From 098f8d2c1c678cd65555a44ba1d06bb02b6ac860 Mon Sep 17 00:00:00 2001
From: Sergey Silkin <ssilkin@webrtc.org>
Date: Sat, 14 Apr 2018 22:25:22 +0200
Subject: [PATCH] Round down when converting allocated bitrate from bits to
 kilobits.
MIME-Version: 1.0
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With rounding to the nearest the result can exceed the allocated
bitrate.

Bug: none
Change-Id: I0260a1640a1454951ca8e48fd447e047ef0271ee
Reviewed-on: https://webrtc-review.googlesource.com/69982
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22879}
---
 common_types.h                                      |  5 ++++-
 .../video_coding/codecs/vp9/svc_rate_allocator.cc   | 13 +++++++++++--
 .../codecs/vp9/svc_rate_allocator_unittest.cc       |  9 +++------
 3 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/common_types.h b/common_types.h
index 5896fd62dc..292db5ee23 100644
--- a/common_types.h
+++ b/common_types.h
@@ -570,7 +570,10 @@ class BitrateAllocation {
   std::vector<uint32_t> GetTemporalLayerAllocation(size_t spatial_index) const;
 
   uint32_t get_sum_bps() const { return sum_; }  // Sum of all bitrates.
-  uint32_t get_sum_kbps() const { return (sum_ + 500) / 1000; }
+  uint32_t get_sum_kbps() const {
+    // Round down to not exceed the allocated bitrate.
+    return sum_ / 1000;
+  }
 
   inline bool operator==(const BitrateAllocation& other) const {
     return memcmp(bitrates_, other.bitrates_, sizeof(bitrates_)) == 0;
diff --git a/modules/video_coding/codecs/vp9/svc_rate_allocator.cc b/modules/video_coding/codecs/vp9/svc_rate_allocator.cc
index 48d22fdfb1..e335550cdc 100644
--- a/modules/video_coding/codecs/vp9/svc_rate_allocator.cc
+++ b/modules/video_coding/codecs/vp9/svc_rate_allocator.cc
@@ -12,6 +12,7 @@
 
 #include <algorithm>
 #include <cmath>
+#include <numeric>
 
 #include "rtc_base/checks.h"
 
@@ -132,17 +133,25 @@ std::vector<size_t> SvcRateAllocator::SplitBitrate(size_t num_layers,
                                                    float rate_scaling_factor) {
   std::vector<size_t> bitrates;
 
-  float denominator = 0.0f;
+  double denominator = 0.0;
   for (size_t layer_idx = 0; layer_idx < num_layers; ++layer_idx) {
     denominator += std::pow(rate_scaling_factor, layer_idx);
   }
 
-  float numerator = std::pow(rate_scaling_factor, num_layers - 1);
+  double numerator = std::pow(rate_scaling_factor, num_layers - 1);
   for (size_t layer_idx = 0; layer_idx < num_layers; ++layer_idx) {
     bitrates.push_back(numerator * total_bitrate / denominator);
     numerator /= rate_scaling_factor;
   }
 
+  const size_t sum = std::accumulate(bitrates.begin(), bitrates.end(), 0);
+  // Ensure the sum of split bitrates doesn't exceed the total bitrate.
+  RTC_DCHECK_LE(sum, total_bitrate);
+
+  // Keep the sum of split bitrates equal to the total bitrate by adding bits,
+  // which were lost due to rounding, to the latest layer.
+  bitrates.back() += total_bitrate - sum;
+
   return bitrates;
 }
 
diff --git a/modules/video_coding/codecs/vp9/svc_rate_allocator_unittest.cc b/modules/video_coding/codecs/vp9/svc_rate_allocator_unittest.cc
index f75dfe104b..e89416694c 100644
--- a/modules/video_coding/codecs/vp9/svc_rate_allocator_unittest.cc
+++ b/modules/video_coding/codecs/vp9/svc_rate_allocator_unittest.cc
@@ -134,12 +134,9 @@ TEST(SvcRateAllocatorTest, BitrateIsCapped) {
 
   EXPECT_EQ(allocation.get_sum_kbps(),
             layers[0].maxBitrate + layers[1].maxBitrate + layers[2].maxBitrate);
-  EXPECT_EQ((allocation.GetSpatialLayerSum(0) + 500) / 1000,
-            layers[0].maxBitrate);
-  EXPECT_EQ((allocation.GetSpatialLayerSum(1) + 500) / 1000,
-            layers[1].maxBitrate);
-  EXPECT_EQ((allocation.GetSpatialLayerSum(2) + 500) / 1000,
-            layers[2].maxBitrate);
+  EXPECT_EQ(allocation.GetSpatialLayerSum(0) / 1000, layers[0].maxBitrate);
+  EXPECT_EQ(allocation.GetSpatialLayerSum(1) / 1000, layers[1].maxBitrate);
+  EXPECT_EQ(allocation.GetSpatialLayerSum(2) / 1000, layers[2].maxBitrate);
 }
 
 }  // namespace webrtc