b=576143 copy and draw over background to avoid alpha extraction when possible r=roc
authorKarl Tomlinson <karlt+@karlt.net>
Fri, 23 Jul 2010 10:39:22 +1200
changeset 48109 8fd528e132d7737b5fb2e5b1e610f59e4d390668
parent 48108 92559a5573bd3ef6159f8dab92931107ff581440
child 48110 02d92df7381df28ba6e5b070cc2d67298f4614ea
push id14574
push userktomlinson@mozilla.com
push dateThu, 22 Jul 2010 23:01:26 +0000
treeherdermozilla-central@02d92df7381d [default view] [failures only]
perfherder[talos] [build metrics] [platform microbench] (compared to previous push)
reviewersroc
bugs576143
milestone2.0b3pre
first release with
nightly linux32
nightly linux64
nightly mac
nightly win32
nightly win64
last release without
nightly linux32
nightly linux64
nightly mac
nightly win32
nightly win64
b=576143 copy and draw over background to avoid alpha extraction when possible r=roc
gfx/thebes/gfxXlibNativeRenderer.cpp
--- a/gfx/thebes/gfxXlibNativeRenderer.cpp
+++ b/gfx/thebes/gfxXlibNativeRenderer.cpp
@@ -35,16 +35,17 @@
  * 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 "gfxXlibNativeRenderer.h"
 
 #include "gfxXlibSurface.h"
+#include "gfxImageSurface.h"
 #include "gfxContext.h"
 #include "cairo-xlib.h"
 #include "cairo-xlib-xrender.h"
 #include <stdlib.h>
 
 #if   HAVE_STDINT_H
 #include <stdint.h>
 #elif HAVE_INTTYPES_H
@@ -55,28 +56,48 @@
 
 #if 0
 #include <stdio.h>
 #define NATIVE_DRAWING_NOTE(m) fprintf(stderr, m)
 #else
 #define NATIVE_DRAWING_NOTE(m) do {} while (0)
 #endif
 
-/* We have three basic strategies available:
-   1) 'direct': cr targets an xlib surface, and other conditions are met: we can
-      pass the underlying drawable directly to the callback
-   2) 'opaque': the image is opaque: we can create a temporary cairo xlib surface,
-      pass its underlying drawable to the callback, and paint the result
-      using cairo
-   3) 'default': create a temporary cairo xlib surface, fill with black, pass its
-      underlying drawable to the callback, copy the results to a cairo
-      image surface, repeat with a white background, update the on-black
+/* We have four basic strategies available:
+
+   1) 'direct': If the target is an xlib surface, and other conditions are met,
+      we can pass the underlying drawable directly to the callback.
+
+   2) 'simple': If the drawing is opaque, or we can draw to a surface with an
+      alpha channel, then we can create a temporary xlib surface, pass its
+      underlying drawable to the callback, and composite the result using
+      cairo.
+
+   3) 'copy-background': If the drawing is not opaque but the target is
+      opaque, and we can draw to a surface with format such that pixel
+      conversion to and from the target format is exact, we can create a
+      temporary xlib surface, copy the background from the target, pass the
+      underlying drawable to the callback, and copy back to the target.
+
+      This strategy is not used if the pixel format conversion is not exact,
+      because that would mean that drawing intended to be very transparent
+      messes with other content.
+
+      The strategy is prefered over simple for non-opaque drawing and opaque
+      targets on the same screen as compositing without alpha is a simpler
+      operation.
+
+   4) 'alpha-extraction': create a temporary xlib surface, fill with black,
+      pass its underlying drawable to the callback, copy the results to a
+      cairo image surface, repeat with a white background, update the on-black
       image alpha values by comparing the two images, then paint the on-black
-      image using cairo
-   Sure would be nice to have an X extension to do 3 for us on the server...
+      image using cairo.
+
+      Sure would be nice to have an X extension or GL to do this for us on the
+      server...
 */
 
 static cairo_bool_t
 _convert_coord_to_int (double coord, PRInt32 *v)
 {
     *v = (PRInt32)coord;
     /* XXX allow some tolerance here? */
     return *v == coord;
@@ -92,48 +113,48 @@ static PRBool
     cairo_rectangle_list_t *cliplist;
     cairo_rectangle_t *clips;
     int i;
     PRBool retval = PR_TRUE;
 
     cliplist = cairo_copy_clip_rectangle_list (cr);
     if (cliplist->status != CAIRO_STATUS_SUCCESS) {
         retval = PR_FALSE;
-        NATIVE_DRAWING_NOTE("TAKING SLOW PATH: non-rectangular clip\n");
+        NATIVE_DRAWING_NOTE("FALLBACK: non-rectangular clip");
         goto FINISH;
     }
 
