Implement computeFastBounds in SkLocalMatrixImageFilter

This also adds a GM based on Jim's fiddle: https://fiddle.skia.org/c/781234e35c208ee03f79b90613117b91
I confirmed that it never flickers when animating in viewer.

Patchset 1 shows the GM being blank (with the computeFastBounds fix
disabled). Patchset 3 shows that with the fix enabled, the GM is not
blank.

Bug: skia:9282
Change-Id: I206f7150c395b0a35ecf0455e4905f72ae057e6b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295558
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>

gm/imagefilterstransformed.cpp

@@ -25,7 +25,9 @@
 #include "include/core/SkTypes.h"
 #include "include/effects/SkGradientShader.h"
 #include "include/effects/SkImageFilters.h"
+#include "tools/Resources.h"
 #include "tools/ToolUtils.h"
+#include "tools/timer/TimeUtils.h"
 
 #include <utility>
 
@@ -161,3 +163,56 @@ DEF_SIMPLE_GM(rotate_imagefilter, canvas, 500, 500) {
         canvas->translate(0, 150);
     }
 }
+
+class ImageFilterMatrixWLocalMatrix : public skiagm::GM {
+public:
+
+    // Start at 132 degrees, since that resulted in a skipped draw before the fix to
+    // SkLocalMatrixImageFilter's computeFastBounds() function.
+    ImageFilterMatrixWLocalMatrix() : fDegrees(132.f) {}
+
+protected:
+    SkString onShortName() override {
+        return SkString("imagefilter_matrix_localmatrix");
+    }
+
+    SkISize onISize() override {
+        return SkISize::Make(512, 512);
+    }
+
+    bool onAnimate(double nanos) override {
+        // Animate the rotation angle to ensure the local matrix bounds modifications work
+        // for a variety of transformations.
+        fDegrees = TimeUtils::Scaled(1e-9f * nanos, 360.f);
+        return true;
+    }
+
+    void onOnceBeforeDraw() override {
+        fImage = GetResourceAsImage("images/mandrill_256.png");
+    }
+
+    void onDraw(SkCanvas* canvas) override {
+        SkMatrix localMatrix;
+        localMatrix.preTranslate(128, 128);
+        localMatrix.preScale(2.0f, 2.0f);
+
+        // This matrix applies a rotate around the center of the image (prior to the simulated
+        // hi-dpi 2x device scale).
+        SkMatrix filterMatrix;
+        filterMatrix.setRotate(fDegrees, 64, 64);
+
+        sk_sp<SkImageFilter> filter =
+                SkImageFilter::MakeMatrixFilter(filterMatrix, kLow_SkFilterQuality, nullptr)
+                             ->makeWithLocalMatrix(localMatrix);
+
+        SkPaint p;
+        p.setImageFilter(filter);
+        canvas->drawImage(fImage.get(), 128, 128, &p);
+    }
+
+private:
+    SkScalar fDegrees;
+    sk_sp<SkImage> fImage;
+};
+
+DEF_GM(return new ImageFilterMatrixWLocalMatrix();)

src/core/SkLocalMatrixImageFilter.cpp

@@ -53,3 +53,15 @@ SkIRect SkLocalMatrixImageFilter::onFilterBounds(const SkIRect& src, const SkMat
                                                  MapDirection dir, const SkIRect* inputRect) const {
     return this->getInput(0)->filterBounds(src, SkMatrix::Concat(ctm, fLocalM), dir, inputRect);
 }
+
+SkRect SkLocalMatrixImageFilter::computeFastBounds(const SkRect& bounds) const {
+    // In order to match the behavior of onFilterBounds, we map 'bounds' by the inverse of our
+    // local matrix, pass that to our child, and then map the result by our local matrix.
+    SkMatrix localInv;
+    if (!fLocalM.invert(&localInv)) {
+        return this->getInput(0)->computeFastBounds(bounds);
+    }
+
+    SkRect localBounds = localInv.mapRect(bounds);
+    return fLocalM.mapRect(this->getInput(0)->computeFastBounds(localBounds));
+}

src/core/SkLocalMatrixImageFilter.h

@@ -19,6 +19,8 @@ class SkLocalMatrixImageFilter : public SkImageFilter_Base {
 public:
     static sk_sp<SkImageFilter> Make(const SkMatrix& localM, sk_sp<SkImageFilter> input);
 
+    SkRect computeFastBounds(const SkRect&) const override;
+
 protected:
     void flatten(SkWriteBuffer&) const override;
     sk_sp<SkSpecialImage> onFilterImage(const Context&, SkIPoint* offset) const override;