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

src/core/SkMatrix.cpp

Index: src/core/SkMatrix.cpp
diff --git a/src/core/SkMatrix.cpp b/src/core/SkMatrix.cpp
index fc698530ec9d2b01a31778d70a5198b5ea61e993..149aca948fb43e877090d04b903f1ad7742a62e2 100644
--- a/src/core/SkMatrix.cpp
+++ b/src/core/SkMatrix.cpp
@@ -1571,19 +1571,19 @@ template <MinMaxOrBoth MIN_MAX_OR_BOTH> bool get_scale_factor(SkMatrix::TypeMask
     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;