lavf/mov.c: offset index timestamps by the minimum pts to make first pts zero

If the videos starts with B frame, then the minimum composition time
as computed by stts + ctts will be non-zero. Hence we need to shift
the DTS, so that the first pts is zero. This was the intention of that
code-block. However it was subtracting by the wrong amount.

For example, for one of the videos in the bug nonFormatted.mp4 we have

stts:
sample_count  duration
960           1001

ctts:
sample_count  duration
1             3003
2             0
1             3003
....

The resulting composition times are :  3003, 1001, 2002, 6006, ...

The minimum composition time or PTS is 1001, which should be used to
offset DTS. However the code block was wrongly using ctts[0] which is
3003. Hence the PTS was negative. This change computes the minimum pts
encountered while fixing the index, and then subtracts it from all the
timestamps after the edit list fixes are applied.

Samples files available from:

https://bugs.chromium.org/p/chromium/issues/detail?id=721451
https://bugs.chromium.org/p/chromium/issues/detail?id=723537

fate-suite/h264/twofields_packet.mp4 is a similar file starting with 2
B frames. Before this change the PTS of first two B-frames was -6006
and -3003, and I am guessing one of them got dropped when being decoded
and remuxed  to the framecrc before, and now it is not being dropped.

Signed-off-by: Sasi Inguva <isasi@google.com>

1 files changed, 40 insertions, 17 deletions

diff --git a/libavformat/mov.c b/libavformat/mov.c
index 3845e63b53..d7d64c3361 100644
--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -3039,6 +3039,9 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
     int64_t edit_list_dts_entry_end = 0;
     int64_t edit_list_start_ctts_sample = 0;
     int64_t curr_cts;
+    int64_t curr_ctts = 0;
+    int64_t min_corrected_pts = -1;
+    int64_t empty_edits_sum_duration = 0;
     int64_t edit_list_index = 0;
     int64_t index;
     int64_t index_ctts_count;
@@ -3053,6 +3056,7 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
     int first_non_zero_audio_edit = -1;
     int packet_skip_samples = 0;
     MOVIndexRange *current_index_range;
+    int i;
 
     if (!msc->elst_data || msc->elst_count <= 0 || nb_old <= 0) {
         return;
@@ -3080,13 +3084,9 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
 
     // If the dts_shift is positive (in case of negative ctts values in mov),
     // then negate the DTS by dts_shift
-    if (msc->dts_shift > 0)
+    if (msc->dts_shift > 0) {
         edit_list_dts_entry_end -= msc->dts_shift;
-
-    // Offset the DTS by ctts[0] to make the PTS of the first frame 0
-    if (ctts_data_old && ctts_count_old > 0) {
-        edit_list_dts_entry_end -= ctts_data_old[0].duration;
-        av_log(mov->fc, AV_LOG_DEBUG, "Offset DTS by ctts[%d].duration: %d\n", 0, ctts_data_old[0].duration);
+        av_log(mov->fc, AV_LOG_DEBUG, "Shifting DTS by %d because of negative CTTS.\n", msc->dts_shift);
     }
 
     start_dts = edit_list_dts_entry_end;
@@ -3100,6 +3100,7 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
         edit_list_dts_entry_end += edit_list_duration;
         num_discarded_begin = 0;
         if (edit_list_media_time == -1) {
+            empty_edits_sum_duration += edit_list_duration;
             continue;
         }
 
@@ -3179,11 +3180,13 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
 
             // frames (pts) before or after edit list
             curr_cts = current->timestamp + msc->dts_shift;
+            curr_ctts = 0;
 
             if (ctts_data_old && ctts_index_old < ctts_count_old) {
-                av_log(mov->fc, AV_LOG_DEBUG, "shifted frame pts, curr_cts: %"PRId64" @ %"PRId64", ctts: %d, ctts_count: %"PRId64"\n",
-                       curr_cts, ctts_index_old, ctts_data_old[ctts_index_old].duration, ctts_count_old);
-                curr_cts += ctts_data_old[ctts_index_old].duration;
+                curr_ctts = ctts_data_old[ctts_index_old].duration;
+                av_log(mov->fc, AV_LOG_DEBUG, "stts: %"PRId64" ctts: %"PRId64", ctts_index: %"PRId64", ctts_count: %"PRId64"\n",
+                       curr_cts, curr_ctts, ctts_index_old, ctts_count_old);
+                curr_cts += curr_ctts;
                 ctts_sample_old++;
                 if (ctts_sample_old == ctts_data_old[ctts_index_old].count) {
                     if (add_ctts_entry(&msc->ctts_data, &msc->ctts_count,
@@ -3244,14 +3247,21 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
                         }
                     }
                 }
-            } else if (edit_list_start_encountered == 0) {
-                edit_list_start_encountered = 1;
-                // Make timestamps strictly monotonically increasing for audio, by rewriting timestamps for
-                // discarded packets.
-                if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO && frame_duration_buffer) {
-                    fix_index_entry_timestamps(st, st->nb_index_entries, edit_list_dts_counter,
-                                               frame_duration_buffer, num_discarded_begin);
-                    av_freep(&frame_duration_buffer);
+            } else {
+                if (min_corrected_pts < 0) {
+                    min_corrected_pts = edit_list_dts_counter + curr_ctts + msc->dts_shift;
+                } else {
+                    min_corrected_pts = FFMIN(min_corrected_pts, edit_list_dts_counter + curr_ctts + msc->dts_shift);
+                }
+                if (edit_list_start_encountered == 0) {
+                    edit_list_start_encountered = 1;
+                    // Make timestamps strictly monotonically increasing for audio, by rewriting timestamps for
+                    // discarded packets.
+                    if (st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO && frame_duration_buffer) {
+                        fix_index_entry_timestamps(st, st->nb_index_entries, edit_list_dts_counter,
+                                                   frame_duration_buffer, num_discarded_begin);
+                        av_freep(&frame_duration_buffer);
+                    }
                 }
             }
 
@@ -3292,6 +3302,19 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
             }
         }
     }
+    // If there are empty edits, then min_corrected_pts might be positive intentionally. So we subtract the
+    // sum duration of emtpy edits here.
+    min_corrected_pts -= empty_edits_sum_duration;
+
+    // If the minimum pts turns out to be greater than zero after fixing the index, then we subtract the
+    // dts by that amount to make the first pts zero.
+    if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO && min_corrected_pts > 0) {
+        av_log(mov->fc, AV_LOG_DEBUG, "Offset DTS by %"PRId64" to make first pts zero.\n", min_corrected_pts);
+        for (i = 0; i < st->nb_index_entries; ++i) {
+            st->index_entries[i].timestamp -= min_corrected_pts;
+        }
+    }
+
     // Update av stream length
     st->duration = edit_list_dts_entry_end - start_dts;