VSE: Fix audaspace not reading ffmpeg files with start offset correctly

The duration and start time for audio strips were not correctly read in
audaspace.

Some video files have a "lead in" section of audio that plays before the
video starts playing back. Before this patch, we would play this lead in
audio at the same time as the video started and thus the audio would not
be in sync anymore.

Now the lead in audio is cut off and the duration should be correctly
calculated with this in mind.

If the audio starts after the video, the audio strip is shifted to
account for this, but it will also lead to cut off audio which might not
be wanted. However we don't have a simple way to solve this at this
point.

Differential Revision: http://developer.blender.org/D11917

1 files changed, 39 insertions, 7 deletions

diff --git a/source/blender/sequencer/intern/strip_add.c b/source/blender/sequencer/intern/strip_add.c
index dab5593be37..cc70cc38ebb 100644
--- a/source/blender/sequencer/intern/strip_add.c
+++ b/source/blender/sequencer/intern/strip_add.c
@@ -382,9 +382,14 @@ Sequence *SEQ_add_image_strip(Main *bmain, Scene *scene, ListBase *seqbase, SeqL
  * \return created strip
  */
 
-Sequence *SEQ_add_sound_strip(Main *bmain, Scene *scene, ListBase *seqbase, SeqLoadData *load_data)
+Sequence *SEQ_add_sound_strip(Main *bmain,
+                              Scene *scene,
+                              ListBase *seqbase,
+                              SeqLoadData *load_data,
+                              const double audio_offset)
 {
   bSound *sound = BKE_sound_new_file(bmain, load_data->path); /* Handles relative paths. */
+  sound->offset_time = audio_offset;
   SoundInfo info;
   bool sound_loaded = BKE_sound_info_get(bmain, sound, &info);
 
@@ -398,14 +403,36 @@ Sequence *SEQ_add_sound_strip(Main *bmain, Scene *scene, ListBase *seqbase, SeqL
     return NULL;
   }
 
-  Sequence *seq = SEQ_sequence_alloc(
-      seqbase, load_data->start_frame, load_data->channel, SEQ_TYPE_SOUND_RAM);
+  /* If this sound it part of a video, then the sound might start after the video.
+   * In this case we need to then offset the start frame of the audio so it syncs up
+   * properly with the video.
+   */
+  int start_frame_offset = info.start_offset * FPS;
+  double start_frame_offset_remainer = (info.start_offset * FPS - start_frame_offset) / FPS;
+
+  if (start_frame_offset_remainer > FLT_EPSILON) {
+    /* We can't represent a fraction of a frame, so skip the first frame fraction of sound so we
+     * start on a "whole" frame.
+     */
+    start_frame_offset++;
+  }
+
+  sound->offset_time += start_frame_offset_remainer;
+
+  Sequence *seq = SEQ_sequence_alloc(seqbase,
+                                     load_data->start_frame + start_frame_offset,
+                                     load_data->channel,
+                                     SEQ_TYPE_SOUND_RAM);
   seq->sound = sound;
   seq->scene_sound = NULL;
 
-  /* We add a very small negative offset here, because
-   * ceil(132.0) == 133.0, not nice with videos, see T47135. */
-  seq->len = MAX2(1, (int)ceil((double)info.length * FPS - 1e-4));
+  /* We round the frame duration as the audio sample lenghts usually does not
+   * line up with the video frames. Therefore we round this number to the
+   * nearsest frame as the audio track usually overshoots or undershoots the
+   * end frame ofthe video by a little bit.
+   * See T47135 for under shoot example.
+   */
+  seq->len = MAX2(1, round((info.length - sound->offset_time) * FPS));
 
   Strip *strip = seq->strip;
   /* We only need 1 element to store the filename. */
@@ -477,7 +504,11 @@ Sequence *SEQ_add_meta_strip(Scene *scene, ListBase *seqbase, SeqLoadData *load_
  * \param load_data: SeqLoadData with information necessary to create strip
  * \return created strip
  */
-Sequence *SEQ_add_movie_strip(Main *bmain, Scene *scene, ListBase *seqbase, SeqLoadData *load_data)
+Sequence *SEQ_add_movie_strip(Main *bmain,
+                              Scene *scene,
+                              ListBase *seqbase,
+                              SeqLoadData *load_data,
+                              double *r_video_start_offset)
 {
   char path[sizeof(load_data->path)];
   BLI_strncpy(path, load_data->path, sizeof(path));
@@ -552,6 +583,7 @@ Sequence *SEQ_add_movie_strip(Main *bmain, Scene *scene, ListBase *seqbase, SeqL
 
   if (anim_arr[0] != NULL) {
     seq->len = IMB_anim_get_duration(anim_arr[0], IMB_TC_RECORD_RUN);
+    *r_video_start_offset = IMD_anim_get_offset(anim_arr[0]);
 
     IMB_anim_load_metadata(anim_arr[0]);