[FFmpeg-devel] [PATCH v4 2/4] libavformat/mxf: DNxUncompressed MXF related changes

[Marton Balint]

On Wed, 11 Sep 2024, Tomas Härdin wrote:

> ons 2024-09-11 klockan 01:08 +0200 skrev martin schitter:
>> 
>> 
>> On 10.09.24 15:14, Tomas Härdin wrote:
>> > > +++ b/libavformat/mxf.c
>> 
>> > > +    { {
>> > > 0x06,0x0E,0x2B,0x34,0x04,0x01,0x01,0x01,0x0E,0x04,0x02,0x01,0x02,
>> > > 0x1E,0x01,0x00 }, 16,      AV_CODEC_ID_DNXUC }, /*
>> > > DNxUncompressed/SMPTE RDD 50 */
>> > 
>> > Are really all 16 bytes significant?
>> 
>> I have to correct myself again:
>> 
>> This Entry in the "PictureEssenceCoding" list of 'mxf.c' should 
>> according to the specification look like:
>> 
>> +    { { 
>> 0x06,0x0E,0x2B,0x34,0x04,0x01,0x01,0x0D,0x04,0x01,0x02,0x02,0x03,0x07
>> ,0x01,0x00 
>> }, 15,      AV_CODEC_ID_DNXUC }, /* DNxUncompressed/SMPTE RDD 50 */
>> 
>> The significant bit count is set to 15, because we do not support any
>> of 
>> the extended fix point format variants.
>
> Ah, didn't know. But this happens often with ULs. One must carefully
> read the wrapping spec!

Actually it should match 14 bytes, because byte 14 defines the codec 
being DNXUC. Byte 15 defines the "profile" of the codec (standard format, 
or extended fixed point formats), but from the demuxer's point of view it 
is irrelevant what profiles / codec features ffmpeg's own decoder 
supports. E.g. the user might use the demuxer only and decode the essence 
on its own.

Regards,
Marton


>
>
>> btw. there is an annoying flaw in the ffmpeg mxf parser:
>> 
>> The very common 'fill' boxes, which are used in DNxUncompressed files
>> to 
>> align 'pack' boxes on 265 byte boundaries, are not recognized by
>> ffmpegs 
>> mxf parser and generate lots of
>> "Dark key 06.0e.2b.34.01.01.01.02.03.01.02.10.01.00.00.00" warnings.
>
> Patch welcome ;) But surely we already deal with KAG fill? Maybe the UL
> for it just has too large a matching length.. But looking at
> mxf_metadata_read_table[] and searching for "fill" it appears we don't?
>
> /Tomas
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