FEC: For multi-frame FEC, allow for the size of the code to be increased,

under certain conditions. This generally improves the FEC recovery for 
     bursty loss under medium-high protection level.
Review URL: https://webrtc-codereview.appspot.com/566012

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2271 4adac7df-926f-26a2-2b94-8c16560cd09d

src/modules/rtp_rtcp/source/forward_error_correction.cc

@@ -157,19 +157,14 @@ int32_t ForwardErrorCorrection::GenerateFEC(
     }
     mediaListIt++;
   }
-  // Result in Q0 with an unsigned round.
-  uint32_t numFecPackets = (numMediaPackets * protectionFactor + (1 << 7)) >> 8;
-  // Generate at least one FEC packet if we need protection.
-  if (protectionFactor > 0 && numFecPackets == 0) {
-    numFecPackets = 1;
-  }
+
+  int numFecPackets = GetNumberOfFecPackets(numMediaPackets, protectionFactor);
   if (numFecPackets == 0) {
     return 0;
   }
-  assert(numFecPackets <= numMediaPackets);
 
   // Prepare FEC packets by setting them to 0.
-  for (uint32_t i = 0; i < numFecPackets; i++) {
+  for (int i = 0; i < numFecPackets; i++) {
     memset(_generatedFecPackets[i].data, 0, IP_PACKET_SIZE);
     _generatedFecPackets[i].length = 0;  // Use this as a marker for untouched
     // packets.
@@ -204,6 +199,18 @@ int32_t ForwardErrorCorrection::GenerateFEC(
   return 0;
 }
 
+int ForwardErrorCorrection::GetNumberOfFecPackets(uint16_t numMediaPackets,
+                                                  uint8_t protectionFactor) {
+  // Result in Q0 with an unsigned round.
+  int numFecPackets = (numMediaPackets * protectionFactor + (1 << 7)) >> 8;
+  // Generate at least one FEC packet if we need protection.
+  if (protectionFactor > 0 && numFecPackets == 0) {
+    numFecPackets = 1;
+  }
+  assert(numFecPackets <= numMediaPackets);
+  return numFecPackets;
+}
+
 void ForwardErrorCorrection::GenerateFecBitStrings(
     const PacketList& mediaPacketList,
     uint8_t* packetMask,

src/modules/rtp_rtcp/source/forward_error_correction.h

@@ -198,6 +198,12 @@ class ForwardErrorCorrection {
    */
   int32_t DecodeFEC(ReceivedPacketList* receivedPacketList,
                     RecoveredPacketList* recoveredPacketList);
+
+  // Get the number of FEC packets, given the number of media packets and the
+  // protection factor.
+  int GetNumberOfFecPackets(uint16_t numMediaPackets,
+                            uint8_t protectionFactor);
+
   /**
    * Gets the size in bytes of the FEC/ULP headers, which must be accounted for
    * as packet overhead.

src/modules/rtp_rtcp/source/producer_fec.cc

@@ -18,7 +18,20 @@ namespace webrtc {
 // Minimum RTP header size in bytes.
 enum { kRtpHeaderSize = 12 };
 enum { kREDForFECHeaderLength = 1 };
-enum { kMaxOverhead = 60 };  // Q8.
+// This controls the maximum amount of excess overhead (actual - target)
+// allowed in order to trigger GenerateFEC(), before |params_.max_fec_frames|
+// is reached. Overhead here is defined as relative to number of media packets.
+enum { kMaxExcessOverhead = 50 };  // Q8.
+// This is the minimum number of media packets required (above some protection
+// level) in order to trigger GenerateFEC(), before |params_.max_fec_frames| is
+// reached.
+enum { kMinimumMediaPackets = 4 };
+// Threshold on the received FEC protection level, above which we enforce at
+// least |kMinimumMediaPackets| packets for the FEC code. Below this
+// threshold |kMinimumMediaPackets| is set to default value of 1.
+enum { kHighProtectionThreshold = 80 };  // Corresponds to ~30 overhead, range
+// is 0 to 255, where 255 corresponds to 100% overhead (relative to number of
+// media packets).
 
 struct RtpPacket {
   WebRtc_UWord16 rtpHeaderLength;
@@ -77,6 +90,7 @@ ProducerFec::ProducerFec(ForwardErrorCorrection* fec)
       num_frames_(0),
       incomplete_frame_(false),
       num_first_partition_(0),
+      minimum_media_packets_fec_(1),
       params_(),
       new_params_() {
   memset(&params_, 0, sizeof(params_));
@@ -100,6 +114,11 @@ void ProducerFec::SetFecParameters(const FecProtectionParams* params,
   // produced.
   new_params_ = *params;
   num_first_partition_ = num_first_partition;
+  if (params->fec_rate > kHighProtectionThreshold) {
+    minimum_media_packets_fec_ = kMinimumMediaPackets;
+  } else {
+    minimum_media_packets_fec_ = 1;
+  }
 }
 
