +11,7 @@
  */
 package org.locationtech.jts.util;
 
+import org.locationtech.jts.algorithm.Angle;
 import org.locationtech.jts.geom.Coordinate;
 import org.locationtech.jts.geom.Envelope;
 import org.locationtech.jts.geom.Geometry;
@@ -222,8 +223,8 @@ public Polygon createEllipse()
     int iPt = 0;
     for (int i = 0; i < nPts; i++) {
         double ang = i * (2 * Math.PI / nPts);
-        double x = xRadius * Math.cos(ang) + centreX;
-        double y = yRadius * Math.sin(ang) + centreY;
+        double x = xRadius * Angle.cosSnap(ang) + centreX;
+        double y = yRadius * Angle.sinSnap(ang) + centreY;
         pts[iPt++] = coord(x, y);
     }
     pts[iPt] = new Coordinate(pts[0]);
@@ -319,16 +320,16 @@ public LineString createArc(
     double centreY = env.getMinY() + yRadius;
 
      double angSize = angExtent;
-     if (angSize <= 0.0 || angSize > 2 * Math.PI)
-       angSize = 2 * Math.PI;
+     if (angSize <= 0.0 || angSize > Angle.PI_TIMES_2)
+       angSize = Angle.PI_TIMES_2;
      double angInc = angSize / (nPts - 1);
 
      Coordinate[] pts = new Coordinate[nPts];
      int iPt = 0;
      for (int i = 0; i < nPts; i++) {
          double ang = startAng + i * angInc;
-         double x = xRadius * Math.cos(ang) + centreX;
-         double y = yRadius * Math.sin(ang) + centreY;
+         double x = xRadius * Angle.cosSnap(ang) + centreX;
+         double y = yRadius * Angle.sinSnap(ang) + centreY;
          pts[iPt++] = coord(x, y);
      }
      LineString line = geomFact.createLineString(pts);
@@ -353,8 +354,8 @@ public Polygon createArcPolygon(double startAng, double angExtent) {
     double centreY = env.getMinY() + yRadius;
 
     double angSize = angExtent;
-    if (angSize <= 0.0 || angSize > 2 * Math.PI)
-      angSize = 2 * Math.PI;
+    if (angSize <= 0.0 || angSize > Angle.PI_TIMES_2)
+      angSize = Angle.PI_TIMES_2;
     double angInc = angSize / (nPts - 1);
     // double check = angInc * nPts;
     // double checkEndAng = startAng + check;
@@ -366,8 +367,8 @@ public Polygon createArcPolygon(double startAng, double angExtent) {
     for (int i = 0; i < nPts; i++) {
       double ang = startAng + angInc * i;
 
-      double x = xRadius * Math.cos(ang) + centreX;
-      double y = yRadius * Math.sin(ang) + centreY;
+      double x = xRadius * Angle.cosSnap(ang) + centreX;
+      double y = yRadius * Angle.sinSnap(ang) + centreY;
       pts[iPt++] = coord(x, y);
     }
     pts[iPt++] = coord(centreX, centreY);

modules/core/src/test/java/org/locationtech/jts/algorithm/AngleTest.java

@@ -158,7 +158,41 @@ public void testAngleBisector() {
 
     assertEquals(45, Math.toDegrees(Angle.bisector(p(13,10), p(10,10), p(10,20))), 0.01);
   }
-
+
+  public void testSinCosSnap() {
+
+    // -720 to 720 degrees with 1 degree increments
+    for (int angdeg = -720; angdeg <= 720; angdeg++) {
+      double ang = Angle.toRadians(angdeg);
+
+      double rSin = Angle.sinSnap(ang);
+      double rCos = Angle.cosSnap(ang);
+
+      double cSin = Math.sin(ang);
+      double cCos = Math.cos(ang);
+      if ( (angdeg % 90) == 0 ) {
+        // not always the same for multiples of 90 degrees
+        assertTrue(Math.abs(rSin - cSin) < 1e-15);
+        assertTrue(Math.abs(rCos - cCos) < 1e-15);
+      } else {
+        assertEquals(rSin, cSin);
+        assertEquals(rCos, cCos);
+      }
+
+    }
+
+    // use radian increments that don't snap to exact degrees or zero
+    for (double angrad = -6.3; angrad < 6.3; angrad += 0.013) {
+
+      double rSin = Angle.sinSnap(angrad);
+      double rCos = Angle.cosSnap(angrad);
+
+      assertEquals(rSin, Math.sin(angrad));
+      assertEquals(rCos, Math.cos(angrad));
+
+    }
+  }
+
   private static Coordinate p(double x, double y) {
     return new Coordinate(x, y);
   }

modules/core/src/test/java/org/locationtech/jts/operation/buffer/BufferTest.java

@@ -11,6 +11,7 @@
  */
 package org.locationtech.jts.operation.buffer;
 
+import org.locationtech.jts.geom.Coordinate;
 import org.locationtech.jts.geom.Geometry;
 import org.locationtech.jts.geom.GeometryCollection;
 import org.locationtech.jts.geom.GeometryFactory;
@@ -602,4 +603,22 @@ private boolean hasHole(Geometry geom) {
     return false;
   }
 
+  /**
+   * See GEOS PR https://github.com/libgeos/geos/pull/978
+   */
+  public void testDefaultBuffer() {
+    Geometry g = read("POINT (0 0)").buffer(1.0);
+    Geometry b = g.getBoundary();
+    Coordinate[] coords = b.getCoordinates();
+    assertEquals(33, coords.length);
+    assertEquals(coords[0].x, 1.0);
+    assertEquals(coords[0].y, 0.0);
+    assertEquals(coords[8].x, 0.0);
+    assertEquals(coords[8].y, -1.0);
+    assertEquals(coords[16].x, -1.0);
+    assertEquals(coords[16].y, 0.0);
+    assertEquals(coords[24].x, 0.0);
+    assertEquals(coords[24].y, 1.0);
+  }
+
 }