iOS HW encoder: Increase data rate limit

webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_encoder.cc

 /*
  *  Copyright (c) 2015 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.
  *
  */
@@ skipping 10 matching lines @@
 #include "RTCUIApplication.h"
 #endif
 #include "libyuv/convert_from.h"
 #include "webrtc/base/checks.h"
 #include "webrtc/base/logging.h"
 #include "webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_nalu.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace internal {
 
+// The ratio between kVTCompressionPropertyKey_DataRateLimits and
+// kVTCompressionPropertyKey_AverageBitRate. The data rate limit is set higher
+// than the average bit rate to avoid undershooting the target.
+const float kLimitToAverageBitRateFactor = 1.5f;
+
 // Convenience function for creating a dictionary.
 inline CFDictionaryRef CreateCFDictionary(CFTypeRef* keys,
                                           CFTypeRef* values,
                                           size_t size) {
   return CFDictionaryCreate(kCFAllocatorDefault, keys, values, size,
                             &kCFTypeDictionaryKeyCallBacks,
                             &kCFTypeDictionaryValueCallBacks);
 }
 
 // Copies characters from a CFStringRef into a std::string.
@@ skipping 471 matching lines @@
   }
 }
 
 void H264VideoToolboxEncoder::SetEncoderBitrateBps(uint32_t bitrate_bps) {
   if (compression_session_) {
     internal::SetVTSessionProperty(compression_session_,
                                    kVTCompressionPropertyKey_AverageBitRate,
                                    bitrate_bps);
 
     // TODO(tkchin): Add a helper method to set array value.
-    int64_t bytes_per_second_value = bitrate_bps / 8;
+    int64_t data_limit_bytes_per_second_value = static_cast<int64_t>(
+        bitrate_bps * internal::kLimitToAverageBitRateFactor / 8);
     CFNumberRef bytes_per_second =
         CFNumberCreate(kCFAllocatorDefault,
                        kCFNumberSInt64Type,
-                       &bytes_per_second_value);
+                       &data_limit_bytes_per_second_value);
     int64_t one_second_value = 1;
     CFNumberRef one_second =
         CFNumberCreate(kCFAllocatorDefault,
                        kCFNumberSInt64Type,
                        &one_second_value);
     const void* nums[2] = { bytes_per_second, one_second };
     CFArrayRef data_rate_limits =
         CFArrayCreate(nullptr, nums, 2, &kCFTypeArrayCallBacks);
     OSStatus status =
         VTSessionSetProperty(compression_session_,
@@ skipping 85 matching lines @@
   if (result != 0) {
     LOG(LS_ERROR) << "Encode callback failed: " << result;
     return;
   }
   bitrate_adjuster_.Update(frame._size);
 }
 
 }  // namespace webrtc
 
 #endif  // defined(WEBRTC_VIDEO_TOOLBOX_SUPPORTED)