Revert "Remove ambient clamp hack for analytic shadows."

This reverts commit d75b34394b5c53b0a5e8144dd28f956fbc8b90b2.

Reason for revert: Breaking GMs

Original change's description:
> Remove ambient clamp hack for analytic shadows.
> 
> The hack was added to make analytic ambient shadows match the geometric
> ones exactly. Removing it does produce a slight difference, but it's
> only visible at full black values (and then, only slightly) and this
> makes room to pass a blend parameter for falloff.
> 
> Also fixes an issue with filling out arcs in the shadow tesselator.
> 
> Bug: skia:7486
> Change-Id: If78d0104df7e18f4320c1658af75576f6a4a8c73
> Reviewed-on: https://skia-review.googlesource.com/122580
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Jim Van Verth <jvanverth@google.com>

TBR=jvanverth@google.com,bsalomon@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: skia:7486
Change-Id: If0a20a358759c08a3a475d60be54a3d62ff7b6ad
Reviewed-on: https://skia-review.googlesource.com/123120
Reviewed-by: Jim Van Verth <jvanverth@google.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/src/gpu/GrRenderTargetContext.cpp b/src/gpu/GrRenderTargetContext.cpp
index aae0248ae4..e26ec9018b 100644
--- a/src/gpu/GrRenderTargetContext.cpp
+++ b/src/gpu/GrRenderTargetContext.cpp
@@ -1024,10 +1024,15 @@ bool GrRenderTargetContext::drawFastShadow(const GrClip& clip,
             // set a large inset to force a fill
             devSpaceInsetWidth = ambientRRect.width();
         }
+        // the fraction of the blur we want to apply is devSpaceInsetWidth/devSpaceAmbientBlur,
+        // which is just 1/umbraRecipAlpha.
+        SkScalar blurClamp = SkScalarInvert(umbraRecipAlpha);
+
         std::unique_ptr<GrDrawOp> op = GrShadowRRectOp::Make(ambientColor, viewMatrix,
                                                              ambientRRect,
                                                              devSpaceAmbientBlur,
-                                                             devSpaceInsetWidth);
+                                                             devSpaceInsetWidth,
+                                                             blurClamp);
         SkASSERT(op);
         this->addDrawOp(clip, std::move(op));
     }