author Robert O'Callahan <>
Fri, 16 Jul 2010 09:08:10 +1200
changeset 47767 1e2f6e38cd981923b9f8d6612ed9a170326dff85
parent 47748 0615ff4efbdce6b19e9467eea816c5e7e111f6d5
permissions -rw-r--r--
Bug 564991. Part 34: Make sure that ThebesLayerBuffers are always allocated as similar surfaces to the widget surface, whenever possible. r=cjones

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#include "gfxContext.h"
#include "gfxASurface.h"
#include "nsRegion.h"

namespace mozilla {
namespace layers {

class ThebesLayer;

 * This class encapsulates the buffer used to retain ThebesLayer contents,
 * i.e., the contents of the layer's GetVisibleRegion().
 * This is a cairo/Thebes surface, but with a literal twist. Scrolling
 * causes the layer's visible region to move. We want to keep
 * reusing the same surface if the region size hasn't changed, but we don't
 * want to keep moving the contents of the surface around in memory. So
 * we use a trick.
 * Consider just the vertical case, and suppose the buffer is H pixels
 * high and we're scrolling down by N pixels. Instead of copying the
 * buffer contents up by N pixels, we leave the buffer contents in place,
 * and paint content rows H to H+N-1 into rows 0 to N-1 of the buffer.
 * Then we can refresh the screen by painting rows N to H-1 of the buffer
 * at row 0 on the screen, and then painting rows 0 to N-1 of the buffer
 * at row H-N on the screen.
 * mBufferRotation.y would be N in this example.
class ThebesLayerBuffer {
  ThebesLayerBuffer() : mBufferRotation(0,0)

   * Wipe out all retained contents. Call this when the entire
   * buffer becomes invalid.
  void Clear()
    mBuffer = nsnull;

   * This is returned by BeginPaint. The caller should draw into mContext.
   * mRegionToDraw must be drawn. mRegionToInvalidate has been invalidated
   * by ThebesLayerBuffer and must be redrawn on the screen.
   * mRegionToInvalidate is set when the buffer has changed from
   * opaque to transparent or vice versa, since the details of rendering can
   * depend on the buffer type.
  struct PaintState {
    nsRefPtr<gfxContext> mContext;
    nsIntRegion mRegionToDraw;
    nsIntRegion mRegionToInvalidate;
   * Pass OPAQUE_CONTENT when we have determined that everything visible
   * in the buffer will be rendered with opaque pixels.
  enum {
   * Start a drawing operation. This returns a PaintState describing what
   * needs to be drawn to bring the buffer up to date in the visible region.
   * This queries aLayer to get the currently valid and visible regions.
   * The returned mContext may be null if mRegionToDraw is empty.
   * Otherwise it must not be null.
   * mRegionToInvalidate will contain mRegionToDraw.
   * @param aReferenceSurface if we need to create a buffer, we'll create
   * a surface that's similar to aReferenceSurface
  PaintState BeginPaint(ThebesLayer* aLayer, gfxASurface* aReferenceSurface,
                        PRUint32 aFlags);
   * Complete the drawing operation. The region to draw must have been drawn
   * before this is called. The contents of the buffer are drawn to aTarget.
  void DrawTo(ThebesLayer* aLayer, PRUint32 aFlags, gfxContext* aTarget, float aOpacity);

   * Get the underlying buffer, if any. This is useful because we can pass
   * in the buffer as the default "reference surface" if there is one.
   * Don't use it for anything else!
  gfxASurface* GetBuffer() { return mBuffer; }

  enum XSide {
  enum YSide {
  nsIntRect GetQuadrantRectangle(XSide aXSide, YSide aYSide);
  void DrawBufferQuadrant(gfxContext* aTarget, XSide aXSide, YSide aYSide, float aOpacity);
  void DrawBufferWithRotation(gfxContext* aTarget, float aOpacity);

  nsRefPtr<gfxASurface> mBuffer;
  /** The area of the ThebesLayer that is covered by the buffer as a whole */
  nsIntRect             mBufferRect;
   * The x and y rotation of the buffer. Conceptually the buffer
   * has its origin translated to mBufferRect.TopLeft() - mBufferRotation,
   * is tiled to fill the plane, and the result is clipped to mBufferRect.
   * So the pixel at mBufferRotation within the buffer is what gets painted at
   * mBufferRect.TopLeft().
   * This is "rotation" in the sense of rotating items in a linear buffer,
   * where items falling off the end of the buffer are returned to the
   * buffer at the other end, not 2D rotation!
  nsIntPoint            mBufferRotation;