H264POC: Allow gaps in frame_num.

media/video/h264_poc.cc

Index: media/video/h264_poc.cc
diff --git a/media/video/h264_poc.cc b/media/video/h264_poc.cc
index 164d704f13dd51eee5759fe65fd7bb0f03abea3e..88a4a9063e569c508f4beca9be6804dde23f9ee4 100644
--- a/media/video/h264_poc.cc
+++ b/media/video/h264_poc.cc
@@ -27,6 +27,7 @@ void H264POC::Reset() {
   ref_pic_order_cnt_lsb_ = 0;
   prev_frame_num_ = 0;
   prev_frame_num_offset_ = 0;
+  prev_poc_ = 0;
 }
 
 // Check if a slice includes memory management control operation 5, which
@@ -62,10 +63,10 @@ bool H264POC::ComputePicOrderCnt(
     return false;
   }
 
-  // TODO(sandersd): Handle |gaps_in_frame_num_value|.
-  if (prev_frame_num_ > 0 && prev_frame_num_ < slice_hdr.frame_num - 1) {
-    DLOG(ERROR) << "Gaps in frame_num are not supported";
-    return false;
+  if (!slice_hdr.idr_pic_flag && slice_hdr.frame_num == prev_frame_num_) {
+    DLOG(WARNING) << "Redundant picture passed to H264POC";
+    *pic_order_cnt = prev_poc_;
+    return true;
   }
 
   bool mmco5 = HasMMCO5(slice_hdr);
@@ -73,11 +74,21 @@ bool H264POC::ComputePicOrderCnt(
   int32_t max_pic_order_cnt_lsb =
       1 << (sps->log2_max_pic_order_cnt_lsb_minus4 + 4);
 
+  if (!slice_hdr.idr_pic_flag &&
+      slice_hdr.frame_num != (prev_frame_num_ + 1) % max_frame_num) {
+    // We don't do any special handling of gaps in |frame_num|, because the
+    // computations below are unaffected by them. In particular, wrapping is
+    // handled the same whether we simulate the missing frames or not.
+    if (!sps->gaps_in_frame_num_value_allowed_flag)
+      DLOG(WARNING) << "Invalid gap in frame_num";
+  }
+
   // Based on T-REC-H.264 8.2.1, "Decoding process for picture order
   // count", available from http://www.itu.int/rec/T-REC-H.264.
   //
   // Reorganized slightly from spec pseudocode to handle MMCO5 when storing
   // state instead of when loading it.
+  int32_t poc;
   switch (sps->pic_order_cnt_type) {
     case 0: {
       int32_t prev_pic_order_cnt_msb = ref_pic_order_cnt_msb_;
@@ -109,7 +120,7 @@ bool H264POC::ComputePicOrderCnt(
       //           (assuming no interlacing).
       int32_t top_foc = pic_order_cnt_msb + slice_hdr.pic_order_cnt_lsb;
       int32_t bottom_foc = top_foc + slice_hdr.delta_pic_order_cnt_bottom;
-      *pic_order_cnt = std::min(top_foc, bottom_foc);
+      poc = std::min(top_foc, bottom_foc);
 
       // Store state.
       prev_frame_num_ = slice_hdr.frame_num;
@@ -178,7 +189,7 @@ bool H264POC::ComputePicOrderCnt(
       int32_t top_foc = expected_pic_order_cnt + slice_hdr.delta_pic_order_cnt0;
       int32_t bottom_foc = top_foc + sps->offset_for_top_to_bottom_field +
                            slice_hdr.delta_pic_order_cnt1;
-      *pic_order_cnt = std::min(top_foc, bottom_foc);
+      poc = std::min(top_foc, bottom_foc);
 
       // Store state.
       prev_frame_num_ = slice_hdr.frame_num;
@@ -202,11 +213,11 @@ bool H264POC::ComputePicOrderCnt(
       // 8-12, 8-13. Derive |temp_pic_order_count| (it's always the
       // |pic_order_cnt|, regardless of interlacing).
       if (slice_hdr.idr_pic_flag)
-        *pic_order_cnt = 0;
+        poc = 0;
       else if (slice_hdr.nal_ref_idc == 0)
-        *pic_order_cnt = 2 * (frame_num_offset + slice_hdr.frame_num) - 1;
+        poc = 2 * (frame_num_offset + slice_hdr.frame_num) - 1;
       else
-        *pic_order_cnt = 2 * (frame_num_offset + slice_hdr.frame_num);
+        poc = 2 * (frame_num_offset + slice_hdr.frame_num);
 
       // Store state.
       prev_frame_num_ = slice_hdr.frame_num;
@@ -222,6 +233,7 @@ bool H264POC::ComputePicOrderCnt(
       return false;
   }
 
+  *pic_order_cnt = prev_poc_ = poc;
   return true;
 }