 RedPacket* ProducerFec::BuildRedPacket(const uint8_t* data_buffer,
@@ -136,12 +155,13 @@ int ProducerFec::AddRtpPacketAndGenerateFec(const uint8_t* data_buffer,
     ++num_frames_;
     incomplete_frame_ = false;
   }
-  // Produce FEC over at most |params_.max_fec_frames| frames, or as soon as
-  // the wasted overhead (actual overhead - requested protection) is less than
-  // |kMaxOverhead|.
+  // Produce FEC over at most |params_.max_fec_frames| frames, or as soon as:
+  // (1) the excess overhead (actual overhead - requested/target overhead) is
+  // less than |kMaxExcessOverhead|, and
+  // (2) at least |minimum_media_packets_fec_| media packets is reached.
   if (!incomplete_frame_ &&
       (num_frames_ == params_.max_fec_frames ||
-          (Overhead() - params_.fec_rate) < kMaxOverhead)) {
+          (ExcessOverheadBelowMax() && MinimumMediaPacketsReached()))) {
     assert(num_first_partition_ <=
            static_cast<int>(ForwardErrorCorrection::kMaxMediaPackets));
     int ret = fec_->GenerateFEC(media_packets_fec_,
@@ -158,6 +178,32 @@ int ProducerFec::AddRtpPacketAndGenerateFec(const uint8_t* data_buffer,
   return 0;
 }
 
+// Returns true if the excess overhead (actual - target) for the FEC is below
+// the amount |kMaxExcessOverhead|. This effects the lower protection level
+// cases and low number of media packets/frame. The target overhead is given by
+// |params_.fec_rate|, and is only achievable in the limit of large number of
+// media packets.
+bool ProducerFec::ExcessOverheadBelowMax() {
+  return ((Overhead() - params_.fec_rate) < kMaxExcessOverhead);
+}
+
+// Returns true if the media packet list for the FEC is at least
+// |minimum_media_packets_fec_|. This condition tries to capture the effect
+// that, for the same amount of protection/overhead, longer codes
+// (e.g. (2k,2m) vs (k,m)) are generally more effective at recovering losses.
+bool ProducerFec::MinimumMediaPacketsReached() {
+  float avg_num_packets_frame = static_cast<float>(media_packets_fec_.size() /
+                                                   num_frames_);
+  if (avg_num_packets_frame < 2.0f) {
+  return (static_cast<int>(media_packets_fec_.size()) >=
+      minimum_media_packets_fec_);
+  } else {
+    // For larger rates (more packets/frame), increase the threshold.
+    return (static_cast<int>(media_packets_fec_.size()) >=
+        minimum_media_packets_fec_ + 1);
+  }
+}
+
 bool ProducerFec::FecAvailable() const {
   return (fec_packets_.size() > 0);
 }
@@ -196,10 +242,8 @@ int ProducerFec::Overhead() const {
   // protection factor produced by video_coding module and how the FEC
   // generation is implemented.
   assert(!media_packets_fec_.empty());
-  int num_fec_packets = params_.fec_rate * media_packets_fec_.size();
-  // Ceil.
-  int rounding = (num_fec_packets % (1 << 8) > 0) ? (1 << 8) : 0;
-  num_fec_packets = (num_fec_packets + rounding) >> 8;
+  int num_fec_packets = fec_->GetNumberOfFecPackets(media_packets_fec_.size(),
+                                                    params_.fec_rate);
   // Return the overhead in Q8.
   return (num_fec_packets << 8) / media_packets_fec_.size();
 }

src/modules/rtp_rtcp/source/producer_fec.h

@@ -54,6 +54,10 @@ class ProducerFec {
                                  int payload_length,
                                  int rtp_header_length);
 
+  bool ExcessOverheadBelowMax();
+
+  bool MinimumMediaPacketsReached();
+
   bool FecAvailable() const;
 
   RedPacket* GetFecPacket(int red_pl_type, int fec_pl_type,
@@ -68,6 +72,7 @@ class ProducerFec {
   int num_frames_;
   bool incomplete_frame_;
   int num_first_partition_;
+  int minimum_media_packets_fec_;
   FecProtectionParams params_;
   FecProtectionParams new_params_;
 };

src/modules/rtp_rtcp/source/producer_fec_unittest.cc

@@ -55,8 +55,14 @@ class ProducerFecTest : public ::testing::Test {
 };
 
 TEST_F(ProducerFecTest, OneFrameFec) {
+  // The number of media packets (|kNumPackets|), number of frames (one for
+  // this test), and the protection factor (|params->fec_rate|) are set to make
+  // sure the conditions for generating FEC are satisfied. This means:
+  // (1) protection factor is high enough so that actual overhead over 1 frame
+  // of packets is within |kMaxExcessOverhead|, and (2) the total number of
+  // media packets for 1 frame is at least |minimum_media_packets_fec_|.
   const int kNumPackets = 4;
-  FecProtectionParams params = {5, false, 3};
+  FecProtectionParams params = {15, false, 3};
   std::list<RtpPacket*> rtp_packets;
   generator_->NewFrame(kNumPackets);
   producer_->SetFecParameters(&params, 0);  // Expecting one FEC packet.
@@ -86,9 +92,17 @@ TEST_F(ProducerFecTest, OneFrameFec) {
 }
 
 TEST_F(ProducerFecTest, TwoFrameFec) {
+  // The number of media packets/frame (|kNumPackets|), the number of frames
+  // (|kNumFrames|), and the protection factor (|params->fec_rate|) are set to
+  // make sure the conditions for generating FEC are satisfied. This means:
+  // (1) protection factor is high enough so that actual overhead over
+  // |kNumFrames| is within |kMaxExcessOverhead|, and (2) the total number of
+  // media packets for |kNumFrames| frames is at least
+  // |minimum_media_packets_fec_|.
   const int kNumPackets = 2;
   const int kNumFrames = 2;
-  FecProtectionParams params = {5, 0, 3};
+
+  FecProtectionParams params = {15, 0, 3};
   std::list<RtpPacket*> rtp_packets;
   producer_->SetFecParameters(&params, 0);  // Expecting one FEC packet.
   uint32_t last_timestamp = 0;