[Matroska-devel] Hi, question about the MKV tags

Steve Lhomme slhomme at matroska.org
Mon Jan 31 13:32:56 CET 2011


The reason for this is that some apps may support attachments and not
tags. The cover art system is much simpler to support than tags (when
done properly).

And yes, when editing tags/cover art you have to check if it can fit
in the original place, if not you have to :
- void the old version
- write the new version at the end
- update the segment size
- update the position of the tags/attachment in the SeekHead section

That's more or less what you'd have to do in any container anyway,
except some may not have a Void element and would always put these
data at the end (not nice for network reading).

On Mon, Jan 31, 2011 at 1:23 PM, Santiago Jimeno <sjimeno at ya.com> wrote:
> Hi all
>
> In the email I sent speaking about CodecSpecs a while ago, I said that I
> would write other about Tags.
> I think the current message provides me this opportunity.
>
> Reading Tags is relatively easy (in spite of Nested Tags) in Visual .NET
> because of the easiness that .NET provides to work with XML Documents.
>
> Editing or re-writing Tags is complicated. But I have it done in my program
> FileList.
> But it's not still completed because of the difficulties that I have found
> with Cover Art.
>
> Cover Art are in a Attachments block (separated of Tags block) that is a bag
> for everything that also includes fonts, text and subtitles and located in
> any place of the file. The result in this case is that, in spite of its
> initial advantages, it doesn't exist a program for editing Tags and I think
> it's for the extreme difficulty that implies to almost modify the whole
> file. I think that to have two different blocks for Tags and Pictures is not
> a good design. In Flac files a similar thing happens.
>
> I am working in two possible solutions.
> 1.- Declaring VOID the block part that contains Pictures and writing them at
> the file end (next to Tags) in a second Attachments block.
> 2.- Including them in a new SimpleTag <TagName> COVERART inside TAGS,
> maintaining its structure. The binary data could be written as <TagBinary>
> or as <TagString> in Base64 so that was compatible with UTF8 (Vorbis
> Comments do it this way).
>
> If somebody wants to test FileList program, one can download it in
> http://personales.ya.com/sjimeno/Download/MATROSKAFileList.zip
> To show edit form --> double click in List Item.
> This program is designed for audio files MKA, although it also reads video
> files MKV. Apply Tags edition is disabled at the moment due to the problem
> above mentioned.
>
> Regards. Santiago Jimeno
>
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-- 
Steve Lhomme
Matroska association Chairman



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