DevTools: optimize screencast for slower devices.

content/browser/devtools/protocol/page_handler.cc

Index: content/browser/devtools/protocol/page_handler.cc
diff --git a/content/browser/devtools/protocol/page_handler.cc b/content/browser/devtools/protocol/page_handler.cc
index c4aac86bb98135ded08b06918c24c75a53e25e93..a14415631b5299188cf89a88d1899e79a9f6e2e9 100644
--- a/content/browser/devtools/protocol/page_handler.cc
+++ b/content/browser/devtools/protocol/page_handler.cc
@@ -96,11 +96,12 @@ PageHandler::PageHandler()
       screencast_quality_(kDefaultScreenshotQuality),
       screencast_max_width_(-1),
       screencast_max_height_(-1),
+      capture_every_nth_frame_(1),
       capture_retry_count_(0),
       has_compositor_frame_metadata_(false),
-      screencast_frame_sent_(0),
-      screencast_frame_acked_(0),
-      processing_screencast_frame_(false),
+      session_id_(0),
+      frame_counter_(0),
+      frames_in_flight_(0),
       color_picker_(new ColorPicker(base::Bind(
           &PageHandler::OnColorPicked, base::Unretained(this)))),
       host_(nullptr),
@@ -288,7 +289,8 @@ Response PageHandler::CanScreencast(bool* result) {
 Response PageHandler::StartScreencast(const std::string* format,
                                       const int* quality,
                                       const int* max_width,
-                                      const int* max_height) {
+                                      const int* max_height,
+                                      const int* every_nth_frame) {
   RenderWidgetHostImpl* widget_host =
       host_ ? host_->GetRenderWidgetHost() : nullptr;
   if (!widget_host)
@@ -301,6 +303,10 @@ Response PageHandler::StartScreencast(const std::string* format,
     screencast_quality_ = kDefaultScreenshotQuality;
   screencast_max_width_ = max_width ? *max_width : -1;
   screencast_max_height_ = max_height ? *max_height : -1;
+  ++session_id_;
+  frame_counter_ = 0;
+  frames_in_flight_ = 0;
+  capture_every_nth_frame_ = every_nth_frame ? *every_nth_frame : 1;
 
   bool visible = !widget_host->is_hidden();
   NotifyScreencastVisibility(visible);
@@ -320,8 +326,9 @@ Response PageHandler::StopScreencast() {
   return Response::FallThrough();
 }
 
-Response PageHandler::ScreencastFrameAck(int frame_number) {
-  screencast_frame_acked_ = frame_number;
+Response PageHandler::ScreencastFrameAck(int session_id) {
+  if (session_id == session_id_)
+    --frames_in_flight_;
   return Response::OK();
 }
 
@@ -372,12 +379,13 @@ void PageHandler::NotifyScreencastVisibility(bool visible) {
 }
 
 void PageHandler::InnerSwapCompositorFrame() {
-  if (screencast_frame_sent_ - screencast_frame_acked_ >
-      kMaxScreencastFramesInFlight || processing_screencast_frame_) {
+  if (!host_ || !host_->GetView())
     return;
-  }
 
-  if (!host_ || !host_->GetView())
+  if (frames_in_flight_ > kMaxScreencastFramesInFlight)
+    return;
+
+  if (++frame_counter_ % capture_every_nth_frame_)
     return;
 
   RenderWidgetHostViewBase* view = static_cast<RenderWidgetHostViewBase*>(
@@ -412,7 +420,6 @@ void PageHandler::InnerSwapCompositorFrame() {
       gfx::ScaleSize(viewport_size_dip, scale)));
 
   if (snapshot_size_dip.width() > 0 && snapshot_size_dip.height() > 0) {
-    processing_screencast_frame_ = true;
     gfx::Rect viewport_bounds_dip(gfx::ToRoundedSize(viewport_size_dip));
     view->CopyFromCompositingSurface(
         viewport_bounds_dip,
@@ -421,6 +428,7 @@ void PageHandler::InnerSwapCompositorFrame() {
                    weak_factory_.GetWeakPtr(),
                    last_compositor_frame_metadata_),
         kN32_SkColorType);
+    frames_in_flight_++;
   }
 }
 
@@ -429,7 +437,6 @@ void PageHandler::ScreencastFrameCaptured(
     const SkBitmap& bitmap,
     ReadbackResponse response) {
   if (response != READBACK_SUCCESS) {
-    processing_screencast_frame_ = false;
     if (capture_retry_count_) {
       --capture_retry_count_;
       base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
@@ -437,6 +444,7 @@ void PageHandler::ScreencastFrameCaptured(
                                 weak_factory_.GetWeakPtr()),
           base::TimeDelta::FromMilliseconds(kFrameRetryDelayMs));
     }
+    --frames_in_flight_;
     return;
   }
   base::PostTaskAndReplyWithResult(
@@ -452,16 +460,18 @@ void PageHandler::ScreencastFrameEncoded(
     const cc::CompositorFrameMetadata& metadata,
     const base::Time& timestamp,
     const std::string& data) {
-  processing_screencast_frame_ = false;
-
   // Consider metadata empty in case it has no device scale factor.
-  if (metadata.device_scale_factor == 0 || !host_ || data.empty())
+  if (metadata.device_scale_factor == 0 || !host_ || data.empty()) {
+    --frames_in_flight_;
     return;
+  }
 
   RenderWidgetHostViewBase* view = static_cast<RenderWidgetHostViewBase*>(
       host_->GetView());
-  if (!view)
+  if (!view) {
+    --frames_in_flight_;
     return;
+  }
 
   gfx::SizeF screen_size_dip =
       gfx::ScaleSize(gfx::SizeF(view->GetPhysicalBackingSize()),
@@ -478,7 +488,7 @@ void PageHandler::ScreencastFrameEncoded(
   client_->ScreencastFrame(ScreencastFrameParams::Create()
       ->set_data(data)
       ->set_metadata(param_metadata)
-      ->set_frame_number(++screencast_frame_sent_));
+      ->set_session_id(session_id_));
 }
 
 void PageHandler::ScreenshotCaptured(DevToolsCommandId command_id,