pjs/gfx/layers/LayerSorter.cpp

/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*-
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 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
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 *
 * Software distributed under the License is distributed on an "AS IS" basis,
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 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Mozilla Corporation code.
 *
 * The Initial Developer of the Original Code is Mozilla Foundation.
 * Portions created by the Initial Developer are Copyright (C) 2011
 * the Initial Developer. All Rights Reserved.
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 * Contributor(s):
 *   Matt Woodrow <mwoodrow@mozilla.com>
 *
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 * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
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#include "LayerSorter.h"
#include "DirectedGraph.h"
#include "limits.h"

namespace mozilla {
namespace layers {

enum LayerSortOrder {
  Undefined,
  ABeforeB,
  BBeforeA,
};

/**
 * Recover the z component from a 2d transformed point by finding the intersection
 * of a line through the point in the z direction and the transformed plane.
 *
 * We want to solve:
 *
 * point = normal . (p0 - l0) / normal . l
 */
static gfxFloat RecoverZDepth(const gfx3DMatrix& aTransform, const gfxPoint& aPoint)
{
    const gfxPoint3D l(0, 0, 1);
    gfxPoint3D l0 = gfxPoint3D(aPoint.x, aPoint.y, 0);
    gfxPoint3D p0 = aTransform.Transform3D(gfxPoint3D(0, 0, 0));
    gfxPoint3D normal = aTransform.GetNormalVector();

    gfxFloat n = normal.DotProduct(p0 - l0); 
    gfxFloat d = normal.DotProduct(l);

    if (!d) {
        return 0;
    }

    return n/d;
}

/**
 * Determine if this transform layer should be drawn before another when they 
 * are both preserve-3d children.
 *
 * We want to find the relative z depths of the 2 layers at points where they
 * intersect when projected onto the 2d screen plane.
 *
 * If the ordering is consistent at all intersection points, then we have
 * a definitive order, otherwise the 2 layers must actually intersect in 3d
 * space, and we just order these arbitrarily.
 */
static LayerSortOrder CompareDepth(Layer* aOne, Layer* aTwo) {
  gfxRect ourRect = aOne->GetEffectiveVisibleRegion().GetBounds();
  gfxRect otherRect = aTwo->GetEffectiveVisibleRegion().GetBounds();

  gfx3DMatrix ourTransform = aOne->GetTransform();
  gfx3DMatrix otherTransform = aTwo->GetTransform();

  // Transform both rectangles and project into 2d space.
  gfxQuad ourTransformedRect = ourTransform.TransformRect(ourRect);
  gfxQuad otherTransformedRect = otherTransform.TransformRect(otherRect);

  // Make a list of all points that are within the other rect.
  nsTArray<gfxPoint> points;
  for (PRUint32 i=0; i<4; i++) {
    if (ourTransformedRect.Contains(otherTransformedRect.mPoints[i])) {
      points.AppendElement(otherTransformedRect.mPoints[i]);
    }
    if (otherTransformedRect.Contains(ourTransformedRect.mPoints[i])) {
      points.AppendElement(ourTransformedRect.mPoints[i]);
    }
  }

  // No intersections, no defined order between these layers.
  if (points.IsEmpty()) {
    return Undefined;
  }

  // Find the relative Z depths of each intersection point and check that the layers are in the same order.
  bool drawBefore = false;
  for (PRUint32 i = 0; i < points.Length(); i++) {
    bool temp = RecoverZDepth(ourTransform, points.ElementAt(i)) <= RecoverZDepth(otherTransform, points.ElementAt(i));
    if (i == 0) {
      drawBefore = temp; 
    } else if (drawBefore != temp) {
      // Mixed ordering means an intersection in 3d space that we can't resolve without plane splitting
      // or depth buffering. Store this as having no defined order for now.
      return Undefined;
    }
  }
  if (drawBefore) {
    return ABeforeB;
  }
  return BBeforeA;
}

