Move common logic between BoxReflectFilterOperation and FEBoxReflect into BoxReflection.

third_party/WebKit/Source/platform/graphics/BoxReflection.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BoxReflection_h
+#define BoxReflection_h
+
+#include "platform/PlatformExport.h"
+#include "wtf/PassRefPtr.h"
+
+class SkImageFilter;
+class SkMatrix;
+
+namespace blink {
+
+class FloatRect;
+
+// A reflection, as created by -webkit-box-reflect. Consists of:
+// * a direction (either vertical or horizontal)
+// * an offset to be applied to the reflection after flipping about the
+//   x- or y-axis, according to the direction
+class PLATFORM_EXPORT BoxReflection {

[Stephen White, 2016/04/13 15:35:51]

<bikeshed> Could this be BoxReflectParams (my preference) or BoxReflectionParams
or BoxReflectionInfo or BoxReflectInfo or something? BoxReflection sounds to me
like it does actual reflection (rendering). </bikeshed>

[jbroman, 2016/04/13 17:59:15]

It could. I'd be OK with BoxReflectParams, though that implies it's a little
more structy (and knows less about math) than this. I would draw a comparison
with Gradient and Pattern.

[Stephen White, 2016/04/13 18:07:35]

OK, I buy that comparison.

+public:
+    enum ReflectionDirection {
+        // Vertically flipped (to appear above or below).
+        VerticalReflection,
+        // Horizontally flipped (to appear to the left or right).
+        HorizontalReflection,
+    };
+
+    BoxReflection(ReflectionDirection direction, float offset)
+        : m_direction(direction), m_offset(offset) {}
+
+    ReflectionDirection direction() const { return m_direction; }
+    float offset() const { return m_offset; }
+
+    // Returns a matrix which maps points between the original content and its
+    // reflection. Reflections are self-inverse, so this matrix can be used to
+    // map in either direction.
+    SkMatrix reflectionMatrix() const;
+
+    // Maps a source rectangle to the destination rectangle it can affect,
+    // including this reflection. Due to the symmetry of reflections, this can
+    // also be used to map from a destination rectangle to the source rectangle
+    // which contributes to it.
+    FloatRect mapRect(const FloatRect&) const;
+
+    // Creates a Skia image filter which reflects content according to the
+    // parameters of this reflection.
+    PassRefPtr<SkImageFilter> createImageFilter(PassRefPtr<SkImageFilter> input) const;

[Stephen White, 2016/04/13 15:35:51]

For dependencies and symmetry, could we keep the creation of the SkImageFilter
in the SkiaImageFilterBuilder? That way, this class stays a simple bag of data
and math. AFAICT, there's no refactoring to be had here, since FEBoxReflect
calls the builder already.

[jbroman, 2016/04/13 17:59:15]

I don't know what you mean by "for dependencies"; these are part of the same
component, so it's allowed to use Skia here.
Could. SkiaImageFilterBuilder seems a little weird, and this logic seems more
closely connected with the math (e.g. the math to make the reflection matrix)
than it is with any other other methods on SkiaImageFilterBuilder (which already
builds a minority of skia image filters).

[Stephen White, 2016/04/13 18:07:35]

True, but everything SkImageFilter-related lives in platform/graphics/filters 
(except for the oddball SVGFEImage, for dependency reasons). I'd like to keep
them together if possible.

[jbroman, 2016/04/13 22:24:52]

Done.

+
+private:
+    ReflectionDirection m_direction;
+    float m_offset;
+};
+
+inline bool operator==(const BoxReflection& a, const BoxReflection& b)
+{
+    return a.direction() == b.direction() && a.offset() == b.offset();
+}
+
+inline bool operator!=(const BoxReflection& a, const BoxReflection& b)
+{
+    return !(a == b);
+}
+
+} // namespace blink
+
+#endif // BoxReflection_h