Re: [Openexr-devel] Adding/updating attribute to existing OpenEXR file

[Ger Hobbelt]

I don't believe there's a possibility to add to your OpenEXR headers
and NOT have to rewrite your files.
And then rewriting (having to move the pixel data in each file) in
file is probably as costly as writing to another file, then 'mv'
moving it over the old one, unless you use memory mapped I/O -- which
I don't know if it is supported by the OpenEXR lib; it might or it
might not.

Reasoning:

Given the OpenEXR file format (headers are at start of file; see also
file layout PDF documentation) 'in-file' adding of extra attributes
would mean enlarging the header.
(I can safely assume you're not working on a platform which supports
'record insertion' in regular files ;-)  (VMS/RMS can IIRC,
UNIX/Windows definitely can not))

Enlarging the header size would mean you'll have to rewrite the file
anyway and it doesn't matter all that much if it's done by your
run-time library code or by your own (though a straight binary copy of
the pixel data _will_ be the fastest when using memory mapped I/O, but
I haven't dug that deep in the OpenEXR lib to know if you can get it
to spit out byte offsets to find the 'end-of-header' ==
'start-of-pixel-data' and just memmove() that pixel data on your own.)

My EUR 0.02, hope it helps,

Ger


On Tue, Aug 26, 2008 at 10:07 PM, Nicholas Yue <address@hidden> wrote:
> From the help message of exrstdattr
>
>       'the program cannot edit an image file "in place" '
>
> I am trying to avoid another copy as I am writing a small app to update
> comments and other production specific attributes on lots of rendered
> sequences.
>
> I'll hunt around further.
>
> Thanks Ken.
>
> Regards
>
>
>
> _______________________________________________
> Openexr-devel mailing list
> address@hidden
> http://lists.nongnu.org/mailman/listinfo/openexr-devel
>
>



-- 
Met vriendelijke groeten / Best regards,

Ger Hobbelt

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