     /* the clip is always in surface backend coordinates (i.e. native backend coords) */
     clips = cliplist->rectangles;
 
     for (i = 0; i < cliplist->num_rectangles; ++i) {
         
         nsIntRect rect;
         if (!_convert_coord_to_int (clips[i].x, &rect.x) ||
             !_convert_coord_to_int (clips[i].y, &rect.y) ||
             !_convert_coord_to_int (clips[i].width, &rect.width) ||
             !_convert_coord_to_int (clips[i].height, &rect.height))
         {
             retval = PR_FALSE;
-            NATIVE_DRAWING_NOTE("TAKING SLOW PATH: non-integer clip\n");
+            NATIVE_DRAWING_NOTE("FALLBACK: non-integer clip");
             goto FINISH;
         }
 
         if (rect == bounds) {
             /* the bounds are entirely inside the clip region so we don't need to clip. */
             *need_clip = PR_FALSE;
             goto FINISH;
         }            
 
         NS_ASSERTION(bounds.Contains(rect),
                      "Was expecting to be clipped to bounds");
 
         if (i >= max_rectangles) {
             retval = PR_FALSE;
-            NATIVE_DRAWING_NOTE("TAKING SLOW PATH: unsupported clip rectangle count\n");
+            NATIVE_DRAWING_NOTE("FALLBACK: unsupported clip rectangle count");
             goto FINISH;
         }
 
         rectangles[i] = rect;
     }
   
     *need_clip = PR_TRUE;
     *num_rectangles = cliplist->num_rectangles;
@@ -152,58 +173,52 @@ FINISH:
  */
 PRBool
 gfxXlibNativeRenderer::DrawDirect(gfxContext *ctx, nsIntSize size,
                                   PRUint32 flags,
                                   Screen *screen, Visual *visual)
 {
     cairo_t *cr = ctx->GetCairo();
 
-    /* Check that the operator is OVER */
-    if (cairo_get_operator (cr) != CAIRO_OPERATOR_OVER) {
-        NATIVE_DRAWING_NOTE("TAKING SLOW PATH: non-OVER operator\n");
-        return PR_FALSE;
-    }
-    
     /* Check that the target surface is an xlib surface. */
     cairo_surface_t *target = cairo_get_group_target (cr);
     if (cairo_surface_get_type (target) != CAIRO_SURFACE_TYPE_XLIB) {
-        NATIVE_DRAWING_NOTE("TAKING SLOW PATH: non-X surface\n");
+        NATIVE_DRAWING_NOTE("FALLBACK: non-X surface");
         return PR_FALSE;
     }
     
     /* Check that the screen is supported.
        Visuals belong to screens, so, if alternate visuals are not supported,
        then alternate screens cannot be supported. */  
     PRBool supports_alternate_visual =
         (flags & DRAW_SUPPORTS_ALTERNATE_VISUAL) != 0;
-    PRBool supports_alternate_screen = supports_alternate_visual
-        && (flags & DRAW_SUPPORTS_ALTERNATE_SCREEN);
+    PRBool supports_alternate_screen = supports_alternate_visual &&
+        (flags & DRAW_SUPPORTS_ALTERNATE_SCREEN);
     if (!supports_alternate_screen &&
         cairo_xlib_surface_get_screen (target) != screen) {
-        NATIVE_DRAWING_NOTE("TAKING SLOW PATH: non-default screen\n");
+        NATIVE_DRAWING_NOTE("FALLBACK: non-default screen");
         return PR_FALSE;
     }
         
     /* Check that there is a visual */
     Visual *target_visual = cairo_xlib_surface_get_visual (target);
     if (!target_visual) {
-        NATIVE_DRAWING_NOTE("TAKING SLOW PATH: no Visual for surface\n");
+        NATIVE_DRAWING_NOTE("FALLBACK: no Visual for surface");
         return PR_FALSE;
     }        
     /* Check that the visual is supported */
     if (!supports_alternate_visual && target_visual != visual) {
         // Only the format of the visual is important (not the GLX properties)
         // for Xlib or XRender drawing.
         XRenderPictFormat *target_format =
             cairo_xlib_surface_get_xrender_format (target);
         if (!target_format ||
             (target_format !=
              XRenderFindVisualFormat (DisplayOfScreen(screen), visual))) {
-            NATIVE_DRAWING_NOTE("TAKING SLOW PATH: unsupported Visual\n");
+            NATIVE_DRAWING_NOTE("FALLBACK: unsupported Visual");
             return PR_FALSE;
         }
     }
   
