cc/trees/damage_tracker.cc

// Copyright 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "cc/trees/damage_tracker.h"

#include <stddef.h>

#include <algorithm>

#include "cc/base/math_util.h"
#include "cc/layers/heads_up_display_layer_impl.h"
#include "cc/layers/layer_impl.h"
#include "cc/layers/render_surface_impl.h"
#include "cc/output/filter_operations.h"
#include "cc/trees/layer_tree_host_common.h"
#include "cc/trees/layer_tree_impl.h"
#include "ui/gfx/geometry/rect_conversions.h"

namespace cc {

scoped_ptr<DamageTracker> DamageTracker::Create() {
  return make_scoped_ptr(new DamageTracker());
}

DamageTracker::DamageTracker()
  : mailboxId_(0) {}

DamageTracker::~DamageTracker() {}

static inline void ExpandRectWithFilters(gfx::Rect* rect,
                                         const FilterOperations& filters) {
  int top, right, bottom, left;
  filters.GetOutsets(&top, &right, &bottom, &left);
  rect->Inset(-left, -top, -right, -bottom);
}

static inline void ExpandDamageRectInsideRectWithFilters(
    gfx::Rect* damage_rect,
    const gfx::Rect& pre_filter_rect,
    const FilterOperations& filters) {
  gfx::Rect expanded_damage_rect = *damage_rect;
  ExpandRectWithFilters(&expanded_damage_rect, filters);
  gfx::Rect filter_rect = pre_filter_rect;
  ExpandRectWithFilters(&filter_rect, filters);

  expanded_damage_rect.Intersect(filter_rect);
  damage_rect->Union(expanded_damage_rect);
}

void DamageTracker::UpdateDamageTrackingState(
    const LayerImplList& layer_list,
    int target_surface_layer_id,
    bool target_surface_property_changed_only_from_descendant,
    const gfx::Rect& target_surface_content_rect,
    LayerImpl* target_surface_mask_layer,
    const FilterOperations& filters) {
  //
  // This function computes the "damage rect" of a target surface, and updates
  // the state that is used to correctly track damage across frames. The damage
  // rect is the region of the surface that may have changed and needs to be
  // redrawn. This can be used to scissor what is actually drawn, to save GPU
  // computation and bandwidth.
  //
  // The surface's damage rect is computed as the union of all possible changes
  // that have happened to the surface since the last frame was drawn. This
  // includes:
  //   - any changes for existing layers/surfaces that contribute to the target
  //     surface
  //   - layers/surfaces that existed in the previous frame, but no longer exist
  //
  // The basic algorithm for computing the damage region is as follows:
  //
  //   1. compute damage caused by changes in active/new layers
  //       for each layer in the layer_list:
  //           if the layer is actually a render_surface:
  //               add the surface's damage to our target surface.
  //           else
  //               add the layer's damage to the target surface.
  //
  //   2. compute damage caused by the target surface's mask, if it exists.
  //
  //   3. compute damage caused by old layers/surfaces that no longer exist
  //       for each leftover layer:
  //           add the old layer/surface bounds to the target surface damage.
  //
  //   4. combine all partial damage rects to get the full damage rect.
  //
  // Additional important points:
  //
  // - This algorithm is implicitly recursive; it assumes that descendant
  //   surfaces have already computed their damage.
  //
  // - Changes to layers/surfaces indicate "damage" to the target surface; If a
  //   layer is not changed, it does NOT mean that the layer can skip drawing.
  //   All layers that overlap the damaged region still need to be drawn. For
  //   example, if a layer changed its opacity, then layers underneath must be
  //   re-drawn as well, even if they did not change.
  //
  // - If a layer/surface property changed, the old bounds and new bounds may
  //   overlap... i.e. some of the exposed region may not actually be exposing
  //   anything. But this does not artificially inflate the damage rect. If the
  //   layer changed, its entire old bounds would always need to be redrawn,
  //   regardless of how much it overlaps with the layer's new bounds, which
  //   also need to be entirely redrawn.
  //
  // - See comments in the rest of the code to see what exactly is considered a
  //   "change" in a layer/surface.
  //
  // - To correctly manage exposed rects, SortedRectMap is maintained:
  //
  //      1. All existing rects from the previous frame are marked as
  //         not updated.
  //      2. The map contains all the layer bounds that contributed to
  //         the previous frame (even outside the previous damaged area). If a
  //         layer changes or does not exist anymore, those regions are then
  //         exposed and damage the target surface. As the algorithm progresses,
  //         entries are updated in the map until only leftover layers
  //         that no longer exist stay marked not updated.
  //
  //      3. After the damage rect is computed, the leftover not marked regions
  //         in a map are used to compute are damaged by deleted layers and
  //         erased from map.
  //

