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diff --git a/image/Downscaler.h b/image/Downscaler.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Downscaler is a high-quality, streaming image downscaler based upon Skia's
+ * scaling implementation.
+ */
+
+#ifndef mozilla_image_Downscaler_h
+#define mozilla_image_Downscaler_h
+
+#include "mozilla/Maybe.h"
+#include "mozilla/UniquePtr.h"
+#include "gfxPoint.h"
+#include "nsRect.h"
+#include "mozilla/gfx/ConvolutionFilter.h"
+#include "mozilla/gfx/Matrix.h"
+
+namespace mozilla {
+namespace image {
+
+/**
+ * DownscalerInvalidRect wraps two invalidation rects: one in terms of the
+ * original image size, and one in terms of the target size.
+ */
+struct DownscalerInvalidRect {
+  nsIntRect mOriginalSizeRect;
+  nsIntRect mTargetSizeRect;
+};
+
+/**
+ * Downscaler is a high-quality, streaming image downscaler based upon Skia's
+ * scaling implementation.
+ *
+ * Decoders can construct a Downscaler once they know their target size, then
+ * call BeginFrame() for each frame they decode. They should write a decoded row
+ * into the buffer returned by RowBuffer(), and then call CommitRow() to signal
+ * that they have finished.
+ *
+
+ * Because invalidations need to be computed in terms of the scaled version of
+ * the image, Downscaler also tracks them. Decoders can call HasInvalidation()
+ * and TakeInvalidRect() instead of tracking invalidations themselves.
+ */
+class Downscaler {
+ public:
+  /// Constructs a new Downscaler which to scale to size @aTargetSize.
+  explicit Downscaler(const nsIntSize& aTargetSize);
+  ~Downscaler();
+
+  const nsIntSize& OriginalSize() const { return mOriginalSize; }
+  const nsIntSize& TargetSize() const { return mTargetSize; }
+  const nsIntSize FrameSize() const {
+    return nsIntSize(mFrameRect.Width(), mFrameRect.Height());
+  }
+  const gfx::MatrixScalesDouble& Scale() const { return mScale; }
+
+  /**
+   * Begins a new frame and reinitializes the Downscaler.
+   *
+   * @param aOriginalSize The original size of this frame, before scaling.
+   * @param aFrameRect The region of  the original image which has data.
+   *                   Every pixel outside @aFrameRect is considered blank and
+   *                   has zero alpha.
+   * @param aOutputBuffer The buffer to which the Downscaler should write its
+   *                      output; this is the same buffer where the Decoder
+   *                      would write its output when not downscaling during
+   *                      decode.
+   * @param aHasAlpha Whether or not this frame has an alpha channel.
+   *                  Performance is a little better if it doesn't have one.
+   * @param aFlipVertically If true, output rows will be written to the output
+   *                        buffer in reverse order vertically, which matches
+   *                        the way they are stored in some image formats.
+   */
+  nsresult BeginFrame(const nsIntSize& aOriginalSize,
+                      const Maybe<nsIntRect>& aFrameRect,
+                      uint8_t* aOutputBuffer, bool aHasAlpha,
+                      bool aFlipVertically = false);
+
+  bool IsFrameComplete() const {
+    return mCurrentInLine >= mOriginalSize.height;
+  }
+
+  /// Retrieves the buffer into which the Decoder should write each row.
+  uint8_t* RowBuffer() {
+    return mRowBuffer.get() + mFrameRect.X() * sizeof(uint32_t);
+  }
+
+  /// Clears the current row buffer.
+  void ClearRow() { ClearRestOfRow(0); }
+
+  /// Clears the current row buffer starting at @aStartingAtCol.
+  void ClearRestOfRow(uint32_t aStartingAtCol);
+
+  /// Signals that the decoder has finished writing a row into the row buffer.
+  void CommitRow();
+
+  /// Returns true if there is a non-empty invalid rect available.
+  bool HasInvalidation() const;
+
+  /// Takes the Downscaler's current invalid rect and resets it.
+  DownscalerInvalidRect TakeInvalidRect();
+
+  /**
+   * Resets the Downscaler's position in the image, for a new progressive pass
+   * over the same frame. Because the same data structures can be reused, this
+   * is more efficient than calling BeginFrame.
+   */
+  void ResetForNextProgressivePass();
+
+ private:
+  void DownscaleInputLine();
+  void ReleaseWindow();
+  void SkipToRow(int32_t aRow);
+
+  nsIntSize mOriginalSize;
+  nsIntSize mTargetSize;
+  nsIntRect mFrameRect;
+  gfx::MatrixScalesDouble mScale;
+
+  uint8_t* mOutputBuffer;
+
+  UniquePtr<uint8_t[]> mRowBuffer;
+  UniquePtr<uint8_t*[]> mWindow;
+
+  gfx::ConvolutionFilter mXFilter;
+  gfx::ConvolutionFilter mYFilter;
+
+  int32_t mWindowCapacity;
+
+  int32_t mLinesInBuffer;
+  int32_t mPrevInvalidatedLine;
+  int32_t mCurrentOutLine;
+  int32_t mCurrentInLine;
+
+  bool mHasAlpha : 1;
+  bool mFlipVertically : 1;
+};
+
+}  // namespace image
+}  // namespace mozilla
+
+#endif  // mozilla_image_Downscaler_h