Item and piece in OPC

[Chris Rae]

Hi all – for reference, here are the definitions of “logical item name” (which really seems to me to actually define “logical item”), “piece” and “zip item” from the T&Ds:

logical item name — An abstraction that allows package implementers to manipulate physical data items
consistently regardless of whether those data items can be mapped to parts or not or whether the package is
laid out with simple ordering or interleaved ordering.

piece — A portion of a part. Pieces of different parts can be interleaved together. The individual pieces are
named using a unique mapping from the part name. Piece name grammar is not equivalent to the part name
grammar. Pieces are not addressable in the package model.

ZIP item — A ZIP item is an atomic set of data in a ZIP archive that becomes a file when the archive is
uncompressed. When a user unzips a ZIP-based package, the user sees an organized set of files and folders.

I’m not sure what you mean about the relationship being 1:1 – a piece, for example, definitely does not always correspond to a whole part. But I might be misunderstanding your wording. Here’s my take on these terms though:

The term “ZIP item” is used only when talking about the ZIP structure and 10.2 makes clear that once the ZIP item is out of the ZIP it’s just a Part [unless interleaving was involved, in which case it’s a set of pieces which will become a Part, as per 10.1.4].

“Piece” is, I think, defined quite clearly in 10.1.4 which describes interleaving and the part of the piece (ahem).

I think that a logical item is what you have when you take a complete physical package item, after applying any interleaving if necessary.

I’m slightly less clear on physical package items and how exactly they relate to ZIP items or logical items. There’s a section in 10.1.3 concerning that which is quite verbose but I’m still slightly unsure of the purpose of logical items (when not used simply to put back together interleaved segments).

I’m not sure whether a ZIP item is a physical package item, or is in fact a further stage on top of that.

This is probably a good topic to dig into in Prague – John’s on vacation until just before we leave, but I’ll try to sit down with him before the meetings to see what his thoughts are on this.

Chris


From: eb2mmrt at gmail.com [mailto:eb2mmrt at gmail.com] On Behalf Of MURATA Makoto
Sent: 04 June 2014 00:18
To: SC34
Subject: Item and piece in OPC

Dear colleagues,

There are four similar (if not identical) terms in OPC.  They are
  1) logical item
  2) physical package item
  3) ZIP item, and
  4) piece

The relationship between any two of them is 1:1.

Then, how are they different from each other?  Here
is my understanding.

- "Logical item" is independent from
  the underlying physical representation

- "ZIP item" is dependent on the ZIP format

- "physical package item" is either "ZIP item"
   or XXX item, where XXX is a particular
   physical representation

- "piece" is used only when interleaving is involved.
   It is not clear whether "piece" is a logical item,
   ZIP item, or physical package item.


Regards,
Makoto
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