windowscodecs: Create a IWICMetadataReader when loading a TIFF frame.

Dmitry Timoshkov dmitry at
Mon Jun 18 09:33:45 CDT 2012

Vincent Povirk <madewokherd at> wrote:

> >> We can't use that stream because we provide separate seek and
> >> read/write methods to libtiff, and libtiff may rely on the stream's
> >> position to not change between operations.
> >
> > libtiff doesn't rely on stream positions, and my use of stream matches
> > MSDN guidelines on writing a WIC enabled codec.
> We should provide the stream interface to libtiff as documented and
> not rely on implementation details.
> Even without libtiff, you will end up using the same stream for
> multiple metadata handlers, each of which could end up using it later,
> unless you specify WICPersistOptionNoCacheStream.

That's not the case of an IFD metadata reader, it creates all the items
on load, so creating multiple metadata readers from the same stream won't

> >> Your GetReaderByIndex method should check whether
> >> This->metadata_reader has been set and return an error if not.
> >
> > No, that's not an error. TIFF always provides metadata since IFD is
> > the core of TIFF. If a metadata reader can't be created it's OK to
> > return 0 metadata items.
> That's fine, but your check in GetCount suggests that having no
> metadata reader is a valid state, and GetReaderByIndex(0) will crash
> in that case. If it's not valid, you shouldn't need to test for it in
> GetCount.

That's a valid point, sorry, I misread your comment first time.

> Also, I just noticed that you're calling create_metadata_reader
> unconditionally from QueryInterface, and not checking in the process
> whether the reader has already been created. That will leak if QI is
> called multiple times.
> I'm not sure doing real work inside QueryInterface is a good idea.
> It's pretty surprising.
> I think it would make more sense to do this when a method on
> IWICMetadataBlockReader is first called, or in Initialize if
> WICDecodeMetadataCacheOnLoad is specified (though doing it in
> Initialize would require creating the frame objects then as well, and
> probably combining their refcounts with the parent).

MSDN suggests different ways of creating a metadata reader:
1. when creating a IWICBitmapFrameDecode
2. when creating a block reader
3. inside of GetReaderByIndex

It's all up to an implementor. I decided to that at #1. Each instance
of IWICBitmapFrameDecode has its own metadata reader, I don't see how
it could be created twice or leaked.


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