mat2.determinant, mat2.inverse

bpl · May 3, 2012 · 99a6ced · 99a6ced
1 parent 5198b9b
commit 99a6ced
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diff --git a/gl-matrix.js b/gl-matrix.js
@@ -2610,6 +2610,39 @@
         return dest;
     };
 
+    /**
+     * Calculates the determinant of a mat2
+     *
+     * @param {mat2} mat mat2 to calculate determinant of
+     *
+     * @returns {Number} determinant of mat
+     */
+    mat2.determinant = function (mat) {
+      return mat[0] * mat[3] - mat[2] * mat[1];
+    };
+
+    /**
+     * Calculates the inverse matrix of a mat2
+     *
+     * @param {mat2} mat mat2 to calculate inverse of
+     * @param {mat2} [dest] mat2 receiving inverse matrix. If not specified result is written to mat
+     *
+     * @param {mat2} dest is specified, mat otherwise, null if matrix cannot be inverted
+     */
+    mat2.inverse = function (mat, dest) {
+        if (!dest) { dest = mat; }
+        var a0 = mat[0], a1 = mat[1], a2 = mat[2], a3 = mat[3];
+        var det = a0 * a3 - a2 * a1;
+        if (!det) return null;
+
+        det = 1.0 / det;
+        dest[0] =  a3 * det;
+        dest[1] = -a1 * det;
+        dest[2] = -a2 * det;
+        dest[3] =  a0 * det;
+        return dest;
+    }
+
     /*
      * Exports
      */

diff --git a/spec/javascripts/mat2_spec.js b/spec/javascripts/mat2_spec.js
@@ -58,5 +58,31 @@ describe("mat2", function() {
       it("should return a", function() { expect(result).toBe(a); });
     });
   });
-
+
+  describe("determinant", function() {
+    it("should produce the correct result", function() {
+      expect(mat2.determinant(a)).toBeEqualish(-2);
+    });
+  });
+
+  describe("inverse", function() {
+    describe("when no inverse exists", function() {
+      it("should be null", function() {
+        expect(mat2.inverse([3, 4, 6, 8])).toBeNull();
+      });
+    });
+
+    describe("with dest", function() {
+      beforeEach(function() { result = mat2.inverse(a = [4, 7, 2, 6], dest); });
+      it("should set dest", function() { expect(dest).toBeEqualish([0.6, -0.7, -0.2, 0.4]); });
+      it("should return dest", function() { expect(result).toBe(dest); });
+      it("should not modify a", function() { expect(a).toBeEqualish([4, 7, 2, 6]); });
+    });
+
+    describe("without dest", function() {
+      beforeEach(function() { result = mat2.inverse(a = [4, 7, 2, 6]); });
+      it("should set a", function() { expect(a).toBeEqualish([0.6, -0.7, -0.2, 0.4]); });
+      it("should return a", function() { expect(result).toBe(a); });
+    });
+  });
 });