Re: [Cellar] On the multiplicity of Info elements

["Sebastian G. <bastik>" <[e-mail]>]

06.01.2016, 08:06 Dave Rice:
> Here is a draft for an EBML Schema Attribute to be used in the
> definition of EBML Elements for what I’m referring to as
> Identically-Recurring Elements.
> 
> ==== A boolean to express if the EBML Element may occur within its
> Parent Element more than once but that each recurrance within that

Is 'recurrance' spelled correctly? (Not a native speaker). 'recurrence'
is suggested as search term suggestion.

> Parent Element MUST be identical both in storage and semantics. Such
> Elements are referred to as Identically-Recurring Elements. In this
> case, identical copies of an EBML Element are permitted to be stored
> multiple times within the same Parent Element in order to increase
> data resillience and optimize the use of EBML in transmission.

'resillience' seems to have an error. 'resilience'.

> Identically-Recurring Elements SHOULD include a CRC-32 Element as a
> Child Element. If a Parent Element contains more than one copy of an
> Identically-Recurring Element which includes a CRC-32 Child Elememnt

s/Elememnt/Element

> then the first instance of the Identically-Recurring Element with a
> valid CRC-32 value should be used for interpretation. If a Parent
> Element contains more than one copy of an Identically-Recurring
> Element which does not contain a CRC-32 Child Elememnt then the first

s/Elememnt/Element

'If a Parent Element contains more than one copy of an
Identically-Recurring Element which does not contain a CRC-32 Child
Element "or if CRC-32 Child Elements are present but neither of those
are valid" then the first instance of the Identically-Recurring Element
should be used for interpretation.'

I took your sentence and added (noted by "") the case something would
contain CRCs, but neither of them would be valid. Feel free to ignore
this suggestion. It might be a rare case.

It is certainly possible to have it as separate sentence. Maybe it makes
it easier to read. Maybe even easier to understand.

> instance of the Identically-Recurring Element should be used for
> interpretation. If the `identical` attribute is not expressed for
> that Element then that Element is considered to not have a
> requirement for identical expression within the same Parent Element.
> The `identical` attribute is only valid if the Element is not set to
> `multiple`, otherwise the `identical` attribute shall be ignored. 
> ====
> 
> The text may be seen in context with the other attributes at
> https://github.com/MediaArea/ebml-specification/blob/d108c14d1f1c748d1f3f50e58d4057208325f892/specification.markdown#ebml-schema-element-attributes
>
>  Some pending questions:
> - Should Identically-Recurring Elements recommend or mandate inclusion of a CRC-32? I suggestion recommend and not mandate for reverse compability.

Backward compatibility seems to be a higher goal than CRCs. I think it
should be even possible to play a file without checking CRCs, which does
not contradict having CRCs in the first place.

Like Timothy B. Terriberry points out, reasons for not having CRCs on
those elements should be given in the specification.

> - Is 'Identically-Recurring Elements’ a decent short name for these types of Elements?

I have no objection. IRE could be used when talking about those
elements, unless it is within official documents.

> - Should the ‘identical’ attribute apply to only Master-elements? I think it could be open. I could see scenarios to place the CRC-32 both at the beginning and end of the Parent Element.

I have not enough knowledge to make an informed statement.

> - I’m thinking that ‘multiple’ means the Element may recur within its Parent Element and means that there are multiple semantics. I suggest that ‘identical’ Elements not be ‘multiple’ as an ‘identical’ Element recurs but only with a single and non-multiple semantic meaning. OK?

I have not enough knowledge to make an informed statement.

> - If there are multiple copies of identical Elements, I wrote in the draft that if there’s a CRC than the first copy with valid CRC be used, else the first copy be used. Potentially we could say first valid copy be used, but then need to say fully what valid means.

I'm neutral, but agree that if just valid is used it would have to be
explained what valid means.

> - I agree that an identical Element should not be used to change the semantics over time.

Agreed.

--
Sebastian