Fully check basis vectors for extent degeneracy.

src/core/SkScalerContext.cpp

 
 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "SkScalerContext.h"
@@ skipping 706 matching lines @@
     this->getSingleMatrix(&A);
 
     // The caller may find the 'total' matrix useful when dealing directly with EM sizes.
     if (A_out) {
         *A_out = A;
     }
 
     // If the 'total' matrix is singular, set the 'scale' to something finite and zero the matrices.
     // All underlying ports have issues with zero text size, so use the matricies to zero.
 
-    // Map the vectors [1,1] and [1,-1] (the EM) through the 'total' matrix.
+    // Map the vectors [0,1], [1,0], [1,1] and [1,-1] (the EM) through the 'total' matrix.
     // If the length of one of these vectors is less than 1/256 then an EM filling square will
     // never affect any pixels.
-    SkVector diag[2] = { { A.getScaleX() + A.getSkewX(), A.getScaleY() + A.getSkewY() },
+    SkVector diag[4] = { { A.getScaleX()               ,                 A.getSkewY() },

[jvanverth1, 2016/02/25 19:00:33]

I think you're multiplying these as row vectors, but it shouldn't matter for the
calculation. LGTM

+                         {                 A.getSkewX(), A.getScaleY()                },
+                         { A.getScaleX() + A.getSkewX(), A.getScaleY() + A.getSkewY() },
                          { A.getScaleX() - A.getSkewX(), A.getScaleY() - A.getSkewY() }, };
     if (diag[0].lengthSqd() <= SK_ScalarNearlyZero * SK_ScalarNearlyZero ||
-        diag[1].lengthSqd() <= SK_ScalarNearlyZero * SK_ScalarNearlyZero)
+        diag[1].lengthSqd() <= SK_ScalarNearlyZero * SK_ScalarNearlyZero ||
+        diag[2].lengthSqd() <= SK_ScalarNearlyZero * SK_ScalarNearlyZero ||
+        diag[3].lengthSqd() <= SK_ScalarNearlyZero * SK_ScalarNearlyZero)
     {
         s->fX = SK_Scalar1;
         s->fY = SK_Scalar1;
         sA->setScale(0, 0);
         if (GsA) {
             GsA->setScale(0, 0);
         }
         if (G_inv) {
             G_inv->reset();
         }
@@ skipping 124 matching lines @@
 
 SkScalerContext* SkTypeface::createScalerContext(const SkDescriptor* desc,
                                                  bool allowFailure) const {
     SkScalerContext* c = this->onCreateScalerContext(desc);
 
     if (!c && !allowFailure) {
         c = new SkScalerContext_Empty(const_cast<SkTypeface*>(this), desc);
     }
     return c;
 }