     cairo_matrix_t matrix;
     cairo_get_matrix (cr, &matrix);
     double device_offset_x, device_offset_y;
     cairo_surface_get_device_offset (target, &device_offset_x, &device_offset_y);
@@ -264,67 +279,184 @@ gfxXlibNativeRenderer::DrawDirect(gfxCon
     if (NS_SUCCEEDED(rv)) {
         cairo_surface_mark_dirty (target);
         return PR_TRUE;
     }
     return PR_FALSE;
 }
 
 static PRBool
-FormatHasAlpha(const XRenderPictFormat *format) {
-    if (!format)
-        return false;
+VisualHasAlpha(Screen *screen, Visual *visual) {
+    // There may be some other visuals format with alpha but usually this is
+    // the only one we care about.
+    return visual->c_class == TrueColor &&
+        visual->bits_per_rgb == 8 &&
+        visual->red_mask == 0xff0000 &&
+        visual->green_mask == 0xff00 &&
+        visual->blue_mask == 0xff &&
+        gfxXlibSurface::DepthOfVisual(screen, visual) == 32;
+}
+
+// Returns whether pixel conversion between visual and format is exact (in
+// both directions).
+static PRBool
+FormatConversionIsExact(Screen *screen, Visual *visual, XRenderPictFormat *format) {
+    if (!format ||
+        visual->c_class != TrueColor ||
+        format->type != PictTypeDirect ||
+        gfxXlibSurface::DepthOfVisual(screen, visual) != format->depth)
+        return PR_FALSE;
 
-    if (format->type != PictTypeDirect)
-        return false;
+    XRenderPictFormat *visualFormat =
+        XRenderFindVisualFormat(DisplayOfScreen(screen), visual);
+
+    if (visualFormat->type != PictTypeDirect )
+        return PR_FALSE;
 
-    return format->direct.alphaMask != 0;
+    const XRenderDirectFormat& a = visualFormat->direct;
+    const XRenderDirectFormat& b = format->direct;
+    return a.redMask == b.redMask &&
+        a.greenMask == b.greenMask &&
+        a.blueMask == b.blueMask;
 }
 
+// The 3 non-direct strategies described above.
+// The surface format and strategy are inter-dependent.
+enum DrawingMethod {
+    eSimple,
+    eCopyBackground,
+    eAlphaExtraction
+};
+
 static already_AddRefed<gfxXlibSurface>
-_create_temp_xlib_surface (cairo_t *cr, nsIntSize size,
-                           PRUint32 flags, Screen *screen, Visual *visual)
+CreateTempXlibSurface (gfxASurface *destination, nsIntSize size,
+                       PRBool canDrawOverBackground,
+                       PRUint32 flags, Screen *screen, Visual *visual,
+                       DrawingMethod *method)
 {
-    Drawable drawable = None;
+    PRBool drawIsOpaque = (flags & gfxXlibNativeRenderer::DRAW_IS_OPAQUE) != 0;
+    PRBool supportsAlternateVisual =
+        (flags & gfxXlibNativeRenderer::DRAW_SUPPORTS_ALTERNATE_VISUAL) != 0;
+    PRBool supportsAlternateScreen = supportsAlternateVisual &&
+        (flags & gfxXlibNativeRenderer::DRAW_SUPPORTS_ALTERNATE_SCREEN);
+
+    cairo_surface_t *target = destination->CairoSurface();
+    cairo_surface_type_t target_type = cairo_surface_get_type (target);
+    cairo_content_t target_content = cairo_surface_get_content (target);
 
-    // For opaque drawing, set up the temp surface for copying to the target.
-    // For non-opaque drawing we read back anyway so just use the
-    // prefered screen and visual.
-    cairo_surface_t *target = cairo_get_group_target (cr);
-    if ((flags & gfxXlibNativeRenderer::DRAW_IS_OPAQUE)
-        && cairo_surface_get_type (target) == CAIRO_SURFACE_TYPE_XLIB) {
+    Screen *target_screen = target_type == CAIRO_SURFACE_TYPE_XLIB ?
+        cairo_xlib_surface_get_screen (target) : screen;
+
+    // When the background has an alpha channel, we need to draw with an alpha
+    // channel anyway, so there is no need to copy the background.  If
+    // doCopyBackground is set here, we'll also need to check below that the
+    // background can copied without any loss in format conversions.
+    PRBool doCopyBackground = !drawIsOpaque && canDrawOverBackground &&
+        target_content == CAIRO_CONTENT_COLOR;
+
+    if (supportsAlternateScreen && screen != target_screen && drawIsOpaque) {
+        // Prefer a visual on the target screen.
+        // (If !drawIsOpaque, we'll need doCopyBackground or an alpha channel.)
+        visual = DefaultVisualOfScreen(target_screen);
+        screen = target_screen;
 
