To find the median value in a set, I need to sort the set from
lowest to highest color values and then find the value in the middle
if the set is odd, or average the 2 middle values if the set is even.
Since this is my first attempt at graphics programming on the GPU I
don't know if it makes sense for this problem (sorting)?
Any advise?
give up?
But seriously. CoreImage doesn't allow data-dependent branching, so
sorting with it isn't really possible. (There's the ternary operator,
but for sorting non-trivial data sets that's going to get
exponentially complex)
You _can_ make CPU-based core image filters, but not through the QC
interface. This doesn't satisfy the "on-gpu" aspect, but it does
satisfy the "is possible" aspect. ;)
If you really really really want/need on-GPU sorting, there are some
academic papers that describe some possible approaches. Due to the
overwhelmingly parallel nature of GPUs, these algorithms are usually
non-trivial from what I've seen, and might not be directly applicable
to CI (CI's a narrow subset of gpu functionality, so you can't
implement everything in CI that you can on a GPU in general). http://mgarland.org/papers.html#gpusort-tr
describes a fairly accessible one (it uses radix sort, which isn't
crazy) -- I think this just needs a few scatter/gather operations to
perform the sort (CI can't do this except in very simple cases -- it
can gather (read from multiple locations on the texture), but it can't
scatter (write to arbitrary locations) from what I've seen done).
--
[ christopher wright ]
[email protected]
http://kineme.net/
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