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Re: [media-source] Allow non-ISO/IEC14496-12 top-level boxes in ISOBMFF Byte Streams

From: David Singer <singer@apple.com>
Date: Fri, 09 Dec 2016 10:32:26 -0800
Cc: public-html-media@w3.org
Message-id: <1D1718FF-E03D-4199-ADEF-FC7AB2F4F847@apple.com>
To: David Evans via GitHub <sysbot+gh@w3.org>
I agree

the presence of a brand that you support gives you permission to decode a file and ignore what you don’t understand.  The spec. should admit the possibility of the existence of boxes that can be ignored.

> On Dec 9, 2016, at 6:31 , David Evans via GitHub <sysbot+gh@w3.org> wrote:
> 
> bbcrddave has just created a new issue for 
> https://github.com/w3c/media-source:
> 
> == Allow non-ISO/IEC14496-12 top-level boxes in ISOBMFF Byte Streams 
> ==
> The [Segment Parser 
> Loop](https://w3c.github.io/media-source/index.html#sourcebuffer-segment-parser-loop)
> states the following:
> 
> "If the input buffer contains bytes that violate the SourceBuffer byte
> stream format specification, then run the append error algorithm and 
> abort this algorithm."
> 
> The [ISO BMFF Byte Stream 
> Format](https://www.w3.org/TR/mse-byte-stream-format-isobmff/) states:
> "Valid top-level boxes defined in ISO/IEC 14496-12 other than ftyp, 
> moov, styp, moof, and mdat are allowed to appear between the end of an
> initialization segment or media segment and before the beginning of a
> new media segment. These boxes MUST be accepted and ignored by the 
> user agent and are not considered part of the media segment in this 
> specification. "
> 
> This appears to imply only valid top-level boxes defined in 14496-12 
> are allowed to appear in a stream, and that strictly compliant 
> implementations should reject input buffers containing any other 
> top-level boxes.
> 
> MPEG-DASH (ISO/IEC 23009-1:2014) specifies a new box (`emsg`) as a 
> mechanism for signalling both generic in-band metadata related to the 
> media and DASH-specific operations. It further constrains this new box
> to be placed before the `moof` - i.e. at the top level.
> 
> It seems that the byte stream format specification needs either to be 
> less strict in general in terms of extensions, or include references 
> other than 14496-12 in the list.
> 
> Note: at least one implementation has already included `emsg` in its 
> list of valid top-level boxes [1].
> 
> See also:
> 1. https://bugs.chromium.org/p/chromium/issues/detail?id=276303
> 2. https://bugzilla.mozilla.org/show_bug.cgi?id=1322587
> 
> 
> Please view or discuss this issue at 
> https://github.com/w3c/media-source/issues/174 using your GitHub 
> account
> 

David Singer
Manager, Software Standards, Apple Inc.
Received on Friday, 9 December 2016 18:32:55 UTC

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