-        Screen *target_screen = cairo_xlib_surface_get_screen (target);
-        PRBool supports_alternate_visual =
-            (flags & gfxXlibNativeRenderer::DRAW_SUPPORTS_ALTERNATE_VISUAL) != 0;
-        PRBool supports_alternate_screen = supports_alternate_visual
-            && (flags & gfxXlibNativeRenderer::DRAW_SUPPORTS_ALTERNATE_SCREEN);
-        if (target_screen == screen || supports_alternate_screen) {
+    } else if (doCopyBackground || (supportsAlternateVisual && drawIsOpaque)) {
+        // Analyse the pixel formats either to check whether we can
+        // doCopyBackground or to see if we can find a better visual for
+        // opaque drawing.
+        Visual *target_visual = NULL;
+        XRenderPictFormat *target_format = NULL;
+        switch (target_type) {
+        case CAIRO_SURFACE_TYPE_XLIB:
+            target_visual = cairo_xlib_surface_get_visual (target);
+            target_format = cairo_xlib_surface_get_xrender_format (target);
+            break;
+        case CAIRO_SURFACE_TYPE_IMAGE: {
+            gfxASurface::gfxImageFormat imageFormat =
+                static_cast<gfxImageSurface*>(destination)->Format();
+            target_visual = gfxXlibSurface::FindVisual(screen, imageFormat);
+            Display *dpy = DisplayOfScreen(screen);
+            if (target_visual) {
+                target_format = XRenderFindVisualFormat(dpy, visual);
+            } else {
+                target_format =
+                    gfxXlibSurface::FindRenderFormat(dpy, imageFormat);
+            }                
+            break;
+        }
+        default:
+            break;
+        }
 
-            if (supports_alternate_visual) {
-                Visual *target_visual = cairo_xlib_surface_get_visual (target);
-                if (target_visual &&
-                    (!FormatHasAlpha(cairo_xlib_surface_get_xrender_format (target)))) {
-                    visual = target_visual;
-                } else if (target_screen != screen) {
-                    visual = DefaultVisualOfScreen (target_screen);
-                }
+        if (supportsAlternateVisual &&
+            (supportsAlternateScreen || screen == target_screen)) {
+            if (target_visual) {
+                visual = target_visual;
+                screen = target_screen;
             }
+        }
+        // Could try harder to match formats across screens for background
+        // copying when !supportsAlternateScreen, if we cared.  Preferably
+        // we'll find a visual below with an alpha channel anyway; if so, the
+        // background won't need to be copied.
 
-            drawable = cairo_xlib_surface_get_drawable (target);
-            screen = target_screen;
+        if (doCopyBackground && visual != target_visual &&
+            !FormatConversionIsExact(screen, visual, target_format)) {
+            doCopyBackground = PR_FALSE;
         }
     }
 
-    if (!drawable) {
-        drawable = RootWindowOfScreen (screen);
+    if (supportsAlternateVisual && !drawIsOpaque &&
+        (screen != target_screen ||
+         !(doCopyBackground || VisualHasAlpha(screen, visual)))) {
+        // Try to find a visual with an alpha channel.
+        Screen *visualScreen =
+            supportsAlternateScreen ? target_screen : screen;
+        Visual *argbVisual =
+            gfxXlibSurface::FindVisual(visualScreen,
+                                       gfxASurface::ImageFormatARGB32);
+        if (argbVisual) {
+            visual = argbVisual;
+            screen = visualScreen;
+        } else if (!doCopyBackground &&
+                   gfxXlibSurface::DepthOfVisual(screen, visual) != 24) {
+            // Will need to do alpha extraction; prefer a 24-bit visual.
+            // No advantage in using the target screen.
+            Visual *rgb24Visual =
+                gfxXlibSurface::FindVisual(screen,
+                                           gfxASurface::ImageFormatRGB24);
+            if (rgb24Visual) {
+                visual = rgb24Visual;
+            }
+        }
     }
-    return gfxXlibSurface::Create(screen, visual,
-                                  gfxIntSize(size.width, size.height),
-                                  drawable);
+
+    Drawable drawable =
+        (screen == target_screen && target_type == CAIRO_SURFACE_TYPE_XLIB) ?
+        cairo_xlib_surface_get_drawable (target) : RootWindowOfScreen(screen);
+
+    nsRefPtr<gfxXlibSurface> surface =
+        gfxXlibSurface::Create(screen, visual,
+                               gfxIntSize(size.width, size.height),
+                               drawable);
+
+    if (drawIsOpaque ||
+        surface->GetContentType() == gfxASurface::CONTENT_COLOR_ALPHA) {
+        NATIVE_DRAWING_NOTE(drawIsOpaque ?
+                            ", SIMPLE OPAQUE\n" : ", SIMPLE WITH ALPHA");
+        *method = eSimple;
+    } else if (doCopyBackground) {
+        NATIVE_DRAWING_NOTE(", COPY BACKGROUND\n");
+        *method = eCopyBackground;
+    } else {
+        NATIVE_DRAWING_NOTE(", SLOW ALPHA EXTRACTION\n");
+        *method = eAlphaExtraction;
+    }
+
+    return surface.forget();
 }
 
