Hello,
Dave Wissenbach wrote on 1/8/03
> The MIME Application/Vnd.pwg-multiplexed document format is
> provided to minimize memory requirements in a printer. This
> requirement to minimize memory usage suggests that referenced
> image data should be thrown away as soon as the images have
> been rendered. Yet I have found no documentation to suggest
> that this is indeed the case, ...
>
> If a given image is repeatedly used in the document, is the
> image to be included repeatedly ...
> ...
> ... I am suggesting that persistence be
> explicitly specified by the sending application by adding an
> extended Content-Disposition header.
The January 20th meeting of the PWG XHTML-Print working group discussed this
proposal. Adding a reference count to the image header would indeed seem to
tell the consumer of the multiplexed document when it can free the resources
allocated for the image. However, it didn't seem to the participants that a
producer of the document would always know how many references would be made
and still be able to locate the image next to the 1st chunk that referenced
the image.
Alternatives were discussed in the meeting. The alternative were having the
producer of the multiplexed document insert a chunk that would indicated
when the document would not need an image any more, or having the producer
insert some XHTML-Print element that would indicate when the consumer could
release the resources for an image. In the first case, using a chunk header
to indicate when the consumer could free an image won't work since the
relative location of a chunk header can not reliably be associated with a
processing point within the document. In the second case, adding new
elements or processing instructions for the purpose of indicating when an
image's resources could be freed was not deemed to be a viable alternative
at this time.
The group decided to reaffirm that a consumer could release the resources
associated with an image after the document's reference to the image was
processed.
I propose that the following paragraph be added to the Section B.2 of the
XHTML-Print spec:
Producers and consumers of Application/Vnd.pwg-multiplexed entities, as
defined in [MIMEMPX], should consider each image as having one and only one
reference. The producer should not make assumptions about the buffering
abilities of the consumer of an Application/Vnd.pwg-multiplexed entity and
include a copy of an image for each reference to it. Consumers may release
the resources associated with an image when the reference to it is
satisfied, and are free to optimize references to identical images in an
implementation dependent manner.
Jim Bigelow,
Editor: XHTML-Print & CSS Print Profile
IEEE-ISTO, Printer Working Group
http://www.pwg.org/xhtml-print
Hewlett-Packard
208-396-2068
jim.bigelow@hp.com
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