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diff --git a/gfx/thebes/gfxAlphaRecovery.h b/gfx/thebes/gfxAlphaRecovery.h
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+++ b/gfx/thebes/gfxAlphaRecovery.h
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+/* -*- Mode: C++; tab-width: 20; 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/. */
+
+#ifndef _GFXALPHARECOVERY_H_
+#define _GFXALPHARECOVERY_H_
+
+#include "gfxTypes.h"
+#include "mozilla/gfx/Rect.h"
+
+class gfxImageSurface;
+
+class gfxAlphaRecovery {
+ public:
+  /**
+   * Some SIMD fast-paths only can be taken if the relative
+   * byte-alignment of images' pointers and strides meets certain
+   * criteria.  Aligning image pointers and strides by
+   * |GoodAlignmentLog2()| below will ensure that fast-paths aren't
+   * skipped because of misalignment.  Fast-paths may still be taken
+   * even if GoodAlignmentLog2() is not met, in some conditions.
+   */
+  static uint32_t GoodAlignmentLog2() { return 4; /* for SSE2 */ }
+
+  /* Given two surfaces of equal size with the same rendering, one onto a
+   * black background and the other onto white, recovers alpha values from
+   * the difference and sets the alpha values on the black surface.
+   * The surfaces must have format RGB24 or ARGB32.
+   * Returns true on success.
+   */
+  static bool RecoverAlpha(gfxImageSurface* blackSurface,
+                           const gfxImageSurface* whiteSurface);
+
+  /* This does the same as the previous function, but uses SIMD
+   * optimizations. Usually this should not be called directly.
+   */
+  template <class Arch>
+  static bool RecoverAlphaGeneric(gfxImageSurface* blackSurface,
+                                  const gfxImageSurface* whiteSurface);
+
+  /** from cairo-xlib-utils.c, modified */
+  /**
+   * Given the RGB data for two image surfaces, one a source image composited
+   * with OVER onto a black background, and one a source image composited with
+   * OVER onto a white background, reconstruct the original image data into
+   * black_data.
+   *
+   * Consider a single color channel and a given pixel. Suppose the original
+   * premultiplied color value was C and the alpha value was A. Let the final
+   * on-black color be B and the final on-white color be W. All values range
+   * over 0-255.
+   *
+   * Then B=C and W=(255*(255 - A) + C*255)/255. Solving for A, we get
+   * A=255 - (W - C). Therefore it suffices to leave the black_data color
+   * data alone and set the alpha values using that simple formula. It shouldn't
+   * matter what color channel we pick for the alpha computation, but we'll
+   * pick green because if we went through a color channel downsample the green
+   * bits are likely to be the most accurate.
+   *
+   * This function needs to be in the header file since it's used by both
+   * gfxRecoverAlpha.cpp and gfxRecoverAlphaGeneric.hpp.
+   */
+
+  static inline uint32_t RecoverPixel(uint32_t black, uint32_t white) {
+    const uint32_t GREEN_MASK = 0x0000FF00;
+    const uint32_t ALPHA_MASK = 0xFF000000;
+
+    /* |diff| here is larger when the source image pixel is more
+       transparent.  If both renderings are from the same source image
+       composited with OVER, then the color values on white will always be
+       greater than those on black, so |diff| would not overflow.  However,
+       overflow may happen, for example, when a plugin plays a video and
+       the image is rapidly changing.  If there is overflow, then behave as
+       if we limit to the difference to
+       >= 0, which will make the rendering opaque.  (Without this overflow
+       will make the rendering transparent.) */
+    uint32_t diff = (white & GREEN_MASK) - (black & GREEN_MASK);
+    /* |diff| is 0xFFFFxx00 on overflow and 0x0000xx00 otherwise, so use
+       this to limit the transparency. */
+    uint32_t limit = diff & ALPHA_MASK;
+    /* The alpha bits of the result */
+    uint32_t alpha = (ALPHA_MASK - (diff << 16)) | limit;
+
+    return alpha | (black & ~ALPHA_MASK);
+  }
+};
+
+#endif /* _GFXALPHARECOVERY_H_ */