  PrepareRectHistoryForUpdate();
  // These functions cannot be bypassed with early-exits, even if we know what
  // the damage will be for this frame, because we need to update the damage
  // tracker state to correctly track the next frame.
  gfx::Rect damage_from_active_layers =
      TrackDamageFromActiveLayers(layer_list, target_surface_layer_id);
  gfx::Rect damage_from_surface_mask =
      TrackDamageFromSurfaceMask(target_surface_mask_layer);
  gfx::Rect damage_from_leftover_rects = TrackDamageFromLeftoverRects();

  gfx::Rect damage_rect_for_this_update;

  if (target_surface_property_changed_only_from_descendant) {
    damage_rect_for_this_update = target_surface_content_rect;
  } else {
    // TODO(shawnsingh): can we clamp this damage to the surface's content rect?
    // (affects performance, but not correctness)
    damage_rect_for_this_update = damage_from_active_layers;
    damage_rect_for_this_update.Union(damage_from_surface_mask);
    damage_rect_for_this_update.Union(damage_from_leftover_rects);

    ExpandRectWithFilters(&damage_rect_for_this_update, filters);
  }

  // Damage accumulates until we are notified that we actually did draw on that
  // frame.
  current_damage_rect_.Union(damage_rect_for_this_update);
}

DamageTracker::RectMapData& DamageTracker::RectDataForLayer(
    int layer_id,
    bool* layer_is_new) {

  RectMapData data(layer_id);

  SortedRectMap::iterator it = std::lower_bound(rect_history_.begin(),
    rect_history_.end(), data);

  if (it == rect_history_.end() || it->layer_id_ != layer_id) {
    *layer_is_new = true;
    it = rect_history_.insert(it, data);
  }

  return *it;
}

gfx::Rect DamageTracker::TrackDamageFromActiveLayers(
    const LayerImplList& layer_list,
    int target_surface_layer_id) {
  gfx::Rect damage_rect;

  for (size_t layer_index = 0; layer_index < layer_list.size(); ++layer_index) {
    // Visit layers in back-to-front order.
    LayerImpl* layer = layer_list[layer_index];

    // We skip damage from the HUD layer because (a) the HUD layer damages the
    // whole frame and (b) we don't want HUD layer damage to be shown by the
    // HUD damage rect visualization.
    if (layer == layer->layer_tree_impl()->hud_layer())
      continue;
    if (LayerTreeHostCommon::RenderSurfaceContributesToTarget<LayerImpl>(
            layer, target_surface_layer_id))
      ExtendDamageForRenderSurface(layer, &damage_rect);
    else
      ExtendDamageForLayer(layer, &damage_rect);
  }

  return damage_rect;
}

gfx::Rect DamageTracker::TrackDamageFromSurfaceMask(
    LayerImpl* target_surface_mask_layer) {
  gfx::Rect damage_rect;

  if (!target_surface_mask_layer)
    return damage_rect;

