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* 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
* 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.
* 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,
static gfxIntSize CalculateBlurRadius(const gfxPoint& aStandardDeviation);
* The spread radius, in pixels.
* The blur radius, in pixels.
* The context of the temporary alpha surface.
* The temporary alpha surface.
* A copy of the dirty rect passed to Init(). This will only be valid if
* mHasDirtyRect is TRUE.
* A rect indicating the area where blurring is unnecessary, and the blur
* algorithm should skip over it.
#endif /* GFX_BLUR_H */