vec2.cross

gl-matrix.js

@@ -2420,6 +2420,35 @@
         return dest;
     };
 
+    /**
+     * Computes the cross product of two vec2's. Note that the cross product must by definition
+     * produce a 3D vector. If a dest vector is given, it will contain the resultant 3D vector.
+     * Otherwise, a scalar number will be returned, representing the vector's Z coordinate, since
+     * its X and Y must always equal 0.
+     *
+     * Examples:
+     *    vec2.cross([1, 2], [3, 4], vec3.create())
+     *    //=> [0, 0, -2]
+     *    
+     *    vec2.cross([1, 2], [3, 4])
+     *    //=> -2
+     *
+     * See http://stackoverflow.com/questions/243945/calculating-a-2d-vectors-cross-product
+     * for some interesting facts.
+     *
+     * @param {vec2} vecA left operand
+     * @param {vec2} vecB right operand
+     * @param {vec2} [dest] optional vec2 receiving result. If not specified a scalar is returned
+     *
+     */
+    vec2.cross = function (vecA, vecB, dest) {
+      var z = vecA[0] * vecB[1] - vecA[1] * vecB[0];
+      if (!dest) return z;
+      dest[0] = dest[1] = 0;
+      dest[2] = z;
+      return dest;
+    };
+
     /*
      * Exports
      */

spec/javascripts/vec2_spec.js

@@ -13,6 +13,23 @@ describe("vec2", function() {
     });
   });
 
+  describe("cross", function() {
+    describe("with dest given", function() {
+      beforeEach(function() { result = vec2.cross(vecA, vecB, dest); });
+      it("should store the cross in dest", function() { expect(dest).toBeEqualish([0, 0, -2]); });
+      it("should return dest", function() { expect(result).toBe(dest); });
+      it("should not alter vecA", function() { expect(vecA).toBeEqualish([1, 2]); });
+      it("should not alter vecB", function() { expect(vecB).toBeEqualish([3, 4]); });
+    });
+
+    describe("with no dest given", function() {
+      beforeEach(function() { result = vec2.cross(vecA, vecB); });
+      it("should return the scalar Z value", function() { expect(result).toBeEqualish(-2); });
+      it("should not alter vecA", function() { expect(vecA).toBeEqualish([1, 2]); });
+      it("should not alter vecB", function() { expect(vecB).toBeEqualish([3, 4]); });
+    });
+  });
+
   describe("normalize", function() {
     describe("with dest given", function() {
       beforeEach(function() { result = vec2.normalize(vecA, dest); });