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diff --git a/docs/design/part-stereo-multiview-video.markdown b/docs/design/part-stereo-multiview-video.markdown new file mode 100644 index 000000000..c9370c5e6 --- /dev/null +++ b/docs/design/part-stereo-multiview-video.markdown @@ -0,0 +1,175 @@ +Design for Stereoscopic & Multiview Video Handling +================================================== + +There are two cases to handle: + +* Encoded video output from a demuxer to parser / decoder or from encoders into a muxer. +* Raw video buffers + +The design below is somewhat based on the proposals from +[bug 611157](https://bugzilla.gnome.org/show_bug.cgi?id=611157) + +Multiview is used as a generic term to refer to handling both +stereo content (left and right eye only) as well as extensions for videos +containing multiple independent viewpoints. + +Encoded Signalling +------------------ +This is regarding the signalling in caps and buffers from demuxers to +parsers (sometimes) or out from encoders. + +For backward compatibility with existing codecs many transports of +stereoscopic 3D content use normal 2D video with 2 views packed spatially +in some way, and put extra new descriptions in the container/mux. + +Info in the demuxer seems to apply to stereo encodings only. For all +MVC methods I know, the multiview encoding is in the video bitstream itself +and therefore already available to decoders. Only stereo systems have been retro-fitted +into the demuxer. + +Also, sometimes extension descriptions are in the codec (e.g. H.264 SEI FPA packets) +and it would be useful to be able to put the info onto caps and buffers from the +parser without decoding. + +To handle both cases, we need to be able to output the required details on +encoded video for decoders to apply onto the raw video buffers they decode. + +*If there ever is a need to transport multiview info for encoded data the +same system below for raw video or some variation should work* + +### Encoded Video: Parameters to transmit in caps +1. multiview-mode (called "Channel Layout" in bug 611157) + * Whether a stream is mono, for a single eye, stereo, mixed-mono-stereo + (switches between mono and stereo - mp4 can do this) + * Uses a buffer flag to mark individual buffers as mono or "not mono" + (single|stereo|multiview) for mixed scenarios. The alternative (not + proposed) is for the demuxer to switch caps for each mono to not-mono + change, and not used a 'mixed' caps variant at all. + * _single_ refers to a stream of buffers that only contain 1 view. + It is different from mono in that the stream is a marked left or right + eye stream for later combining in a mixer or when displaying. + * _multiple_ marks a stream with multiple independent views encoded. + It is included in this list for completeness. As noted above, there's + currently no scenario that requires marking encoded buffers as MVC. +2. Frame-packing arrangements / view sequence orderings + * Possible frame packings: side-by-side, side-by-side-quincunx, + column-interleaved, row-interleaved, top-bottom, checker-board + * bug 611157 - sreerenj added side-by-side-full and top-bottom-full but + I think that's covered by suitably adjusting pixel-aspect-ratio. If + not, they can be added later. + * _top-bottom_, _side-by-side_, _column-interleaved_, _row-interleaved_ are as the names suggest. + * _checker-board_, samples are left/right pixels in a chess grid +-+-+-/-+-+-+ + * _side-by-side-quincunx_. Side By Side packing, but quincunx sampling - + 1 pixel offset of each eye needs to be accounted when upscaling or displaying + * there may be other packings (future expansion) + * Possible view sequence orderings: frame-by-frame, frame-primary-secondary-tracks, sequential-row-interleaved + * _frame-by-frame_, each buffer is left, then right view etc + * _frame-primary-secondary-tracks_ - the file has 2 video tracks (primary and secondary), one is left eye, one is right. + Demuxer info indicates which one is which. + Handling this means marking each stream as all-left and all-right views, decoding separately, and combining automatically (inserting a mixer/combiner in playbin) + -> *Leave this for future expansion* + * _sequential-row-interleaved_ Mentioned by sreerenj in bug patches, I can't find a mention of such a thing. Maybe it's in MPEG-2 + -> *Leave this for future expansion / deletion* +3. view encoding order + * Describes how to decide which piece of each frame corresponds to left or right eye + * Possible orderings left, right, left-then-right, right-then-left + - Need to figure out how we find the correct frame in the demuxer to start decoding when seeking in frame-sequential streams + - Need a buffer flag for marking the first buffer of a group. +4. "Frame layout flags" + * flags for view specific interpretation + * horizontal-flip-left, horizontal-flip-right, vertical-flip-left, vertical-flip-right + Indicates that one or more views has been encoded in a flipped orientation, usually due to camera with mirror or displays with mirrors. + * This should be an actual flags field. Registered GLib flags types aren't generally well supported in our caps - the type might not be loaded/registered yet when parsing a caps string, so they can't be used in caps templates in the registry. + * It might be better just