 PRBool
 gfxXlibNativeRenderer::DrawOntoTempSurface(gfxXlibSurface *tempXlibSurface,
                                            nsIntPoint offset)
 {
     cairo_surface_t *temp_xlib_surface = tempXlibSurface->CairoSurface();
     cairo_surface_flush (temp_xlib_surface);
@@ -451,86 +583,114 @@ gfxXlibNativeRenderer::Draw(gfxContext* 
     unsigned char *black_data;
     unsigned char *white_data;
   
     if (result) {
         result->mSurface = NULL;
         result->mUniformAlpha = PR_FALSE;
         result->mUniformColor = PR_FALSE;
     }
-    
-    PRBool matrixIsIntegerTranslation =
-        !ctx->CurrentMatrix().HasNonIntegerTranslation();
+
+    PRBool drawIsOpaque = (flags & DRAW_IS_OPAQUE) != 0;
+    gfxMatrix matrix = ctx->CurrentMatrix();
+
+    // We can only draw direct or onto a copied background if pixels align and
+    // native drawing is compatible with the current operator.  (The matrix is
+    // actually also pixel-exact for flips and right-angle rotations, which
+    // would permit copying the background but not drawing direct.)
+    PRBool matrixIsIntegerTranslation = !matrix.HasNonIntegerTranslation();
+    PRBool canDrawOverBackground = matrixIsIntegerTranslation &&
+        ctx->CurrentOperator() == gfxContext::OPERATOR_OVER;
 
     // The padding of 0.5 for non-pixel-exact transformations used here is
     // the same as what _cairo_pattern_analyze_filter uses.
     const gfxFloat filterRadius = 0.5;
     gfxRect affectedRect(0.0, 0.0, size.width, size.height);
     if (!matrixIsIntegerTranslation) {
         // The filter footprint means that the affected rectangle is a
         // little larger than the drawingRect;
         affectedRect.Outset(filterRadius);
 
-        NATIVE_DRAWING_NOTE("TAKING SLOW PATH: matrix not integer translation\n");
+        NATIVE_DRAWING_NOTE("FALLBACK: matrix not integer translation");
+    } else if (!canDrawOverBackground) {
+        NATIVE_DRAWING_NOTE("FALLBACK: unsupported operator");
     }
+
     // Clipping to the region affected by drawing allows us to consider only
     // the portions of the clip region that will be affected by drawing.
     gfxRect clipExtents;
     {
         gfxContextAutoSaveRestore autoSR(ctx);
         ctx->Clip(affectedRect);
 
         clipExtents = ctx->GetClipExtents();
         if (clipExtents.IsEmpty())
             return; // nothing to do
 
-        if (matrixIsIntegerTranslation &&
+        if (canDrawOverBackground &&
             DrawDirect(ctx, size, flags, screen, visual))
             return;
     }
 
     nsIntRect drawingRect(nsIntPoint(0, 0), size);
-    PRBool drawIsOpaque = (flags & DRAW_IS_OPAQUE) != 0;
-    if (drawIsOpaque || !result) {
-        // Drawing need only be performed within the clip extents
-        // (and padding for the filter).
-        if (!matrixIsIntegerTranslation) {
-            // The source surface may need to be a little larger than the clip
-            // extents due to the filter footprint.  
-            clipExtents.Outset(filterRadius);
-        }
-        clipExtents.RoundOut();
+    // Drawing need only be performed within the clip extents
+    // (and padding for the filter).
+    if (!matrixIsIntegerTranslation) {
+        // The source surface may need to be a little larger than the clip
+        // extents due to the filter footprint.
+        clipExtents.Outset(filterRadius);
+    }
+    clipExtents.RoundOut();
 
