special-case matrix invert for translate and scale

update unittest to use diff scale-x and scale-y values, and tests for non-invertible scale matrices
Review URL: https://codereview.appspot.com/7027055

git-svn-id: http://skia.googlecode.com/svn/trunk@7019 2bbb7eff-a529-9590-31e7-b0007b416f81

2 files changed, 43 insertions, 3 deletions

diff --git a/src/core/SkMatrix.cpp b/src/core/SkMatrix.cpp
index 5c9547e8a7..6cccc39ece 100644
--- a/src/core/SkMatrix.cpp
+++ b/src/core/SkMatrix.cpp
@@ -850,7 +850,42 @@ bool SkMatrix::asAffine(SkScalar affine[6]) const {
 
 bool SkMatrix::invertNonIdentity(SkMatrix* inv) const {
     SkASSERT(!this->isIdentity());
-    int         isPersp = this->hasPerspective();
+
+    TypeMask mask = this->getType();
+
+    if (0 == (mask & ~(kScale_Mask | kTranslate_Mask))) {
+        if (inv) {
+            if (mask & kScale_Mask) {
+                SkScalar invX = fMat[kMScaleX];
+                SkScalar invY = fMat[kMScaleY];
+                if (0 == invX || 0 == invY) {
+                    return false;
+                }
+                invX = SkScalarInvert(invX);
+                invY = SkScalarInvert(invY);
+
+                // Must be careful when writing to inv, since it may be the
+                // same memory as this.
+
+                inv->fMat[kMSkewX] = inv->fMat[kMSkewY] =
+                inv->fMat[kMPersp0] = inv->fMat[kMPersp1] = 0;
+
+                inv->fMat[kMScaleX] = invX;
+                inv->fMat[kMScaleY] = invY;
+                inv->fMat[kMPersp2] = kMatrix22Elem;
+                inv->fMat[kMTransX] = -SkScalarMul(fMat[kMTransX], invX);
+                inv->fMat[kMTransY] = -SkScalarMul(fMat[kMTransY], invY);
+
+                inv->setTypeMask(mask | kRectStaysRect_Mask);
+            } else {
+                // translate only
+                inv->setTranslate(-fMat[kMTransX], -fMat[kMTransY]);
+            }
+        }
+        return true;
+    }
+    
+    int         isPersp = mask & kPerspective_Mask;
     int         shift;
     SkDetScalar scale = sk_inv_determinant(fMat, isPersp, &shift);
 
diff --git a/tests/MatrixTest.cpp b/tests/MatrixTest.cpp
index 894278e84b..bba2e08683 100644
--- a/tests/MatrixTest.cpp
+++ b/tests/MatrixTest.cpp
@@ -384,13 +384,13 @@ static void TestMatrix(skiatest::Reporter* reporter) {
     iden1.setConcat(mat, inverse);
     REPORTER_ASSERT(reporter, is_identity(iden1));
 
-    mat.setScale(SkIntToScalar(2), SkIntToScalar(2));
+    mat.setScale(SkIntToScalar(2), SkIntToScalar(4));
     REPORTER_ASSERT(reporter, mat.invert(&inverse));
     iden1.setConcat(mat, inverse);
     REPORTER_ASSERT(reporter, is_identity(iden1));
     test_flatten(reporter, mat);
 
-    mat.setScale(SK_Scalar1/2, SK_Scalar1/2);
+    mat.setScale(SK_Scalar1/2, SkIntToScalar(2));
     REPORTER_ASSERT(reporter, mat.invert(&inverse));
     iden1.setConcat(mat, inverse);
     REPORTER_ASSERT(reporter, is_identity(iden1));
@@ -407,6 +407,11 @@ static void TestMatrix(skiatest::Reporter* reporter) {
     test_flatten(reporter, mat);
     test_flatten(reporter, iden2);
 
+    mat.setScale(0, SK_Scalar1);
+    REPORTER_ASSERT(reporter, !mat.invert(&inverse));
+    mat.setScale(SK_Scalar1, 0);
+    REPORTER_ASSERT(reporter, !mat.invert(&inverse));
+
     // rectStaysRect test
     {
         static const struct {