to use a hex value / integer + +Buffer representation for raw video +----------------------------------- +* Transported as normal video buffers with extra metadata +* The caps define the overall buffer width/height, with GstVideoMeta to extract the individual views. +* pixel-aspect-ratio adjusted if needed to double the overall width/height +* video sinks that don't know about multiview extensions yet should show the '0th' GstVideoMeta and + therefore the 'primary' view in a backwards compatible way for simple frame-packed transports. + For frame-sequence outputs, things might look weird, but just adding multiview-mode to the sink caps + can disallow those transports. +* _row-interleaved_ packing is actually just side-by-side memory layout with half frame width, twice + the height, so can be handled by adjusting the overall caps and strides +* Other exotic layouts need new pixel formats defined (checker-board, column-interleaved, side-by-side-quincunx) +* _Frame-by-frame_ - one view per buffer, but with alternating metas marking which buffer is which left/right/other view and using a new buffer flag as described above + to mark the start of a group of corresponding frames. +* New video caps addition as for encoded buffers + +### Proposed Caps fields +Combining the requirements above and collapsing the combinations into mnemonics: + +* multiview-mode = + mono | left | right | sbs | sbs-quin | col | row | topbot | checkers | + frame-by-frame | mixed-sbs | mixed-sbs-quin | mixed-col | mixed-row | + mixed-topbot | mixed-checkers | mixed-frame-by-frame | multiview-frames mixed-multiview-frames +* multiview-flags = + + 0x0000 none + + 0x0001 right-view-first + + 0x0002 left-h-flipped + + 0x0004 left-v-flipped + + 0x0008 right-h-flipped + + 0x0010 right-v-flipped + +### Proposed new buffer flags +Add two new GST_VIDEO_BUFFER flags in video-frame.h and make it clear that those +flags can apply to encoded video buffers too. wtay says that's currently the +case anyway, but the documentation should say it. + +**GST_VIDEO_BUFFER_FLAG_MULTIPLE_VIEW** - Marks a buffer as representing non-mono content, although it may be a single (left or right) eye view. +**GST_VIDEO_BUFFER_FLAG_FIRST_IN_BUNDLE** - for frame-sequential methods of transport, mark the "first" of a left/right/other group of frames + +### A new GstMultiviewMeta +This provides a place to describe all provided views in a buffer / stream, +and through Meta negotiation to inform decoders about which views to decode if +not all are wanted. + +* Logical labels/names and mapping to GstVideoMeta numbers +* Standard view labels LEFT/RIGHT, and non-standard ones (strings) + +Outputting stereo content +------------------------- +The initial implementation for output will be stereo content in glimagesink + +### Output Considerations with OpenGL +* If we have support for stereo GL buffer formats, we can output separate left/right eye images and let the hardware take care of display. +* Otherwise, glimagesink needs to render one window with left/right in a suitable frame packing + and that will only show correctly in fullscreen on a device set for the right 3D packing -> requires app intervention to set the video mode. +* Which could be done manually on the TV, or with HDMI 1.4 by setting the right video mode for the screen to inform the TV or third option, we + support rendering to two separate overlay areas on the screen - one for left eye, one for right which can be supported using the 'splitter' element and 2 output sinks or, better, add a 2nd window overlay for split stereo output +* Intel hardware doesn't do stereo GL buffers - only nvidia and AMD, so initial implementation won't include that + +## Other elements for handling multiview content +* videooverlay interface extensions + * Element message to communicate the presence of stereoscopic information to the app + * New API for the app to set rendering options for stereo/multiview content + * This might be best implemented as a **multiview-disposition GstContext**, so that + the pipeline can share app preferences for (in particular) downmixing + to mono for output, or in the sink +* Converter element + * convert different view layouts + * Render to anaglyphs of different types (magenta/green, red/blue, etc) and output as mono +* Mixer element + * take 2 video streams and output as stereo + * later take n video streams + * share code with the converter, it just takes input from n pads instead of one. +* Splitter element + * Output one pad per view + +### Implementing MVC handling in decoders / parsers (and encoders) +Things to do to implement MVC handling + +1. Parsing SEI in h264parse and setting caps (patches available in + bugzilla for parsing, see below) +2. Integrate gstreamer-vaapi MVC support with this proposal +3. Help with [libav MVC implementation](https://wiki.libav.org/Blueprint/MVC) +4. generating SEI in H.264 encoder +5. Support for MPEG2 MVC extensions + +### Relevant bugs +[bug 685215](https://bugzilla.gnome.org/show_bug.cgi?id=685215) - codecparser h264: Add initial MVC parser +[bug 696135](https://bugzilla.gnome.org/show_bug.cgi?id=696135) - h264parse: Add mvc stream parsing support +[bug 732267](https://bugzilla.gnome.org/show_bug.cgi?id=732267) - h264parse: extract base stream from MVC or SVC encoded streams + |