I think the new API should have multi-page support.  It would be useful for 
pdf, djvu, multi-page tiff, and for looking at individual frames of animated 
gif, png, etc., to the extent that we write plugins for those.

I wrote a QQuickImageProvider for PDF (using poppler), but requestImage takes 
only an “id” string to identify the image.  So I had to make up a composite id 
containing both the filename and the page number.  (much like a URL format 
would be, if you had to request multiple pages from a web server)  So that’s 
the next API that needs to have some support for paging.

Can “frame” be considered the same as “page”?  But I see that you also have a 
subimage concept in mind.

Sometimes (as in icon files, or in any kind of file that also includes a 
smaller preview image) the sub images would be the same thing at different 
resolution, right?  That is orthogonal to the idea of paging: a PDF file can 
have multiple pages, and also embedded thumbnails for each page.  In order for 
an image format plugin to be enough to write a PDF reader application, it would 
need to support paging, and also getting the thumbnails for the pages.

In QQuickImageProvider::requestImage, requestedSize can also be given.  So 
that’s one way to specify a thumbnail: if the available thumbnail is near 
enough to requestedSize, return that instead.  The API anyway does not 
guarantee that you get exactly the size that you ask for.

Eventually we could get to the point that a plain Image in QML could be used as 
a PDF viewer, all by itself.  It would also need a paging API.

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