#ifdef DEBUG
static bool gDumpLayerSortList = getenv("MOZ_DUMP_LAYER_SORT_LIST") != 0;

static void DumpLayerList(nsTArray<Layer*>& aLayers)
{
  for (PRUint32 i = 0; i < aLayers.Length(); i++) {
    fprintf(stderr, "%p, ", aLayers.ElementAt(i));
  }
  fprintf(stderr, "\n");
}

static void DumpEdgeList(DirectedGraph<Layer*>& aGraph)
{
  nsTArray<DirectedGraph<Layer*>::Edge> edges = aGraph.GetEdgeList();
  
  for (PRUint32 i = 0; i < edges.Length(); i++) {
    fprintf(stderr, "From: %p, To: %p\n", edges.ElementAt(i).mFrom, edges.ElementAt(i).mTo);
  }
}
#endif

// The maximum number of layers that we will attempt to sort. Anything
// greater than this will be left unsorted. We should consider enabling
// depth buffering for the scene in this case.
#define MAX_SORTABLE_LAYERS 100

void SortLayersBy3DZOrder(nsTArray<Layer*>& aLayers)
{
  PRUint32 nodeCount = aLayers.Length();
  if (nodeCount > MAX_SORTABLE_LAYERS) {
    return;
  }
  DirectedGraph<Layer*> graph;

#ifdef DEBUG
  if (gDumpLayerSortList) {
    fprintf(stderr, " --- Layers before sorting: --- \n");
    DumpLayerList(aLayers);
  }
#endif

  // Iterate layers and determine edges.
  for (PRUint32 i = 0; i < nodeCount; i++) {
    for (PRUint32 j = i + 1; j < nodeCount; j++) {
      Layer* a = aLayers.ElementAt(i);
      Layer* b = aLayers.ElementAt(j);
      LayerSortOrder order = CompareDepth(a, b);
      if (order == ABeforeB) {
        graph.AddEdge(a, b);
      } else if (order == BBeforeA) {
        graph.AddEdge(b, a);
      }
    }
  }

#ifdef DEBUG
  if (gDumpLayerSortList) {
    fprintf(stderr, " --- Edge List: --- \n");
    DumpEdgeList(graph);
  }
#endif

  // Build a new array using the graph.
  nsTArray<Layer*> noIncoming;
  nsTArray<Layer*> sortedList;

  // Make a list of all layers with no incoming edges.
  noIncoming.AppendElements(aLayers);
  const nsTArray<DirectedGraph<Layer*>::Edge>& edges = graph.GetEdgeList();
  for (PRUint32 i = 0; i < edges.Length(); i++) {
    noIncoming.RemoveElement(edges.ElementAt(i).mTo);
  }

  // Move each item without incoming edges into the sorted list,
  // and remove edges from it.
  while (!noIncoming.IsEmpty()) {
    PRUint32 last = noIncoming.Length() - 1;

    Layer* layer = noIncoming.ElementAt(last);

    noIncoming.RemoveElementAt(last);
    sortedList.AppendElement(layer);

    nsTArray<DirectedGraph<Layer*>::Edge> outgoing;
    graph.GetEdgesFrom(layer, outgoing);
    for (PRUint32 i = 0; i < outgoing.Length(); i++) {
      DirectedGraph<Layer*>::Edge edge = outgoing.ElementAt(i);
      graph.RemoveEdge(edge);
      if (!graph.NumEdgesTo(edge.mTo)) {
        // If this node also has no edges now, add it to the list
        noIncoming.AppendElement(edge.mTo);
      }
    }

    // If there are no nodes without incoming edges, but there
    // are still edges, then we have a cycle.
    if (noIncoming.IsEmpty() && graph.GetEdgeCount()) {
      // Find the node with the least incoming edges.
      PRUint32 minEdges = UINT_MAX;
      Layer* minNode = nsnull;
      for (PRUint32 i = 0; i < aLayers.Length(); i++) {
        PRUint32 edgeCount = graph.NumEdgesTo(aLayers.ElementAt(i));
        if (edgeCount && edgeCount < minEdges) {
          minEdges = edgeCount;
          minNode = aLayers.ElementAt(i);
        }
      }

      // Remove all of them!
      graph.RemoveEdgesTo(minNode);
      noIncoming.AppendElement(minNode);
    }
  }
  NS_ASSERTION(!graph.GetEdgeCount(), "Cycles detected!");
#ifdef DEBUG
  if (gDumpLayerSortList) {
    fprintf(stderr, " --- Layers after sorting: --- \n");
    DumpLayerList(sortedList);
  }
#endif

  aLayers.Clear();
  aLayers.AppendElements(sortedList);
}

}
}