gecko/gfx/thebes/gfxAlphaRecovery.cpp

/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Thebes gfx.
 *
 * The Initial Developer of the Original Code is Mozilla Foundation.
 * Portions created by the Initial Developer are Copyright (C) 2007
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *   Vladimir Vukicevic <vladimir@pobox.com>
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either the GNU General Public License Version 2 or later (the "GPL"), or
 * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
 * of those above. If you wish to allow use of your version of this file only
 * under the terms of either the GPL or the LGPL, and not to allow others to
 * use your version of this file under the terms of the MPL, indicate your
 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK ***** */

#include "gfxAlphaRecovery.h"

#include "gfxImageSurface.h"


/** 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.
 */

static inline PRUint32
RecoverPixel(PRUint32 black, PRUint32 white)
{
    const PRUint32 GREEN_MASK = 0x0000FF00;
    const PRUint32 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.) */
    PRUint32 diff = (white & GREEN_MASK) - (black & GREEN_MASK);
    /* |diff| is 0xFFFFxx00 on overflow and 0x0000xx00 otherwise, so use this
        to limit the transparency. */
    PRUint32 limit = diff & ALPHA_MASK;
    /* The alpha bits of the result */
    PRUint32 alpha = (ALPHA_MASK - (diff << 16)) | limit;

    return alpha | (black & ~ALPHA_MASK);
}

/* static */ PRBool
gfxAlphaRecovery::RecoverAlpha(gfxImageSurface* blackSurf,
                               const gfxImageSurface* whiteSurf,
                               Analysis* analysis)
{
    gfxIntSize size = blackSurf->GetSize();

    if (size != whiteSurf->GetSize() ||
        (blackSurf->Format() != gfxASurface::ImageFormatARGB32 &&
         blackSurf->Format() != gfxASurface::ImageFormatRGB24) ||
        (whiteSurf->Format() != gfxASurface::ImageFormatARGB32 &&
         whiteSurf->Format() != gfxASurface::ImageFormatRGB24))
        return PR_FALSE;

    blackSurf->Flush();
    whiteSurf->Flush();

    unsigned char* blackData = blackSurf->Data();
    unsigned char* whiteData = whiteSurf->Data();

    /* Get the alpha value of 'first' */
    PRUint32 first;
    if (size.width == 0 || size.height == 0) {
        first = 0;
    } else {
        if (!blackData || !whiteData)
            return PR_FALSE;

        first = RecoverPixel(*reinterpret_cast<PRUint32*>(blackData),
                             *reinterpret_cast<PRUint32*>(whiteData));
    }

    PRUint32 deltas = 0;
    for (PRInt32 i = 0; i < size.height; ++i) {
        PRUint32* blackPixel = reinterpret_cast<PRUint32*>(blackData);
        const PRUint32* whitePixel = reinterpret_cast<PRUint32*>(whiteData);
        for (PRInt32 j = 0; j < size.width; ++j) {
            PRUint32 recovered = RecoverPixel(blackPixel[j], whitePixel[j]);
            blackPixel[j] = recovered;
            deltas |= (first ^ recovered);
        }
        blackData += blackSurf->Stride();
        whiteData += whiteSurf->Stride();
    }

    blackSurf->MarkDirty();
    
    if (analysis) {
        analysis->uniformAlpha = (deltas >> 24) == 0;
        analysis->uniformColor = PR_FALSE;
        if (analysis->uniformAlpha) {
            double d_first_alpha = first >> 24;
            analysis->alpha = d_first_alpha/255.0;
            /* we only set uniformColor when the alpha is already uniform.
               it's only useful in that case ... and if the alpha was nonuniform
               then computing whether the color is uniform would require unpremultiplying
               every pixel */
            analysis->uniformColor = deltas == 0;
            if (analysis->uniformColor) {
                if (d_first_alpha == 0.0) {
                    /* can't unpremultiply, this is OK */
                    analysis->r = analysis->g = analysis->b = 0.0;
                } else {
                    analysis->r = (first & 0xFF)/d_first_alpha;
                    analysis->g = ((first >> 8) & 0xFF)/d_first_alpha;
                    analysis->b = ((first >> 16) & 0xFF)/d_first_alpha;
                }
            }
        }
    }

    return PR_TRUE;
}