Add option to keep original timestamps in MP4 transmuxer.

Closes videojs#168

lib/mp4/transmuxer.js

@@ -120,15 +120,20 @@ sumFrameByteLengths = function(array) {
  * Constructs a single-track, ISO BMFF media segment from AAC data
  * events. The output of this stream can be fed to a SourceBuffer
  * configured with a suitable initialization segment.
+ * @param options {object} transmuxer options object
+ * @param options.keepOriginalTimestamps {boolean} If true, keep the timestamps
+ *        in the source; false to adjust the first segment to start at 0.
  */
-AudioSegmentStream = function(track) {
+AudioSegmentStream = function(track, options) {
   var
     adtsFrames = [],
     sequenceNumber = 0,
     earliestAllowedDts = 0,
     audioAppendStartTs = 0,
     videoBaseMediaDecodeTime = Infinity;
 
+  options = options || {};
+
   AudioSegmentStream.prototype.init.call(this);
 
   this.push = function(data) {
@@ -170,7 +175,8 @@ AudioSegmentStream = function(track) {
     }
 
     frames = this.trimAdtsFramesByEarliestDts_(adtsFrames);
-    track.baseMediaDecodeTime = calculateTrackBaseMediaDecodeTime(track);
+    track.baseMediaDecodeTime =
+      calculateTrackBaseMediaDecodeTime(track, options.keepOriginalTimestamps);
 
     this.prefixWithSilence_(track, frames);
 
@@ -320,6 +326,8 @@ AudioSegmentStream.prototype = new Stream();
  * @param options {object} transmuxer options object
  * @param options.alignGopsAtEnd {boolean} If true, start from the end of the
  *        gopsToAlignWith list when attempting to align gop pts
+ * @param options.keepOriginalTimestamps {boolean} If true, keep the timestamps
+ *        in the source; false to adjust the first segment to start at 0.
  */
 VideoSegmentStream = function(track, options) {
   var
@@ -474,7 +482,8 @@ VideoSegmentStream = function(track, options) {
     // Concatenate the video data and construct the mdat
     mdat = mp4.mdat(this.concatenateNalData_(gops));
 
-    track.baseMediaDecodeTime = calculateTrackBaseMediaDecodeTime(track);
+    track.baseMediaDecodeTime =
+      calculateTrackBaseMediaDecodeTime(track, options.keepOriginalTimestamps);
 
     this.trigger('processedGopsInfo', gops.map(function(gop) {
       return {
@@ -978,20 +987,23 @@ clearDtsInfo = function(track) {
  * DTS the transmuxer has ever seen and the minimum DTS for the
  * current track
  */
-calculateTrackBaseMediaDecodeTime = function(track) {
+calculateTrackBaseMediaDecodeTime = function(track, keepOriginalTimestamps) {
   var
     baseMediaDecodeTime,
     scale,
-    // Calculate the distance, in time, that this segment starts from the start
-    // of the timeline (earliest time seen since the transmuxer initialized)
-    timeSinceStartOfTimeline = track.minSegmentDts - track.timelineStartInfo.dts;
+    minSegmentDts = track.minSegmentDts;
+
+  // Optionally adjust the time so the first segment starts at zero.
+  if (!keepOriginalTimestamps) {
+    minSegmentDts -= track.timelineStartInfo.dts;
+  }
 
   // track.timelineStartInfo.baseMediaDecodeTime is the location, in time, where
   // we want the start of the first segment to be placed
   baseMediaDecodeTime = track.timelineStartInfo.baseMediaDecodeTime;
 
   // Add to that the distance this segment is from the very first
-  baseMediaDecodeTime += timeSinceStartOfTimeline;
+  baseMediaDecodeTime += minSegmentDts;
 
   // baseMediaDecodeTime must not become negative
   baseMediaDecodeTime = Math.max(0, baseMediaDecodeTime);
@@ -1243,7 +1255,7 @@ Transmuxer = function(options) {
         };
         // hook up the audio segment stream to the first track with aac data
         pipeline.coalesceStream.numberOfTracks++;
-        pipeline.audioSegmentStream = new AudioSegmentStream(audioTrack);
+        pipeline.audioSegmentStream = new AudioSegmentStream(audioTrack, options);
         // Set up the final part of the audio pipeline
         pipeline.adtsStream
           .pipe(pipeline.audioSegmentStream)
@@ -1354,7 +1366,7 @@ Transmuxer = function(options) {
         if (audioTrack && !pipeline.audioSegmentStream) {
           // hook up the audio segment stream to the first track with aac data
           pipeline.coalesceStream.numberOfTracks++;
-          pipeline.audioSegmentStream = new AudioSegmentStream(audioTrack);
+          pipeline.audioSegmentStream = new AudioSegmentStream(audioTrack, options);
 
           // Set up the final part of the audio pipeline
           pipeline.adtsStream