gecko/gfx/2d/Blur.h

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#include "mozilla/gfx/Rect.h"
#include "mozilla/gfx/Point.h"

namespace mozilla {
namespace gfx {

/**
 * 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, and blurs the contents.
 *
 * 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 AlphaBoxBlur
{
public:

  /** Constructs a box blur and initializes the backing 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.
   */
  AlphaBoxBlur(const Rect& aRect,
               const IntSize& aSpreadRadius,
               const IntSize& aBlurRadius,
               const Rect* aDirtyRect,
               const Rect* aSkipRect);

  ~AlphaBoxBlur();

  /**
   * Return the pointer to memory allocated by the constructor for the 8-bit
   * alpha surface you need to be blurred. After you draw to this surface, call
   * Blur(), below, to have its contents blurred.
   */
  unsigned char* GetData();

  /**
   * Return the size, in pixels, of the 8-bit alpha surface backed by the
   * pointer returned by GetData().
   */
  IntSize GetSize();

  /**
   * Return the stride, in bytes, of the 8-bit alpha surface backed by the
   * pointer returned by GetData().
   */
  int32_t GetStride();

  /**
   * Returns the device-space rectangle the 8-bit alpha surface covers.
   */
  IntRect GetRect();

  /**
   * Return a pointer to a dirty rect, as passed in to the constructor, or NULL
   * if none was passed in.
   */
  Rect* GetDirtyRect();

  /**
   * Perform the blur in-place on the surface backed by the pointer returned by
   * GetData().
   */
  void Blur();

  /**
   * 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 AlphaBoxBlur's
   * constructor, above.
   */
  static IntSize CalculateBlurRadius(const Point& aStandardDeviation);

private:

  /**
   * A rect indicating the area where blurring is unnecessary, and the blur
   * algorithm should skip over it.
   */
  IntRect mSkipRect;

  /**
   * The device-space rectangle the the backing 8-bit alpha surface covers.
   */
  IntRect mRect;

  /**
   * A copy of the dirty rect passed to the constructor. This will only be valid if
   * mHasDirtyRect is true.
   */
  Rect mDirtyRect;

  /**
   * The spread radius, in pixels.
   */
  IntSize mSpreadRadius;

  /**
   * The blur radius, in pixels.
   */
  IntSize mBlurRadius;

  /**
   * A pointer to the backing 8-bit alpha surface.
   */
  unsigned char* mData;

  /**
   * The stride of the data contained in mData.
   */
  int32_t mStride;

  /**
   * Whether mDirtyRect contains valid data.
   */
  bool mHasDirtyRect;
};

}
}