It has been agreed upon* that OpenRaster will have fallback image data for advanced features, such as layer groups (and in the future text layers and vector layers). However, how exactly this mechanism will work is not specified. *at the LGM2010 OpenRaster BoF
Just noting that output="data/foo.png" seems to be the mechanism implied by the emerging http://www.freedesktop.org/wiki/Specifications/OpenRaster/Draft/EffectsSpecification . I'm not keen on the attribute name, but it seems to be a sensible convention.
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