[Matroska-devel] Several (minor) issues or underspecified areas in the MKV spec
mjbshaw at google.com
Mon Oct 5 18:47:06 CEST 2015
On Mon, Oct 5, 2015 at 8:03 AM, Dave Rice <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)?
- Shouldn't EBMLMaxSizeLength have a range of > 0?
- 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)?
- 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
- 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”
- 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?
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