Bug 1321412 - Add support for partial prerendering to ShouldPrerenderTransformedContent(). r=mattwoodrow

This is behind a pref and not enabled by default yet.

MozReview-Commit-ID: HKbP02PkdI9

--HG--
extra : rebase_source : 3f724f12d467dfb6e3ac1c44841e8e452a7d4b7e

gfx/thebes/gfxPrefs.h

@@ -529,6 +529,7 @@ private:
   DECL_GFX_PREF(Live, "layers.draw-mask-debug",                DrawMaskLayer, bool, false);
   DECL_GFX_PREF(Live, "layers.geometry.opengl.enabled",        OGLLayerGeometry, bool, false);
 
+  DECL_GFX_PREF(Live, "layout.animation.prerender.partial", PartiallyPrerenderAnimatedContent, bool, false);
   DECL_GFX_PREF(Live, "layout.animation.prerender.viewport-ratio-limit-x", AnimationPrerenderViewportRatioLimitX, float, 1.125f);
   DECL_GFX_PREF(Live, "layout.animation.prerender.viewport-ratio-limit-y", AnimationPrerenderViewportRatioLimitY, float, 1.125f);
   DECL_GFX_PREF(Live, "layout.animation.prerender.absolute-limit-x", AnimationPrerenderAbsoluteLimitX, uint32_t, 4096);

layout/generic/nsFrame.cpp

@@ -2214,6 +2214,10 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
     case nsDisplayTransform::FullPrerender:
       allowAsyncAnimation = true;
       break;
+    case nsDisplayTransform::PartialPrerender:
+      allowAsyncAnimation = true;
+      MOZ_FALLTHROUGH;
+      // fall through to the NoPrerender case
     case nsDisplayTransform::NoPrerender:
       if (overflow.IsEmpty() && !extend3DContext) {
         return;

layout/painting/nsDisplayList.cpp

@@ -6226,6 +6226,20 @@ nsDisplayTransform::CanUseAsyncAnimations(nsDisplayListBuilder* aBuilder)
   return mAllowAsyncAnimation;
 }
 
+static nsRect ComputePartialPrerenderArea(const nsRect& aDirtyRect,
+                                          const nsRect& aOverflow,
+                                          const nsSize& aPrerenderSize)
+{
+  // Simple calculation for now: center the pre-render area on the dirty rect,
+  // and clamp to the overflow area. Later we can do more advanced things like
+  // redistributing from one axis to another, or from one side to another.
+  nscoord xExcess = aPrerenderSize.width - aDirtyRect.width;
+  nscoord yExcess = aPrerenderSize.height - aDirtyRect.height;
+  nsRect result = aDirtyRect;
+  result.Inflate(xExcess / 2, yExcess / 2);
+  return result.MoveInsideAndClamp(aOverflow);
+}
+
 /* static */ auto
 nsDisplayTransform::ShouldPrerenderTransformedContent(nsDisplayListBuilder* aBuilder,
                                                       nsIFrame* aFrame,
@@ -6266,6 +6280,9 @@ nsDisplayTransform::ShouldPrerenderTransformedContent(nsDisplayListBuilder* aBui
   if (frameSize <= maxSize) {
     *aDirtyRect = overflow;
     return FullPrerender;
+  } else if (gfxPrefs::PartiallyPrerenderAnimatedContent()) {
+    *aDirtyRect = ComputePartialPrerenderArea(*aDirtyRect, overflow, maxSize);
+    return PartialPrerender;
   }
 
   EffectCompositor::SetPerformanceWarning(

layout/painting/nsDisplayList.h

@@ -4065,7 +4065,8 @@ class nsDisplayTransform: public nsDisplayItem
 public:
   enum PrerenderDecision {
     NoPrerender,
-    FullPrerender
+    FullPrerender,
+    PartialPrerender
   };
 
   /**
@@ -4281,8 +4282,11 @@ public:
                                                uint32_t aFlags,
                                                const nsRect* aBoundsOverride = nullptr);
   /**
-   * Return FullPrerender when we should try to prerender the entire contents of the
+   * Decide whether we should prerender some or all of the contents of the
    * transformed frame even when it's not completely visible (yet).
+   * Return FullPrerender if the entire contents should be prerendered,
+   * PartialPrerender if some but not all of the contents should be prerendered,
+   * or NoPrerender if only the visible area should be rendered.
    * |aDirtyRect| is updated to the area that should be prerendered.
    */
   static PrerenderDecision ShouldPrerenderTransformedContent(nsDisplayListBuilder* aBuilder,

modules/libpref/init/all.js

@@ -2738,6 +2738,7 @@ pref("dom.animations.offscreen-throttling", true);
 
 // Prefs to control the maximum area to pre-render when animating a large
 // element on the compositor.
+pref("layout.animation.prerender.partial", false);
 pref("layout.animation.prerender.viewport-ratio-limit-x", "1.125");
 pref("layout.animation.prerender.viewport-ratio-limit-y", "1.125");
 pref("layout.animation.prerender.absolute-limit-x", 4096);