GStreamer now has support for I-frame-only (aka keyframe) trick mode playback of DASH streams. It works only on DASH streams with ISOBMFF (aka MP4) fragments, and only if these contain all the required information. This is something I wanted to blog about since many months already, and it’s even included in the GStreamer 1.10 release already.
When trying to play back a DASH stream with rates that are much higher than real-time (say 32x), or playing the streams in reverse, you can easily run into various problems. This is something that was already supported by GStreamer in older versions, for both DASH streams as well as local files or HLS streams but it’s far from ideal. What would happen is that you usually run out of available network bandwidth (you need to be able to download the stream 32x faster than usual), or out of CPU/GPU resources (it needs to be decoded 32x faster than usual) and even if all that works, there’s no point in displaying 960 (30fps at 32x) frames per second.
To get around that, GStreamer 1.10 can now (if explicitly requested with GST_SEEK_FLAG_TRICKMODE_KEY_UNITS) only download and decode I-frames. Depending on the distance of I-frames in the stream and the selected playback speed, this looks more or less smooth. Also depending on that, this might still yield to many frames to be downloaded or decoded in real-time, so GStreamer also measures the distance between I-frames, how fast data can be downloaded and whether decoders and sinks can catch up to decide whether to skip over a couple of I-frames and maybe only download every third I-frame.
If you want to test this, grab the playback-test from GStreamer, select the trickmode key-units mode, and seek in a DASH stream while providing a higher positive or negative (reverse) playback rate.
Let us know if you run into any problems with any specific streams!
Short Implementation Overview
From an implementation point of view this works by having the DASH element in GStreamer (dashdemux) not only download the ISOBMFF fragments but also parses the headers of each to get the positions and distances of each I-frame in the fragment. Based on that it then decides which ones to download or whether to skip ahead one or more fragments. The ISOBMFF headers are then passed to the MP4 demuxer (qtdemux), followed by discontinuous buffers that only contain the actual I-frames and nothing else. While this sounds rather simple from an high-level point of view, getting this all right in the details was the result of a couple of months of work by Edward Hervey and myself.
Currently the heuristics for deciding which I-frames to download and how much to skip ahead are rather minimal, but it’s working fine in many situations already. A lot of tuning can still be done though, and some streams are working less well than others which can also be improved.