[FFmpeg-devel] [PATCH] AVI metadata retrieval improvements

[Thilo Borgmann]

Am 21.03.14 15:30, schrieb Michael Niedermayer:
> On Thu, Mar 20, 2014 at 11:12:27PM +0100, Thilo Borgmann wrote:
>> Am 04.03.14 17:16, schrieb gregory.wolfe at kodakalaris.com:
>>> This is the second of two changes I've made as part of our upgrade to
>>> the latest FFmpeg development branch.
>>> 
>>> This patch enhances two aspects of metadata retrieval from AVI files.
>>> I've attached before/after command line output from ffmpeg for each
>>> modification.  Test video files that can be used to generate the
>>> before/after output have been uploaded to the FFmpeg FTP server.
>>> 
>>> 
>>> Patched file:  libavformat/avidec.c
>>> Description (from commit message):
>>> 
>>> Added function avi_extract_stream_metadata().  Some cameras (e.g., Fuji)
>>> store stream metadata following the "strd" stream data tag.  Currently,
>>> avi_read_header() calls ff_get_extradata() to read and save this data in
>>> the codec's "extradata" slot.  This new function extracts metadata from
>>> "extradata" by first looking for the AVIF tag, then extracting metadata
>>> from the EXIF tags which follow it.
>> 
>> I've rewritten almost everything to use existing EXIF functions.
>> 
>> Patch attached, but there are 2 issues I need advise for:
>> 
>> a) Current EXIF is in lavc only and relies on lavc/tiff.h and lavc/bytestream,
>> so these have to be moved (to lavu)?
> 
> using just a header with macros/inline functions is fine
> using ff_* functions from other libs is not as ff_ is not exported
> using avpriv_* functions in lavf from lavc is ok but the ABI/API
> of such functions is then part of the libavcodec ABI/API, non public
> but still, so care has to be taken with future changes to these
> functions so their ABI/API isnt broken

Ok these are the rules, but what should I do since there are ff_* functions
used? My guess was to move to libavu because metadata decoding seems to be
needed by formats and codecs, but I don't know if there is a better solution? Or
make exif tag decoding a public av_* funtion in lavc?

About the avpriv_request stuff, I could make it easy and not request samples for
avi's featuring these two tags, but I think we actually are interested in such
files. I really wonder that such requests are used nowhere in lavf. So same
move/solution for these like for the exif metadata stuff that would also pull
these tiff tags with it?


>> b) I can't see anything about the endianess in AVI files. How to determine or is
>> it always little-endian?
> 
> the avi fileformat itself is always LE, ive no idea though about the
> content of various chunks, i guess  either
> autodetect if its trivial or
> assume its always LE if all files we have are LE

I will check if there's something in the padding. A usual header is not present. Gregory says at least for these 300 files its alwas le=1. 

-Thilo