Make sure temporal layered screenshare frames are sent in at least 2s.

webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc

 /* Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */
 
 #include "webrtc/modules/video_coding/codecs/vp8/screenshare_layers.h"
@@ skipping 29 matching lines @@
 const int ScreenshareLayers::kTl1Flags =
     VP8_EFLAG_NO_REF_ARF | VP8_EFLAG_NO_UPD_ARF | VP8_EFLAG_NO_UPD_LAST;
 
 // Allow predicting from only TL0 to allow participants to switch to the high
 // bitrate stream. This means predicting only from the LAST reference frame, but
 // only updating GF to not corrupt TL0.
 const int ScreenshareLayers::kTl1SyncFlags =
     VP8_EFLAG_NO_REF_ARF | VP8_EFLAG_NO_REF_GF | VP8_EFLAG_NO_UPD_ARF |
     VP8_EFLAG_NO_UPD_LAST;
 
+// Always emit a frame with certain interval, even if bitrate targets have
+// been exceeded.
+const int ScreenshareLayers::kMaxFrameIntervalMs = 2000;
+
 ScreenshareLayers::ScreenshareLayers(int num_temporal_layers,
                                      uint8_t initial_tl0_pic_idx,
                                      Clock* clock)
     : clock_(clock),
       number_of_temporal_layers_(num_temporal_layers),
       last_base_layer_sync_(false),
       tl0_pic_idx_(initial_tl0_pic_idx),
       active_layer_(-1),
       last_timestamp_(-1),
       last_sync_timestamp_(-1),
+      last_emitted_tl0_timestamp_(-1),
       min_qp_(-1),
       max_qp_(-1),
       max_debt_bytes_(0),
       frame_rate_(-1) {
   RTC_CHECK_GT(num_temporal_layers, 0);
   RTC_CHECK_LE(num_temporal_layers, 2);
 }
 
 ScreenshareLayers::~ScreenshareLayers() {
   UpdateHistograms();
 }
 
 int ScreenshareLayers::CurrentLayerId() const {
   // Codec does not use temporal layers for screenshare.
   return 0;
 }
 
 int ScreenshareLayers::EncodeFlags(uint32_t timestamp) {
   if (number_of_temporal_layers_ <= 1) {
     // No flags needed for 1 layer screenshare.
     return 0;
   }
 
   if (stats_.first_frame_time_ms_ == -1)
     stats_.first_frame_time_ms_ = clock_->TimeInMilliseconds();
 
   int64_t unwrapped_timestamp = time_wrap_handler_.Unwrap(timestamp);
   int flags = 0;
-
   if (active_layer_ == -1 ||
       layers_[active_layer_].state != TemporalLayer::State::kDropped) {
+    if (last_emitted_tl0_timestamp_ != -1 &&
+        (unwrapped_timestamp - last_emitted_tl0_timestamp_) / 90 >
+            kMaxFrameIntervalMs) {
+      // Too long time has passed since the last frame was emitted, cancel debt.
+      layers_[0].debt_bytes_ = 0;

[stefan-webrtc, 2016/04/11 10:29:22]

Is it correct to cancel the whole debt? Instead we could set it to
max_debt_bytes_ - 1 so that we don't end up sending a burst of TL0 frames after
this has triggered?

[sprang_webrtc, 2016/04/11 11:02:53]

You mean if the frames would end up being reeealy tiny? I find that unlikely :)
But sure, we can add that as a precaution.

+    }
     if (layers_[0].debt_bytes_ > max_debt_bytes_) {
       // Must drop TL0, encode TL1 instead.
       if (layers_[1].debt_bytes_ > max_debt_bytes_) {
         // Must drop both TL0 and TL1.
         active_layer_ = -1;
       } else {
         active_layer_ = 1;
       }
     } else {
       active_layer_ = 0;
     }
   }
 
   switch (active_layer_) {
     case 0:
       flags = kTl0Flags;
+      last_emitted_tl0_timestamp_ = unwrapped_timestamp;
       break;
     case 1:
       if (TimeToSync(unwrapped_timestamp)) {
         last_sync_timestamp_ = unwrapped_timestamp;
         flags = kTl1SyncFlags;
       } else {
         flags = kTl1Flags;
       }
       break;
     case -1:
       flags = -1;
       ++stats_.num_dropped_frames_;
       break;
     default:
       flags = -1;
       RTC_NOTREACHED();
   }
 
-  // Make sure both frame droppers leak out bits.
   int64_t ts_diff;
   if (last_timestamp_ == -1) {
     ts_diff = kOneSecond90Khz / (frame_rate_ <= 0 ? 5 : frame_rate_);
   } else {
     ts_diff = unwrapped_timestamp - last_timestamp_;
   }
-
+  // Make sure both frame droppers leak out bits.
   layers_[0].UpdateDebt(ts_diff / 90);
   layers_[1].UpdateDebt(ts_diff / 90);
   last_timestamp_ = timestamp;
   return flags;
 }
 
 bool ScreenshareLayers::ConfigureBitrates(int bitrate_kbps,
                                           int max_bitrate_kbps,
                                           int framerate,
                                           vpx_codec_enc_cfg_t* cfg) {
@@ skipping 191 matching lines @@
       RTC_HISTOGRAM_COUNTS_10000("WebRTC.Video.Screenshare.Layer1.Qp",
                                  stats_.tl1_qp_sum_ / stats_.num_tl1_frames_);
       RTC_HISTOGRAM_COUNTS_10000(
           "WebRTC.Video.Screenshare.Layer1.TargetBitrate",
           stats_.tl1_target_bitrate_sum_ / stats_.num_tl1_frames_);
     }
   }
 }
 
 }  // namespace webrtc