  // Currently, if there is any change to the mask, we choose to damage the
  // entire surface. This could potentially be optimized later, but it is not
  // expected to be a common case.
  if (target_surface_mask_layer->LayerPropertyChanged() ||
      !target_surface_mask_layer->update_rect().IsEmpty()) {
    damage_rect = gfx::Rect(target_surface_mask_layer->bounds());
  }

  return damage_rect;
}

void DamageTracker::PrepareRectHistoryForUpdate() {
  mailboxId_++;
}

gfx::Rect DamageTracker::TrackDamageFromLeftoverRects() {
  // After computing damage for all active layers, any leftover items in the
  // current rect history correspond to layers/surfaces that no longer exist.
  // So, these regions are now exposed on the target surface.

  gfx::Rect damage_rect;
  SortedRectMap::iterator cur_pos = rect_history_.begin();
  SortedRectMap::iterator copy_pos = cur_pos;

  // Loop below basically implements std::remove_if loop with and extra
  // processing (adding deleted rect to damage_rect) for deleted items.
  // cur_pos iterator runs through all elements of the vector, but copy_pos
  // always points to the element after the last not deleted element. If new
  // not deleted element found then it is copied to the *copy_pos and copy_pos
  // moved to the next position.
  // If there are no deleted elements then copy_pos iterator is in sync with
  // cur_pos and no copy happens.
  while (cur_pos < rect_history_.end()) {
    if (cur_pos->mailboxId_ == mailboxId_) {
      if (cur_pos != copy_pos)
        *copy_pos = *cur_pos;

      ++copy_pos;
    } else {
      damage_rect.Union(cur_pos->rect_);
    }

    ++cur_pos;
  }

  if (copy_pos != rect_history_.end())
    rect_history_.erase(copy_pos, rect_history_.end());

  // If the vector has excessive storage, shrink it
  if (rect_history_.capacity() > rect_history_.size() * 4)
    SortedRectMap(rect_history_).swap(rect_history_);

  return damage_rect;
}

void DamageTracker::ExtendDamageForLayer(LayerImpl* layer,
                                         gfx::Rect* target_damage_rect) {
  // There are two ways that a layer can damage a region of the target surface:
  //   1. Property change (e.g. opacity, position, transforms):
  //        - the entire region of the layer itself damages the surface.
  //        - the old layer region also damages the surface, because this region
  //          is now exposed.
  //        - note that in many cases the old and new layer rects may overlap,
  //          which is fine.
  //
  //   2. Repaint/update: If a region of the layer that was repainted/updated,
  //      that region damages the surface.
  //
  // Property changes take priority over update rects.
  //
  // This method is called when we want to consider how a layer contributes to
  // its target RenderSurface, even if that layer owns the target RenderSurface
  // itself. To consider how a layer's target surface contributes to the
  // ancestor surface, ExtendDamageForRenderSurface() must be called instead.

  bool layer_is_new = false;
  RectMapData& data = RectDataForLayer(layer->id(), &layer_is_new);
  gfx::Rect old_rect_in_target_space = data.rect_;

  gfx::Rect rect_in_target_space = layer->GetEnclosingRectInTargetSpace();
  data.Update(rect_in_target_space, mailboxId_);

  if (layer_is_new || layer->LayerPropertyChanged()) {
    // If a layer is new or has changed, then its entire layer rect affects the
    // target surface.
    target_damage_rect->Union(rect_in_target_space);

    // The layer's old region is now exposed on the target surface, too.
    // Note old_rect_in_target_space is already in target space.
    target_damage_rect->Union(old_rect_in_target_space);
    return;
  }

  // If the layer properties haven't changed, then the the target surface is
  // only affected by the layer's damaged area, which could be empty.
  gfx::Rect damage_rect =
      gfx::UnionRects(layer->update_rect(), layer->damage_rect());
  damage_rect.Intersect(gfx::Rect(layer->bounds()));
  if (!damage_rect.IsEmpty()) {
    gfx::Rect damage_rect_in_target_space =
        MathUtil::MapEnclosingClippedRect(layer->DrawTransform(), damage_rect);
    target_damage_rect->Union(damage_rect_in_target_space);
  }
}

