Hi Adrien!
Using the visual word approach I use in Pastec would enable the matching
of modified images but would also require a lot more resources. Thus, while
your hash is 256 bits long, an image signature in the Pastec index is
approximately 8 KB.
8 KB still isn't too bad. It sounds like it could be useful.
Similarly, I guess that the search complexity of your hash approach is o(1)
while in Pastec this is much more complicated: first tf-idf ranking and
then two geometrical rerankings...
Close to o(1) at least. How does Pastec scale to many images? You
mentioned having about 400,000 currently, which is still a rather fair
number, but what about the full ~22M of Wikimedia Commons? I'm
assuming that since tf-idf is a well known method for text mining,
there are well understood and optimised algorithms to search. Perhaps
something like Elasticsearch would be useful right away too?
That would be an advantage, since with our blockhash, we've had to
implement relevant search algorithms ourselves lacking existing
implementations.
One problem that we see and which was discussed recently on the
commons-l mailing list, is the possibility of using approaches like
yours and ours to identify duplicate images in Commons. We've
generated a list of 21274 duplicate pairs, but some of them aren't
actually duplicates, just very similar. Most commonly this is map
data, like [1] and [2], where just a specific region differ.
I'm hypothesizing that your ORB detection would have better success
there, since it would hopefully detect the colored area as a feature
and be able to distinguish the two from each other.
In general, my feeling is that your work with ORB and our work with
Blockhashes complement each other nicely. They work with different use
cases, but have the same purpose, so being able to search using both
would sometimes be an advantage. What is your strategy for scaling
beyond your existing 400,000 images and is there some way we can
cooperate on this? As we go about hashing additional sets (Flickr is a
prime candidate), it would be interesting for us if we could generate
both our blockhash and your ORB visual words signature in an easy way,
since we any way retrieve the images.
[1]
https://commons.wikimedia.org/wiki/File:Locator_map_Puerto_Rico_Trujillo_Alto.png
[2] https://commons.wikimedia.org/wiki/File:Locator_map_Puerto_Rico_Carolina.png
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