-        nsIntRect intExtents(PRInt32(clipExtents.X()),
-                             PRInt32(clipExtents.Y()),
-                             PRInt32(clipExtents.Width()),
-                             PRInt32(clipExtents.Height()));
-        drawingRect.IntersectRect(drawingRect, intExtents);
-    }
+    nsIntRect intExtents(PRInt32(clipExtents.X()),
+                         PRInt32(clipExtents.Y()),
+                         PRInt32(clipExtents.Width()),
+                         PRInt32(clipExtents.Height()));
+    drawingRect.IntersectRect(drawingRect, intExtents);
     gfxPoint offset(drawingRect.x, drawingRect.y);
 
-    cairo_t *cr = ctx->GetCairo();
+    DrawingMethod method;
+    nsRefPtr<gfxASurface> target = ctx->CurrentSurface();
     nsRefPtr<gfxXlibSurface> tempXlibSurface = 
-        _create_temp_xlib_surface (cr, drawingRect.Size(),
-                                   flags, screen, visual);
-    if (tempXlibSurface == NULL)
+        CreateTempXlibSurface(target, drawingRect.Size(),
+                              canDrawOverBackground, flags, screen, visual,
+                              &method);
+    if (!tempXlibSurface)
         return;
   
+    if (drawingRect.Size() != size || method == eCopyBackground) {
+        // Only drawing a portion, or copying background,
+        // so won't return a result.
+        result = NULL;
+    }
+
     nsRefPtr<gfxContext> tmpCtx;
     if (!drawIsOpaque) {
         tmpCtx = new gfxContext(tempXlibSurface);
-        tmpCtx->SetOperator(gfxContext::OPERATOR_CLEAR);
+        if (method == eCopyBackground) {
+            tmpCtx->SetOperator(gfxContext::OPERATOR_SOURCE);
+            tmpCtx->SetSource(target, -(offset + matrix.GetTranslation()));
+            // The copy from the tempXlibSurface to the target context should
+            // use operator SOURCE, but that would need a mask to bound the
+            // operation.  Here we only copy opaque backgrounds so operator
+            // OVER will behave like SOURCE masked by the surface.
+            NS_ASSERTION(tempXlibSurface->GetContentType()
+                         == gfxASurface::CONTENT_COLOR,
+                         "Don't copy background with a transparent surface");
+        } else {
+            tmpCtx->SetOperator(gfxContext::OPERATOR_CLEAR);
+        }
         tmpCtx->Paint();
     }
 
     if (!DrawOntoTempSurface(tempXlibSurface, -drawingRect.TopLeft())) {
         return;
     }
   
-    if (drawIsOpaque) {
+    if (method != eAlphaExtraction) {
         ctx->SetSource(tempXlibSurface, offset);
         ctx->Paint();
         if (result) {
             result->mSurface = tempXlibSurface;
             /* fill in the result with what we know, which is really just what our
                assumption was */
             result->mUniformAlpha = PR_TRUE;
             result->mColor.a = 1.0;
@@ -556,16 +716,17 @@ gfxXlibNativeRenderer::Draw(gfxContext* 
         cairo_surface_status (black_image_surface) == CAIRO_STATUS_SUCCESS &&
         cairo_surface_status (white_image_surface) == CAIRO_STATUS_SUCCESS &&
         black_data != NULL && white_data != NULL) {
         cairo_surface_flush (black_image_surface);
         cairo_surface_flush (white_image_surface);
         _compute_alpha_values ((uint32_t*)black_data, (uint32_t*)white_data, width, height, result);
         cairo_surface_mark_dirty (black_image_surface);
         
+        cairo_t *cr = ctx->GetCairo();
         cairo_set_source_surface (cr, black_image_surface, offset.x, offset.y);
         /* if the caller wants to retrieve the rendered image, put it into
            a 'similar' surface, and use that as the source for the drawing right
            now. This means we always return a surface similar to the surface
            used for 'cr', which is ideal if it's going to be cached and reused.
            We do not return an image if the result has uniform color and alpha. */
         if (result && (!result->mUniformAlpha || !result->mUniformColor)) {
             cairo_surface_t *target = cairo_get_group_target (cr);