Fix the assert and clamping in SkMatrix::get*Scale[s]().

src/core/SkMatrix.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 "SkFloatBits.h"
 #include "SkMatrix.h"
 #include "SkNx.h"
@@ skipping 1553 matching lines @@
         } else if (kMax_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
             results[0] = apluscdiv2 + x;
         } else {
             results[0] = apluscdiv2 - x;
             results[1] = apluscdiv2 + x;
         }
     }
     if (!SkScalarIsFinite(results[0])) {
         return false;
     }
-    if (results[0] < 0 && results[0] > -SK_ScalarNearlyZero) {
+    SkASSERT(results[0] >= -SK_ScalarNearlyZero);

[reed1, 2016/07/18 14:10:06]

Thanks, this is better. Lets add a comment why we're checking for -nearly.
Something about numerical stability (esp. if you can reference where in the
above math the slight error might come from.)

Speaking of which, can you add the assert right after the questionable line?
e.g. right after the dot-prod or whereever?

+    if (results[0] < 0) {
         results[0] = 0;
     }
-    SkASSERT(results[0] >= 0);
     results[0] = SkScalarSqrt(results[0]);
     if (kBoth_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
         if (!SkScalarIsFinite(results[1])) {
             return false;
         }
-        if (results[1] < 0 && results[1] > -SK_ScalarNearlyZero) {
+        SkASSERT(results[1] >= -SK_ScalarNearlyZero);
+        if (results[1] < 0) {
             results[1] = 0;
         }
-        SkASSERT(results[1] >= 0);
         results[1] = SkScalarSqrt(results[1]);
     }
     return true;
 }
 
 SkScalar SkMatrix::getMinScale() const {
     SkScalar factor;
     if (get_scale_factor<kMin_MinMaxOrBoth>(this->getType(), fMat, &factor)) {
         return factor;
     } else {
@@ skipping 286 matching lines @@
     const SkScalar m10 = -m01;
     const SkScalar m11 = m00;
     const SkScalar m12 = fTy;
 
     quad[0].set(m02, m12);
     quad[1].set(m00 * width + m02, m10 * width + m12);
     quad[2].set(m00 * width + m01 * height + m02, m10 * width + m11 * height + m12);
     quad[3].set(m01 * height + m02, m11 * height + m12);
 #endif
 }