Add barycentric coordinate support for ray vs. triangle collision

Agent-Logs-Url: https://github.com/jMonkeyEngine/jmonkeyengine/sessions/75a9c930-942a-438a-b24e-9cc4c7daa5a2

Co-authored-by: riccardobl <4943530+riccardobl@users.noreply.github.com>

Author: Copilot

jme3-core/src/main/java/com/jme3/collision/CollisionResult.java

@@ -32,6 +32,7 @@
 package com.jme3.collision;
 
 import com.jme3.math.Triangle;
+import com.jme3.math.Vector2f;
 import com.jme3.math.Vector3f;
 import com.jme3.scene.Geometry;
 import com.jme3.scene.Mesh;
@@ -50,6 +51,7 @@ public class CollisionResult implements Comparable<CollisionResult> {
     private Vector3f contactNormal;
     private float distance;
     private int triangleIndex;
+    private Vector2f contactBaryCoords;
 
     public CollisionResult(Geometry geometry, Vector3f contactPoint, float distance, int triangleIndex) {
         this.geometry = geometry;
@@ -86,6 +88,10 @@ public void setTriangleIndex(int index) {
         this.triangleIndex = index;
     }
 
+    public void setContactBaryCoords(Vector2f baryCoords) {
+        this.contactBaryCoords = baryCoords;
+    }
+
     public Triangle getTriangle(Triangle store) {
         if (store == null)
             store = new Triangle();
@@ -135,13 +141,28 @@ public int getTriangleIndex() {
         return triangleIndex;
     }
 
+    /**
+     * Returns the barycentric coordinates of the contact point within the
+     * hit triangle, or null if barycentric coordinates are not available.
+     *
+     * <p>The returned vector stores (u, v) where u is the weight of the
+     * second triangle vertex, v is the weight of the third triangle vertex,
+     * and the weight of the first triangle vertex is (1 - u - v).
+     *
+     * @return the barycentric (u, v) coordinates, or null
+     */
+    public Vector2f getContactBaryCoords() {
+        return contactBaryCoords;
+    }
+
     @Override
     public String toString() {
         return "CollisionResult[geometry=" + geometry
                                 + ", contactPoint=" + contactPoint
                                 + ", contactNormal=" + contactNormal
                                 + ", distance=" + distance
                                 + ", triangleIndex=" + triangleIndex
+                                + ", contactBaryCoords=" + contactBaryCoords
                                 + "]";
     }
 }

jme3-core/src/main/java/com/jme3/collision/bih/BIHNode.java

@@ -39,6 +39,7 @@
 import com.jme3.math.Matrix4f;
 import com.jme3.math.Ray;
 import com.jme3.math.Triangle;
+import com.jme3.math.Vector2f;
 import com.jme3.math.Vector3f;
 import com.jme3.util.TempVars;
 import java.io.IOException;
@@ -401,13 +402,14 @@ public final int intersectWhere(Ray r,
             for (int i = node.leftIndex; i <= node.rightIndex; i++) {
                 tree.getTriangle(i, v1, v2, v3);
 
-                float t = r.intersects(v1, v2, v3);
+                Vector2f baryCoords = new Vector2f();
+                float t = r.intersects(v1, v2, v3, baryCoords);
                 if (!Float.isInfinite(t)) {
                     if (worldMatrix != null) {
                         worldMatrix.mult(v1, v1);
                         worldMatrix.mult(v2, v2);
                         worldMatrix.mult(v3, v3);
-                        float t_world = new Ray(o, d).intersects(v1, v2, v3);
+                        float t_world = new Ray(o, d).intersects(v1, v2, v3, baryCoords);
                         t = t_world;
                     }
 
@@ -422,6 +424,7 @@ public final int intersectWhere(Ray r,
                         CollisionResult cr = new CollisionResult(contactPoint, worldSpaceDist);
                         cr.setContactNormal(contactNormal);
                         cr.setTriangleIndex(tree.getTriangleIndex(i));
+                        cr.setContactBaryCoords(baryCoords);
                         results.addCollision(cr);
                         cols++;
                     }

jme3-core/src/main/java/com/jme3/math/Ray.java

@@ -336,6 +336,90 @@ public float intersects(Vector3f v0, Vector3f v1, Vector3f v2) {
         return Float.POSITIVE_INFINITY;
     }
 
