gecko-dev/gfx/thebes/gfxBlur.h

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 * 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
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 * The Original Code is gfx thebes code.
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 * The Initial Developer of the Original Code is Mozilla Foundation.
 * Portions created by the Initial Developer are Copyright (C) 2008
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 * Contributor(s):
 *   Eric Butler <zantifon@gmail.com>
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#ifndef GFX_BLUR_H
#define GFX_BLUR_H

#include "gfxContext.h"
#include "gfxImageSurface.h"
#include "gfxTypes.h"
#include "gfxUtils.h"
#include "nsRect.h"

/**
 * Implementation of a triple box blur approximation of a Gaussian blur.
 *
 * A Gaussian blur is good for blurring because, when done independently
 * in the horizontal and vertical directions, it matches the result that
 * would be obtained using a different (rotated) set of axes.  A triple
 * box blur is a very close approximation of a Gaussian.
 *
 * Creates an 8-bit alpha channel context for callers to draw in,
 * spreads the contents of that context, blurs the contents, and applies
 * it as an alpha mask on a different existing context.
 * 
 * A spread N makes each output pixel the maximum value of all source
 * pixels within a square of side length 2N+1 centered on the output pixel.
 * 
 * A temporary surface is created in the Init function. The caller then draws
 * any desired content onto the context acquired through GetContext, and lastly
 * calls Paint to apply the blurred content as an alpha mask.
 */
class THEBES_API gfxAlphaBoxBlur
{
public:
    gfxAlphaBoxBlur();

    ~gfxAlphaBoxBlur();

    /**
     * Constructs a box blur and initializes the temporary surface.
     * @param aRect The coordinates of the surface to create in device units.
     *
     * @param aBlurRadius The blur radius in pixels.  This is the radius of
     *   the entire (triple) kernel function.  Each individual box blur has
     *   radius approximately 1/3 this value, or diameter approximately 2/3
     *   this value.  This parameter should nearly always be computed using
     *   CalculateBlurRadius, below.
     *
     * @param aDirtyRect A pointer to a dirty rect, measured in device units, if available.
     *  This will be used for optimizing the blur operation. It is safe to pass NULL here.
     *
     * @param aSkipRect A pointer to a rect, measured in device units, that represents an area
     *  where blurring is unnecessary and shouldn't be done for speed reasons. It is safe to
     *  pass NULL here.
     */
    gfxContext* Init(const gfxRect& aRect,
                     const gfxIntSize& aSpreadRadius,
                     const gfxIntSize& aBlurRadius,
                     const gfxRect* aDirtyRect,
                     const gfxRect* aSkipRect);

    /**
     * Returns the context that should be drawn to supply the alpha mask to be
     * blurred. If the returned surface is null, then there was an error in
     * its creation.
     */
    gfxContext* GetContext()
    {
        return mContext;
    }

    /**
     * Premultiplies the image by the given alpha.
     */
    void PremultiplyAlpha(gfxFloat alpha);

    /**
     * Does the actual blurring/spreading and mask applying. Users of this
     * object must have drawn whatever they want to be blurred onto the internal
     * gfxContext returned by GetContext before calling this.
     *
     * @param aDestinationCtx The graphics context on which to apply the
     *  blurred mask.
     */
    void Paint(gfxContext* aDestinationCtx, const gfxPoint& offset = gfxPoint(0.0, 0.0));

    /**
     * Calculates a blur radius that, when used with box blur, approximates
     * a Gaussian blur with the given standard deviation.  The result of
     * this function should be used as the aBlurRadius parameter to Init,
     * above.
     */
    static gfxIntSize CalculateBlurRadius(const gfxPoint& aStandardDeviation);

protected:
    /**
     * The spread radius, in pixels.
     */
    gfxIntSize mSpreadRadius;
    /**
     * The blur radius, in pixels.
     */
    gfxIntSize mBlurRadius;

    /**
     * The context of the temporary alpha surface.
     */
    nsRefPtr<gfxContext> mContext;

    /**
     * The temporary alpha surface.
     */
    nsRefPtr<gfxImageSurface> mImageSurface;

    /**
     * A copy of the dirty rect passed to Init(). This will only be valid if
     * mHasDirtyRect is TRUE.
     */
    gfxRect mDirtyRect;
    /**
     * A rect indicating the area where blurring is unnecessary, and the blur
     * algorithm should skip over it.
     */
    nsIntRect mSkipRect;

    PRPackedBool mHasDirtyRect;
};

#endif /* GFX_BLUR_H */