void DamageTracker::ExtendDamageForRenderSurface(
    LayerImpl* layer,
    gfx::Rect* target_damage_rect) {
  // There are two ways a "descendant surface" can damage regions of the "target
  // surface":
  //   1. Property change:
  //        - a surface's geometry can change because of
  //            - changes to descendants (i.e. the subtree) that affect the
  //              surface's content rect
  //            - changes to ancestor layers that propagate their property
  //              changes to their entire subtree.
  //        - just like layers, both the old surface rect and new surface rect
  //          will damage the target surface in this case.
  //
  //   2. Damage rect: This surface may have been damaged by its own layer_list
  //      as well, and that damage should propagate to the target surface.
  //

  RenderSurfaceImpl* render_surface = layer->render_surface();

  bool surface_is_new = false;
  RectMapData& data = RectDataForLayer(layer->id(), &surface_is_new);
  gfx::Rect old_surface_rect = data.rect_;

  // The drawableContextRect() already includes the replica if it exists.
  gfx::Rect surface_rect_in_target_space =
      gfx::ToEnclosingRect(render_surface->DrawableContentRect());
  data.Update(surface_rect_in_target_space, mailboxId_);

  gfx::Rect damage_rect_in_local_space;
  if (surface_is_new || render_surface->SurfacePropertyChanged()) {
    // The entire surface contributes damage.
    damage_rect_in_local_space = render_surface->content_rect();

    // The surface's old region is now exposed on the target surface, too.
    target_damage_rect->Union(old_surface_rect);
  } else {
    // Only the surface's damage_rect will damage the target surface.
    damage_rect_in_local_space =
        render_surface->damage_tracker()->current_damage_rect();
  }

  // If there was damage, transform it to target space, and possibly contribute
  // its reflection if needed.
  if (!damage_rect_in_local_space.IsEmpty()) {
    const gfx::Transform& draw_transform = render_surface->draw_transform();
    gfx::Rect damage_rect_in_target_space = MathUtil::MapEnclosingClippedRect(
        draw_transform, damage_rect_in_local_space);
    target_damage_rect->Union(damage_rect_in_target_space);

    if (layer->replica_layer()) {
      const gfx::Transform& replica_draw_transform =
          render_surface->replica_draw_transform();
      target_damage_rect->Union(MathUtil::MapEnclosingClippedRect(
          replica_draw_transform, damage_rect_in_local_space));
    }
  }

  // If there was damage on the replica's mask, then the target surface receives
  // that damage as well.
  if (layer->replica_layer() && layer->replica_layer()->mask_layer()) {
    LayerImpl* replica_mask_layer = layer->replica_layer()->mask_layer();

    bool replica_is_new = false;
    RectMapData& data =
        RectDataForLayer(replica_mask_layer->id(), &replica_is_new);

    const gfx::Transform& replica_draw_transform =
        render_surface->replica_draw_transform();
    gfx::Rect replica_mask_layer_rect = MathUtil::MapEnclosingClippedRect(
        replica_draw_transform, gfx::Rect(replica_mask_layer->bounds()));
    data.Update(replica_mask_layer_rect, mailboxId_);

    // In the current implementation, a change in the replica mask damages the
    // entire replica region.
    if (replica_is_new ||
        replica_mask_layer->LayerPropertyChanged() ||
        !replica_mask_layer->update_rect().IsEmpty())
      target_damage_rect->Union(replica_mask_layer_rect);
  }

  // If the layer has a background filter, this may cause pixels in our surface
  // to be expanded, so we will need to expand any damage at or below this
  // layer. We expand the damage from this layer too, as we need to readback
  // those pixels from the surface with only the contents of layers below this
  // one in them. This means we need to redraw any pixels in the surface being
  // used for the blur in this layer this frame.
  if (layer->background_filters().HasFilterThatMovesPixels()) {
    ExpandDamageRectInsideRectWithFilters(target_damage_rect,
                                          surface_rect_in_target_space,
                                          layer->background_filters());
  }
}

}  // namespace cc