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..4ddd7acee7cca3ac24b7e41b2c9149c14a4d9620 100644
--- a/media/video/h264_poc.cc
+++ b/media/video/h264_poc.cc
@@ -62,17 +62,20 @@ 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;
-  }
-
   bool mmco5 = HasMMCO5(slice_hdr);
   int32_t max_frame_num = 1 << (sps->log2_max_frame_num_minus4 + 4);
   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) {

[Pawel Osciak, 2016/05/20 00:21:31]

Not sure if this is intentional, but this changes the logic so that frame_num
has to be strictly larger than prev_frame_num, although them being equal is
sometimes valid (even without gaps).

[sandersd (OOO until July 31), 2016/05/20 00:39:09]

My understanding of the spec is that this only happens when there is interlacing
(not implemented by H264POC yet) or redundant pictures (these are not passed to
H264POC).

Would you prefer that H264POC can handle (or internally ignore) redundant
pictures?

[Pawel Osciak, 2016/05/20 07:50:12]

I just thought that not changing the condition would keep the existing behavior,
but please feel free to make a decision based on what is supported by VTVDA.

+    // 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.
   //