Plugin implementation help

Here's what I want to do: given an entry in
$datadir/category/entry.txt and an image.png in the
same directory, I want to replace all instances of
<img src="image.png"/> with something like
<img src="$someurl/category/image.png"/>. I think this
should go into a postformat filter, since it makes
sense to apply this substitution after all the other
filters have run.

I've got the code that does the replacement, but
I can't seem to find a convenient way of knowing what
"category" is from within the cb_postformat callback.

Is there a better place where this should be?
Where is the "category" available? Is there a simpler
way to do what I want?

Thanks in advance.

Igor


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