[Matroska-devel] Several (minor) issues or underspecified areas in the MKV spec

Dave Rice dave at dericed.com
Fri Oct 16 08:12:08 CEST 2015

Based on these comments I added several PR's to the EBML Specification. Please review.

> On Oct 5, 2015, at 12:47 PM, Michael Bradshaw <mjbshaw at google.com> wrote:
> On Mon, Oct 5, 2015 at 8:03 AM, Dave Rice <dave at dericed.com <mailto:dave at dericed.com>> wrote:
> I'm working on the EBML specification (the one being drafted on GitHub) quite a bit. What are the questions to EBML?
> Preface: some of these are weird corner cases that are extremely unlikely to occur for anyone doing anything sane. That said, I think parsers should consistently (or even gracefully) handle the insane, and in order to do that I think these corner cases should be clarified in the spec.
> Shouldn't EBMLMaxIDLength have a range of > 3 (given that the EBML element has an ID length of 4)?
I wrote some clarifications here https://github.com/Matroska-Org/ebml-specification/pull/25 <https://github.com/Matroska-Org/ebml-specification/pull/25>.
> Shouldn't EBMLMaxSizeLength have a range of > 0?
https://github.com/Matroska-Org/ebml-specification/pull/26 <https://github.com/Matroska-Org/ebml-specification/pull/26>.
> Can a global element (i.e. Void, CRC-32) occur before an EBML element? If so, are they considered part of the document (as is, it seems like an EBML document is implicitly defined as everything between an EBML header and then next EBML header (or EOF), in which case they are not considered part of the EBML document)?

I started a definition of EBML Document here: https://github.com/Matroska-Org/ebml-specification/pull/28 <https://github.com/Matroska-Org/ebml-specification/pull/28>. CRC (at least in the EBML Spec) is level 1+ so couldn't occur before an EBML element unless inside another valid level 0 element that is itself preceded by another EBML Header.
> How should a EBMLMaxSizeLength > 8 be handled if it occurs after the element that needs it (specific edge case: DocType has a size length of 9, but DocType occurs before EBMLMaxSizeLength in the header; how should that be handled?) (alternate edge case: a Void element occurring in (or before) an EBML element with a size length is > 8 and occurring before EBMLMaxSizeLength). Should the spec explicitly require parsers to parse as if EBMLMaxSizeLength is 8 unless and until explicitly told otherwise?
> Do the limitations of EBMLMaxSizeLength apply to the document immediately? That is, if EBMLMaxSizeLength is 1, does that apply to elements in the EBML header immediately after it is encountered, meaning that if DocType followed it it must have a length < 127?
> Typo in the EBML spec in the Length definition for the Binary data type: “A Master-element” should be “A Byte Element”
Thanks. https://github.com/Matroska-Org/ebml-specification/pull/29 <https://github.com/Matroska-Org/ebml-specification/pull/29>.
> The EBML spec says that the Reserved ID (all bits set to 1) is the only ID that may change the Length Descriptor (the count of leading zeroes + 1). What exactly does it mean to "change the Length Descriptor?" Does this mean a Length Descriptor can be > 4 (even if EBMLMaxIDLength = 4) iff the ID is the Reserved ID?
Dave Rice

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