Circular shadow fixes for Flutter.

* Fix spot shadow placement for SkSpotShadowMaskFilter.
* Make sure we don't try to render an oval as a plain RRect
  due to floating point error.
* Use fast path for uncached circles.
* Make sure ShadowMaskFilters can handle near-circles.

Change-Id: Ia9967a00a6e1c980a1c0a7ba8248f09fde61a3b7
Reviewed-on: https://skia-review.googlesource.com/13969
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>

2 files changed, 25 insertions, 22 deletions

diff --git a/src/effects/shadows/SkAmbientShadowMaskFilter.cpp b/src/effects/shadows/SkAmbientShadowMaskFilter.cpp
index e6a8de4141..84f9836b52 100644
--- a/src/effects/shadows/SkAmbientShadowMaskFilter.cpp
+++ b/src/effects/shadows/SkAmbientShadowMaskFilter.cpp
@@ -163,9 +163,8 @@ bool SkAmbientShadowMaskFilterImpl::directFilterMaskGPU(GrContext* context,
     }
 
     // if circle
-    // TODO: switch to SkScalarNearlyEqual when either oval renderer is updated or we
-    // have our own GeometryProc.
-    if (path.isOval(nullptr) && path.getBounds().width() == path.getBounds().height()) {
+    if (path.isOval(nullptr) && SkScalarNearlyEqual(path.getBounds().width(),
+                                                    path.getBounds().height())) {
         SkRRect rrect = SkRRect::MakeOval(path.getBounds());
         return this->directFilterRRectMaskGPU(context, rtContext, std::move(paint), clip,
                                               SkMatrix::I(), strokeRec, rrect, rrect);
diff --git a/src/effects/shadows/SkSpotShadowMaskFilter.cpp b/src/effects/shadows/SkSpotShadowMaskFilter.cpp
index ee82342ecc..93c7e9c0fa 100644
--- a/src/effects/shadows/SkSpotShadowMaskFilter.cpp
+++ b/src/effects/shadows/SkSpotShadowMaskFilter.cpp
@@ -180,9 +180,8 @@ bool SkSpotShadowMaskFilterImpl::directFilterMaskGPU(GrContext* context,
     }
 
     // if circle
-    // TODO: switch to SkScalarNearlyEqual when either oval renderer is updated or we
-    // have our own GeometryProc.
-    if (path.isOval(nullptr) && path.getBounds().width() == path.getBounds().height()) {
+    if (path.isOval(nullptr) && SkScalarNearlyEqual(path.getBounds().width(),
+                                                    path.getBounds().height())) {
         SkRRect rrect = SkRRect::MakeOval(path.getBounds());
         return this->directFilterRRectMaskGPU(context, rtContext, std::move(paint), clip,
                                               SkMatrix::I(), strokeRec, rrect, rrect);
@@ -211,7 +210,7 @@ bool SkSpotShadowMaskFilterImpl::directFilterRRectMaskGPU(GrContext*,
     if (SkStrokeRec::kFill_Style != strokeRec.getStyle()) {
         return false;
     }
-    // Fast path only supports simple rrects with circular corners.
+    // Fast path only supports simple rrects with near-circular corners.
     SkASSERT(devRRect.allCornersCircular());
     if (!rrect.isRect() && !rrect.isOval() && !rrect.isSimple()) {
         return false;
@@ -236,7 +235,9 @@ bool SkSpotShadowMaskFilterImpl::directFilterRRectMaskGPU(GrContext*,
     if (fSpotAlpha > 0.0f) {
         float zRatio = SkTPin(fOccluderHeight / (fLightPos.fZ - fOccluderHeight), 0.0f, 0.95f);
 
-        SkScalar srcSpaceSpotRadius = 2.0f * fLightRadius * zRatio;
+        SkScalar devSpaceSpotRadius = 2.0f * fLightRadius * zRatio;
+        // handle scale of radius and pad due to CTM
+        const SkScalar srcSpaceSpotRadius = devSpaceSpotRadius / scaleFactor;
 
         SkRRect spotRRect;
         if (isRect) {
@@ -250,18 +251,18 @@ bool SkSpotShadowMaskFilterImpl::directFilterRRectMaskGPU(GrContext*,
         const SkScalar scale = fLightPos.fZ / (fLightPos.fZ - fOccluderHeight);
         spotRRect.transform(SkMatrix::MakeScale(scale, scale), &spotShadowRRect);
 
-        SkPoint center = SkPoint::Make(spotShadowRRect.rect().centerX(),
-                                       spotShadowRRect.rect().centerY());
+        SkPoint spotOffset = SkPoint::Make(zRatio*(-fLightPos.fX), zRatio*(-fLightPos.fY));
+        // Adjust for the effect of the scale.
+        spotOffset.fX += scale*viewMatrix[SkMatrix::kMTransX];
+        spotOffset.fY += scale*viewMatrix[SkMatrix::kMTransY];
+        // This offset is in dev space, need to transform it into source space.
         SkMatrix ctmInverse;
         if (!viewMatrix.invert(&ctmInverse)) {
             SkDebugf("Matrix is degenerate. Will not render spot shadow!\n");
             //**** TODO: this is not good
             return true;
         }
-        SkPoint lightPos2D = SkPoint::Make(fLightPos.fX, fLightPos.fY);
-        ctmInverse.mapPoints(&lightPos2D, 1);
-        const SkPoint spotOffset = SkPoint::Make(zRatio*(center.fX - lightPos2D.fX),
-                                                 zRatio*(center.fY - lightPos2D.fY));
+        ctmInverse.mapPoints(&spotOffset, 1);
 
         // We want to extend the stroked area in so that it meets up with the caster
         // geometry. The stroked geometry will, by definition already be inset half the
@@ -292,17 +293,20 @@ bool SkSpotShadowMaskFilterImpl::directFilterRRectMaskGPU(GrContext*,
         } else {
             // Since we can't have unequal strokes, inset the shadow rect so the inner
             // and outer edges of the stroke will land where we want.
-            SkRect insetRect = spotShadowRRect.rect().makeInset(insetAmount / 2.0f,
-                                                                insetAmount / 2.0f);
-            SkScalar insetRad = SkTMax(spotShadowRRect.getSimpleRadii().fX - insetAmount / 2.0f,
-                                       minRadius);
-            spotShadowRRect = SkRRect::MakeRectXY(insetRect, insetRad, insetRad);
+            insetAmount *= 0.5f;
+            SkRect insetRect = spotShadowRRect.rect().makeInset(insetAmount, insetAmount);
+            // If the shadowRRect was an oval then its inset will also be one.
+            // We set it explicitly to avoid errors.
+            if (spotShadowRRect.isOval()) {
+                spotShadowRRect = SkRRect::MakeOval(insetRect);
+            } else {
+                SkScalar insetRad = SkTMax(spotShadowRRect.getSimpleRadii().fX - insetAmount,
+                                           minRadius);
+                spotShadowRRect = SkRRect::MakeRectXY(insetRect, insetRad, insetRad);
+            }
             spotStrokeRec.setStrokeStyle(strokeWidth, false);
         }
 
-        // handle scale of radius and pad due to CTM
-        const SkScalar devSpaceSpotRadius = srcSpaceSpotRadius * scaleFactor;
-
         spotShadowRRect.offset(spotOffset.fX, spotOffset.fY);
 
         rtContext->drawShadowRRect(clip, std::move(paint), viewMatrix, spotShadowRRect,