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 <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"

namespace cc {

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

DamageTracker::DamageTracker()
    : current_rect_history_(new RectMap),
      next_rect_history_(new RectMap) {}

DamageTracker::~DamageTracker() {}

static inline void ExpandRectWithFilters(
    gfx::RectF* 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::RectF* damage_rect,
    const gfx::RectF& pre_filter_rect,
    const FilterOperations& filters) {
  gfx::RectF expanded_damage_rect = *damage_rect;
  ExpandRectWithFilters(&expanded_damage_rect, filters);
  gfx::RectF 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,
    gfx::Rect target_surface_content_rect,
    LayerImpl* target_surface_mask_layer,
    const FilterOperations& filters,
    SkImageFilter* filter) {
  //
  // 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, two RectMaps are maintained:
  //
  //      1. The "current" 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 removed from the map until it has only leftover layers
  //         that no longer exist.
  //
  //      2. The "next" map starts out empty, and as the algorithm progresses,
  //         every layer/surface that contributes to the surface is added to the
  //         map.
  //
  //      3. After the damage rect is computed, the two maps are swapped, so
  //         that the damage tracker is ready for the next frame.
  //

  // 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::RectF damage_from_active_layers =
      TrackDamageFromActiveLayers(layer_list, target_surface_layer_id);
  gfx::RectF damage_from_surface_mask =
      TrackDamageFromSurfaceMask(target_surface_mask_layer);
  gfx::RectF damage_from_leftover_rects = TrackDamageFromLeftoverRects();

  gfx::RectF 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);

    if (filters.HasFilterThatMovesPixels()) {
      ExpandRectWithFilters(&damage_rect_for_this_update, filters);
    } else if (filter) {
      // TODO(senorblanco):  Once SkImageFilter reports its outsets, use
      // those here to limit damage.
      damage_rect_for_this_update = target_surface_content_rect;
    }
  }

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

  // The next history map becomes the current map for the next frame. Note this
  // must happen every frame to correctly track changes, even if damage
  // accumulates over multiple frames before actually being drawn.
  swap(current_rect_history_, next_rect_history_);
}

gfx::RectF DamageTracker::RemoveRectFromCurrentFrame(int layer_id,
                                                     bool* layer_is_new) {
  RectMap::iterator iter = current_rect_history_->find(layer_id);
  *layer_is_new = iter == current_rect_history_->end();
  if (*layer_is_new)
    return gfx::RectF();

  gfx::RectF ret = iter->second;
  current_rect_history_->erase(iter);
  return ret;
}

void DamageTracker::SaveRectForNextFrame(int layer_id,
                                         const gfx::RectF& target_space_rect) {
  // This layer should not yet exist in next frame's history.
  DCHECK_GT(layer_id, 0);
  DCHECK(next_rect_history_->find(layer_id) == next_rect_history_->end());
  (*next_rect_history_)[layer_id] = target_space_rect;
}

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

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

  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::RectF(gfx::PointF(),
                             target_surface_mask_layer->bounds());
  }

  return damage_rect;
}

gfx::RectF 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::RectF damage_rect = gfx::RectF();

  for (RectMap::iterator it = current_rect_history_->begin();
       it != current_rect_history_->end();
       ++it)
    damage_rect.Union(it->second);

  current_rect_history_->clear();

  return damage_rect;
}

static bool LayerNeedsToRedrawOntoItsTargetSurface(LayerImpl* layer) {
  // If the layer does NOT own a surface but has SurfacePropertyChanged,
  // this means that its target surface is affected and needs to be redrawn.
  // However, if the layer DOES own a surface, then the SurfacePropertyChanged
  // flag should not be used here, because that flag represents whether the
  // layer's surface has changed.
  if (layer->render_surface())
    return layer->LayerPropertyChanged();
  return layer->LayerPropertyChanged() || layer->LayerSurfacePropertyChanged();
}

void DamageTracker::ExtendDamageForLayer(LayerImpl* layer,
                                         gfx::RectF* 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;
  gfx::RectF old_rect_in_target_space =
      RemoveRectFromCurrentFrame(layer->id(), &layer_is_new);

  gfx::RectF rect_in_target_space = MathUtil::MapClippedRect(
      layer->draw_transform(),
      gfx::RectF(gfx::PointF(), layer->content_bounds()));
  SaveRectForNextFrame(layer->id(), rect_in_target_space);

  if (layer_is_new || LayerNeedsToRedrawOntoItsTargetSurface(layer)) {
    // 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);
  } else if (!layer->update_rect().IsEmpty()) {
    // If the layer properties haven't changed, then the the target surface is
    // only affected by the layer's update area, which could be empty.
    gfx::RectF update_content_rect =
        layer->LayerRectToContentRect(layer->update_rect());
    gfx::RectF update_rect_in_target_space =
        MathUtil::MapClippedRect(layer->draw_transform(), update_content_rect);
    target_damage_rect->Union(update_rect_in_target_space);
  }
}

void DamageTracker::ExtendDamageForRenderSurface(
    LayerImpl* layer, gfx::RectF* 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;
  gfx::RectF old_surface_rect = RemoveRectFromCurrentFrame(layer->id(),
                                                           &surface_is_new);

  // The drawableContextRect() already includes the replica if it exists.
  gfx::RectF surface_rect_in_target_space =
      render_surface->DrawableContentRect();
  SaveRectForNextFrame(layer->id(), surface_rect_in_target_space);

  gfx::RectF damage_rect_in_local_space;
  if (surface_is_new ||
      render_surface->SurfacePropertyChanged() ||
      layer->LayerSurfacePropertyChanged()) {
    // 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::RectF damage_rect_in_target_space =
        MathUtil::MapClippedRect(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::MapClippedRect(
          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;
    RemoveRectFromCurrentFrame(replica_mask_layer->id(), &replica_is_new);

    const gfx::Transform& replica_draw_transform =
        render_surface->replica_draw_transform();
    gfx::RectF replica_mask_layer_rect = MathUtil::MapClippedRect(
        replica_draw_transform,
        gfx::RectF(gfx::PointF(), replica_mask_layer->bounds()));
    SaveRectForNextFrame(replica_mask_layer->id(), replica_mask_layer_rect);

    // 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