[x264-devel] Appending an iFrame to the start of a h264 video?

[Cary Fitzhugh]

Ok - so - for anyone else who is looking about. I'll just record my findings.

Look at the type of NAL.  If it is type 1,2,3, 4 or 5 - it is a video NAL.

Of those, the first is a non IDR type.  The second is a partition A, then B, then C type, then an IDR type.
I don't know what B or C are, nor really what the others are.  But i haven't seen anything but non IDRs in my adventures 
- and the nonIDR and A type have the slice_header which is what you want to look at to tell the type of the frame.

In the slice header - which is the next thing in the stream after the NAL header - there are a few things to do.
You have to decode the first_mb_in_slice unsigned value.
Then next is the slice type unsigned value.

Slice types are:
0    P
1    B
2    I
3    SP
4    SI
5    P
6    B
7    I
8    SP
9    SI

How to decode those values...  I know that Mp4Box and FFmpeg have code which decodes those Exp Golomb values. 
(Interestingly they look the same, yet one says it is FFMPEG code and the other says it is MP4Box code...)

I believe the AVC specific way to decode exp golomb values is to do the following: (AVC specific b/c the k value is 0) 
(Look at wikipedia)

count the number of 0 bits before you hit your first 1 value. Save that as N.
Put the next N+1 bits into a variable
Then subtract 1 from that value.

Thanks,
Cary FitzHugh

Tomas Carnecky wrote:
> Make sure ffmpeg is producing raw .264 files (with no headers or such). 
> A simple check is to see if the first three of four bytes of the file 
> are (0x00) 0x00 0x00 0x01.
> 
> tom
> 
> Cary Fitzhugh wrote:
>> Thanks for the reply!
>>
>> I'll keep looking if you all think it is possible.
>> I may be getting tripped up by ffmpeg adding some header information onto the front. I'm not sure.
>>
>> At least I have a clue that it may be possible.
>>
>> Thanks so much,
>> Cary
>>
>> Alex Giladi wrote:
>>> "cat movie1.264 movie2.264 > longer_movie.264 " should suffice, AFAIK.
>>>
>>> On Wed, Jul 9, 2008 at 12:41 PM, Cary Fitzhugh <cfitzhugh at echo360.com> wrote:
>>>> Hello --
>>>>
>>>> I'm trying to figure out if there is a way to concatenate two h264 files without reencoding.
>>>>
>>>> I am making edits of longer h264 videos.
>>>> In order to make sure that my video starts on an iFrame - I want to use ffmpeg to encode a single frame at the start
>>>> time.  Then in a subsequent call - do a -vcodec copy on the rest of the edit.
>>>>
>>>> This provides me with 2 h264 movies.  One is 1 frame long and is an iframe.  The second movie is the rest of the edit.
>>>>
>>>> I do not understand the h264 codec well enough - which is why I'm writing you all.  It makes sense in my head, that if I
>>>> were to just concatenate the raw h264 files together and wrap it I should have a valid h264 file.
>>>>
>>>> In my movies I do not have B frames, just P and I frames.  My understanding is that P frames are only moving forward.
>>>> So prepending that initial i frame onto the front of the edit should work fine.
>>>>
>>>> I tried this, and when I combined the two it got very swirly and messed up on vlc.
>>>>
>>>> ANy ideas if this is possible?  I would really like to do this no matter how convoluted I need to get.  When I'm editing
>>>> 1 - 4 hour movies all the time I would rather not wait 20% of real time to make an edit - just for that first iframe.
>>>>
>>>> Thanks for any insight into this.
>>>>
>>>> Cary FitzHugh
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>>>>
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