+    /**
+     * Test for an intersection between the ray and the given triangle and,
+     * if an intersection exists, store the barycentric coordinates of the
+     * contact point in <code>baryCoords</code>.
+     *
+     * <p>The stored (u, v) barycentric coordinates mean: u is the weight
+     * of <code>v1</code>, v is the weight of <code>v2</code>, and
+     * (1&nbsp;-&nbsp;u&nbsp;-&nbsp;v) is the weight of <code>v0</code>.
+     *
+     * @param v0 first vertex of the triangle (not null, unaffected)
+     * @param v1 second vertex of the triangle (not null, unaffected)
+     * @param v2 third vertex of the triangle (not null, unaffected)
+     * @param baryCoords storage for the barycentric (u, v) coordinates of the
+     *     contact point (modified on hit, may be null)
+     * @return the distance along the ray to the intersection, or
+     *     {@link Float#POSITIVE_INFINITY} if there is no intersection
+     */
+    public float intersects(Vector3f v0, Vector3f v1, Vector3f v2, Vector2f baryCoords) {
+        float edge1X = v1.x - v0.x;
+        float edge1Y = v1.y - v0.y;
+        float edge1Z = v1.z - v0.z;
+
+        float edge2X = v2.x - v0.x;
+        float edge2Y = v2.y - v0.y;
+        float edge2Z = v2.z - v0.z;
+
+        float normX = ((edge1Y * edge2Z) - (edge1Z * edge2Y));
+        float normY = ((edge1Z * edge2X) - (edge1X * edge2Z));
+        float normZ = ((edge1X * edge2Y) - (edge1Y * edge2X));
+
+        float dirDotNorm = direction.x * normX + direction.y * normY + direction.z * normZ;
+
+        float diffX = origin.x - v0.x;
+        float diffY = origin.y - v0.y;
+        float diffZ = origin.z - v0.z;
+
+        float sign;
+        if (dirDotNorm > FastMath.FLT_EPSILON) {
+            sign = 1;
+        } else if (dirDotNorm < -FastMath.FLT_EPSILON) {
+            sign = -1f;
+            dirDotNorm = -dirDotNorm;
+        } else {
+            // ray and triangle/quad are parallel
+            return Float.POSITIVE_INFINITY;
+        }
+
+        float diffEdge2X = ((diffY * edge2Z) - (diffZ * edge2Y));
+        float diffEdge2Y = ((diffZ * edge2X) - (diffX * edge2Z));
+        float diffEdge2Z = ((diffX * edge2Y) - (diffY * edge2X));
+
+        float dirDotDiffxEdge2 = sign * (direction.x * diffEdge2X
+                + direction.y * diffEdge2Y
+                + direction.z * diffEdge2Z);
+
+        if (dirDotDiffxEdge2 >= 0.0f) {
+            diffEdge2X = ((edge1Y * diffZ) - (edge1Z * diffY));
+            diffEdge2Y = ((edge1Z * diffX) - (edge1X * diffZ));
+            diffEdge2Z = ((edge1X * diffY) - (edge1Y * diffX));
+
+            float dirDotEdge1xDiff = sign * (direction.x * diffEdge2X
+                    + direction.y * diffEdge2Y
+                    + direction.z * diffEdge2Z);
+
+            if (dirDotEdge1xDiff >= 0.0f) {
+                if (dirDotDiffxEdge2 + dirDotEdge1xDiff <= dirDotNorm) {
+                    float diffDotNorm = -sign * (diffX * normX + diffY * normY + diffZ * normZ);
+                    if (diffDotNorm >= 0.0f) {
+                        // ray intersects triangle
+                        float inv = 1f / dirDotNorm;
+                        float t = diffDotNorm * inv;
+                        if (baryCoords != null) {
+                            baryCoords.set(dirDotDiffxEdge2 * inv,
+                                           dirDotEdge1xDiff * inv);
+                        }
+                        return t;
+                    }
+                }
+            }
+        }
+
+        return Float.POSITIVE_INFINITY;
+    }
+
     /**
      * <code>intersectWherePlanar</code> determines if the Ray intersects a
      * quad defined by the specified points and if so it stores the point of
@@ -393,13 +477,16 @@ public int collideWith(Collidable other, CollisionResults results) {
             return bv.collideWith(this, results);
         } else if (other instanceof AbstractTriangle) {
             AbstractTriangle tri = (AbstractTriangle) other;
-            float d = intersects(tri.get1(), tri.get2(), tri.get3());
+            Vector2f baryCoords = new Vector2f();
+            float d = intersects(tri.get1(), tri.get2(), tri.get3(), baryCoords);
             if (Float.isInfinite(d) || Float.isNaN(d)) {
                 return 0;
             }
 
             Vector3f point = new Vector3f(direction).multLocal(d).addLocal(origin);
-            results.addCollision(new CollisionResult(point, d));
+            CollisionResult cr = new CollisionResult(point, d);
+            cr.setContactBaryCoords(baryCoords);
+            results.addCollision(cr);
             return 1;
         } else {
             throw new UnsupportedCollisionException();