I'm trying to configure a "Windows Media Audio Standard" DMO codec to compress in single-pass, constant bit-rate mode (CBR). Unfortunately I can not find on the MSDN documentation how can I pass the desired bit-rate to the encoder object.
In other words, I'm looking for the equivalent of MFPKEY_RMAX which seems to identify be the desired bit-rate setting for two-pass Variable Bit-rate encoding, but for single-pass, CBR encoding.
Finally found it.
The key I required is MF_MT_AUDIO_AVG_BYTES_PER_SECOND and is documented here:
Choose the encoding bit rate.
For CBR encoding, you must know the bit rate at which you want to encode the stream before the encoding session begins. You must set the bit rate during while you are configuring the encoder. To do this, while you are performing media type negotiation, check the MF_MT_AUDIO_AVG_BYTES_PER_SECOND attribute (for audio streams) or the MF_MT_AVG_BITRATE attribute (for video streams) of the available output media types and choose an output media type that has the average bit rate closest to the target bit rate you want to achieve. For more information, see Media Type Negotiation on the Encoder.
Related
Every once and a while this comes up and I've never been able to find a good solution but I figured I'd ask the experts: is there a video frame parser in FFMPEG that can determine frame type WITHOUT resorting to decoding?
Each codec has its particular syntax, and the decoder is the bespoke component that can work with that syntax. Now, is there an operational mode where the decoders analyze and log surface level parameters of the payload without entering the codepath to generate raster? In general, no.
There is a -debug option (http://www.ffmpeg.org/ffmpeg-all.html#Codec-Options), which when set, certain decoders, mainly the native MPEG decoders, will log some metadata, but this will invoke the decoder. For modern codecs, there's the trace_headers bitstream filter which can be used in streamcopy mode. This will parse and print all the parameter sets and slice headers. You can dump these to file and inspect them.
According to the description, it places global headers in extradata instead of every keyframe.
But what's the actual purpose? Is it useful e.g. for streaming?
I guess that the resulting video file will be marginally shorter, but more prone to corruption (no redundancy = file unplayable if main header corrupted or only partially downloaded)?
Or maybe it somehow improves decoding a little bit? As headers are truly global and cannot change from keyframe to keyframe?
Thanks!
By extradata FFmpeg means out-of-band, as opposed to in-band. The behavior of the flag is format specific. This is useful for headers that are not expected to change because it reduces the overhead.
Example: for H.264 in MP4 the SPS and PPS are stored in the avcC atom. For the same H.264 stream in let's say MPEG-TS the sequences are repeated in the bitstream.
I'm going through the HEVC decoder integrated in FFMPEG. I'm actually trying to understand its flow and working.
By flow, i mean the part in code where it is reading various parameters of the input .bin file. Like where is it reading the resolution, where is it deciding the fps that it needs to play, the output display format that is yuv420p etc.
Initially What i suspected is the demuxer of hevc situated at /libavformat/hevcdec.c In this file does the input file reading work. There is a probe function which is used to detect which decoder to select while decoding the input bin stream. Further we have a FF_DEF_RAWVIDEO_DEMUXER. Is it in this function that the resolution and other parameters read from the input file?
Secondly what i suspect is the hevc parser situated at: /libavcodec/hevc_parser.c but here i think it is just parsing the frame data, that is finding end of frame. So, is this assumption of mine right?
Any suggestions or any predictions will be really helpful to me. Please provide your valuable suggestions. Thanks in advance.
To understand more specifically what is going on in decoder, it's better to start your study with HEVC/H.265 standard (http://www.itu.int/rec/T-REC-H.265). It contains all the information you need to know to find the location of resolution, fps, etc.
If you want to get more details from FFMPEG, here are some hints:
/libavcodec/hevc_parser.c contains H.265 Annex B parser, which converts byte stream into series of NAL units. Each NAL unit has its own format and should be parsed depending on its NAL unit type.
If you are looking for basic properties of video sequence, you may be interested in SPS (Sequence Parameter Set) parsing. Its format is described in section 7.3.2.2.1 of the standard and there is a function ff_hevc_decode_nal_sps located in /libavcodec/hevc_ps.c which extracts SPS parameters from the bitstream.
Note: I was talking about FFMPEG version 2.5.3. Code structure can be different for other versions.
I am encoding video to WMV using Media Foundation SDK. I see that the number of B frames can be set using a property, but I have no clue how/where to set it.
That property is called MFPKEY_NUMBFRAMES and is described here:
http://msdn.microsoft.com/en-us/library/windows/desktop/ff819354%28v=vs.85%29.aspx
Our code does roughly the following:
call MFStartup
call MFCreateAttributes once so we can set muxer, video and audio attributes
configure the IMFAttributes created in the previous step, for example by setting the video bitrate: pVideoOverrides->SetUINT32(MF_MT_AVG_BITRATE, m_iVideoBitrateBPS);
create sink writer by calling IMFReadWriteClassFactory::CreateInstanceFromURL
for each frame, call WriteSample on the sink writer
call MFShutdown
Am I supposed to set the b-frames property on the IMFAttribute on which I also set the video bitrate?
The property is applicable to Windows Media Video 9 Encoder. That is, you need to locate it on your topology and adjust the property there. Other topology elements (e.g. multiplexer) might accept other properties, but this one has no effect there.
MSDN gives you step by st4ep instructions in Configuring a WMV Encoder and where it says
To specify the target bitrate, set the MF_MT_AVG_BITRATE attribute on the media type.
You can also alter other encoder properties. There is also step by step detailed Tutorial: 1-Pass Windows Media Encoding which shows the steps of the entire process.
what settings are necessary to use h264 lossless encoding? I'm not talking about ffmpeg commandline tool, but about the c api.
I manage to encode video with lossy h264, but I don't know how to set the encoder to lossless.?
I code in MSVC++ and use precompiled libraries. online i found some .ffpreset files (for example libx264-lossless_max.ffpreset) which don't seem to be part of the precompiled version i use (at least they are not in the presets folder). I'm not sure whether ffpreset files are somehow compiled into the library, or whether they can be loaded into the encoder somehow? if so, how would I use such a preset file?
you need to set the constant quality mode, and set the desired quality to 0.
in x264 commandline this is by x264 --crf 0.
--crf Quality-based VBR (0-51, 0=lossless) [23.0]
in ffmpeg commandline this is done similarly
To have a constant quality (but a variable bitrate), use the option
’-qscale n’ when ’n’ is between 1 (excellent quality) and 31 (worst
quality).
if you want to do this programatically, just read the source code to see how the specific options I outlned affect the AVCodecContext structure