I need to identify each frame type within a h264 bitstream over RTP. So far, I've been able to identify basically everything that wireshark can detect from the bitstream, including:
Sequence parameter set
NAL and Fragmentation unit headers, including the nal_ref_idc, and nal type
NAL Unit payload, including the slice_type bit.
From what I understand, nal_ref_idc can be combined with the slice_type bit to identify the slice_type - that is, I, P or B. But I'm struggling to understand how that is identified.
Finally, I'm not sure how to identify the type of the frame from this. At first I thought that the slices were the same as frames, but that isn't the case. How can I tell, or estimate which slices belong to the same frame, and then identify the frame type?
Thanks!
Related
I would like to know how one could generate a wavetable out of a wav file for example.
I know a wavetable can be used in web audio api with setPerdiodic wave and I know how to use it.
But what do I need to do to create my own wavetables? I read about inverse FFT, but I did find nearly nothing. I don't need any code just an idea or a formula of how to get the wavetable from an wav file to a Buffer.
There are a few constraints here and I'm not sure how good the result will be.
Your wav file source can't be too long; the PeriodicWave object
only supports arrays up to size 8192 or so.
I'm going to assume your waveform is intended to be periodic. If the
last sample and the first aren't reasonably close to each other,
there will be a hard-to-reproduce jump.
The waveform must have zero mean, so if it doesn't you should remove
the mean.
With that taken care of, select a power of two greater than the length
of your wave file (not strictly needed, but most FFTs expect powers of
two). Zero-pad the wave file if the length is not a power of two.
Then compute the the FFT. You'll either get an array of complex
numbers or two arrays. Separate these out to real and imaginary
arrays and use them for contructing the PeriodicWave.
I am trying to cut a H.264 video stream without decoding and re-encoding. To find a cutting point in the video stream:
Do I first detect the I-frame and then capture the video for the desired time?
Am I right or I have to look for a combination of I, P, and B frames?
Typically H.264 bitstreams start with a sequence parameter set (SPS), a picture parameter set (PPS), followed by an IDR frame in H.264 bitstream which is then followed by other arbitrary frames (P, B, etc). The parameter sets are required to initialise the decoder correctly.
Therefore to be able to decode each segment you're cutting, each segment should ideally begin with the parameter sets, but whether the each IDR is preceded by parameter sets is both codec and codec configuration dependent.
You'll be able to determine your requirements by looking at the NAL unit types of the bitstream you're wanting to cut.
However it is possible to supply a decoder out of band with the SPS and PPS. In that case they would be able to decode the bitstream starting at an IDR.
You don't have to look for combinations of I, P, B frames, just make sure you have the parameter sets, and your segment begins with an IDR.
I'd like to do some stuff with h.264 data recorded from Android phone.
My colleague told me there should be 4 bytes right after mdat wich specifies NALU size, then one byte with NALU metadata and then the raw data, and then (after NALU size), another 4 bytes with another NALU size and so on.
But I have a lot of zeros right after mdat: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 mdat atom are just ftyp mp42, isom mp42 and free atoms. All other atoms (moov, ...) are at the end of the file (that's what Android does, when it writes to socket and not to the file). But If necessary, I've got PPS and SPS from other file with same camera and encoder settings recorded just a seond before this, just to get those PPS and SPS data.
So how exactly can i get NALUs from that?
You can't. The moov atom contains information required to parse the mdat. Without it the mdat has little value. For instance, the first NALU does not need to start at the begining of the mdat, It can start anywhere within the mdat. The byte it starts at is recorded in (I believe) the stco box. If the file has audio, you will find audio and video mixed within mdat with no way to determine what is what without the chunk offsets. In addition, if the video has B frames, there is no way to determine render order without the cts, again only available in the moov. And Technically, the nalu size does not need to be 4 bytes and you cant know that without the moov. I recommend not used mp4. Use a streamable container such as ts or flv. Now if you can make some assumption about the code that is producing the file; Like the chunk offset is always the same, and there is no b frames, you can hard code these values. But is not guaranteed to work after a software update.
I have extracted several nal units from hard disk. I want to know which of them is valid nal unit or not. Is there any tool or code that can validate the structure or syntax of single h264 AVC nal unit.
It depends. First you need to figure out what the NAL type is by the first byte. If the NAL is an SPS or PPS you can basically decode that as-is and see if the result is sane.
If the NAL is an actual coded slice, you will need at least three NALs to decode it. The corresponding SPS, PPS and the coded slice. You can decode the first few elements of the slice header without the SPS and PPS, but then you would need the corresponding SPS and PPS based on the PPS ID in the slice header to continue.
There were some command line tools (maybe h264_parse) that would dump this type of header information for you, or you can hack the reference decoder to help you out.
http://iphome.hhi.de/suehring/tml/
In the end the only way to know if your NAL is "good" is to either match it up with the bitstream you started out with or fully decode it and verify the resulting picture output as bit-exact.
Checking the NAL byte length and maybe a checksum or CRC of each NAL can be helpful too, but no such mechanism exists in the bitstream, you'd have to add that on.
Does H.264 buffer contains Time stamp and decoding time stamp information.
when we get the H.264 nalu data does that contain timing information in it?
If you mean raw H.264 NAL units than no they don't contain timing information if mean something like PTS/DTS. Timestamps are on higher level in containers like MKV/MP4/TS. The only time related information in H.264 specs afaik are num_units_in_tick/time_scale in VUI that can be used to finding FPS in case of constant frame rate (fixed_frame_rate_flag = 1), and some fields in Picture timing SEI but as they are optional and not really well specified so nobody really use them.