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112 lines
5.0 KiB
C++
112 lines
5.0 KiB
C++
/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 4 -*-
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef _GFXALPHARECOVERY_H_
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#define _GFXALPHARECOVERY_H_
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#include "mozilla/SSE.h"
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#include "gfxTypes.h"
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#include "mozilla/gfx/Rect.h"
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class gfxImageSurface;
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class gfxAlphaRecovery {
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public:
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/**
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* Some SIMD fast-paths only can be taken if the relative
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* byte-alignment of images' pointers and strides meets certain
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* criteria. Aligning image pointers and strides by
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* |GoodAlignmentLog2()| below will ensure that fast-paths aren't
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* skipped because of misalignment. Fast-paths may still be taken
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* even if GoodAlignmentLog2() is not met, in some conditions.
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*/
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static uint32_t GoodAlignmentLog2() { return 4; /* for SSE2 */ }
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/* Given two surfaces of equal size with the same rendering, one onto a
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* black background and the other onto white, recovers alpha values from
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* the difference and sets the alpha values on the black surface.
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* The surfaces must have format RGB24 or ARGB32.
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* Returns true on success.
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*/
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static bool RecoverAlpha (gfxImageSurface *blackSurface,
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const gfxImageSurface *whiteSurface);
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#ifdef MOZILLA_MAY_SUPPORT_SSE2
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/* This does the same as the previous function, but uses SSE2
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* optimizations. Usually this should not be called directly. Be sure to
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* check mozilla::supports_sse2() before calling this function.
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*/
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static bool RecoverAlphaSSE2 (gfxImageSurface *blackSurface,
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const gfxImageSurface *whiteSurface);
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/**
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* A common use-case for alpha recovery is to paint into a
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* temporary "white image", then paint onto a subrect of the
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* surface, the "black image", into which alpha-recovered pixels
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* are eventually to be written. This function returns a rect
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* aligned so that recovering alpha for that rect will hit SIMD
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* fast-paths, if possible. It's not always possible to align
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* |aRect| so that fast-paths will be taken.
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*
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* The returned rect is always a superset of |aRect|.
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*/
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static mozilla::gfx::IntRect AlignRectForSubimageRecovery(const mozilla::gfx::IntRect& aRect,
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gfxImageSurface* aSurface);
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#else
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static mozilla::gfx::IntRect AlignRectForSubimageRecovery(const mozilla::gfx::IntRect& aRect,
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gfxImageSurface*)
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{ return aRect; }
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#endif
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/** from cairo-xlib-utils.c, modified */
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/**
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* Given the RGB data for two image surfaces, one a source image composited
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* with OVER onto a black background, and one a source image composited with
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* OVER onto a white background, reconstruct the original image data into
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* black_data.
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*
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* Consider a single color channel and a given pixel. Suppose the original
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* premultiplied color value was C and the alpha value was A. Let the final
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* on-black color be B and the final on-white color be W. All values range
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* over 0-255.
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*
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* Then B=C and W=(255*(255 - A) + C*255)/255. Solving for A, we get
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* A=255 - (W - C). Therefore it suffices to leave the black_data color
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* data alone and set the alpha values using that simple formula. It shouldn't
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* matter what color channel we pick for the alpha computation, but we'll
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* pick green because if we went through a color channel downsample the green
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* bits are likely to be the most accurate.
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*
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* This function needs to be in the header file since it's used by both
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* gfxRecoverAlpha.cpp and gfxRecoverAlphaSSE2.cpp.
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*/
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static inline uint32_t
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RecoverPixel(uint32_t black, uint32_t white)
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{
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const uint32_t GREEN_MASK = 0x0000FF00;
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const uint32_t ALPHA_MASK = 0xFF000000;
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/* |diff| here is larger when the source image pixel is more transparent.
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If both renderings are from the same source image composited with OVER,
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then the color values on white will always be greater than those on
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black, so |diff| would not overflow. However, overflow may happen, for
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example, when a plugin plays a video and the image is rapidly changing.
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If there is overflow, then behave as if we limit to the difference to
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>= 0, which will make the rendering opaque. (Without this overflow
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will make the rendering transparent.) */
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uint32_t diff = (white & GREEN_MASK) - (black & GREEN_MASK);
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/* |diff| is 0xFFFFxx00 on overflow and 0x0000xx00 otherwise, so use this
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to limit the transparency. */
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uint32_t limit = diff & ALPHA_MASK;
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/* The alpha bits of the result */
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uint32_t alpha = (ALPHA_MASK - (diff << 16)) | limit;
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return alpha | (black & ~ALPHA_MASK);
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}
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};
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#endif /* _GFXALPHARECOVERY_H_ */
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