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You may want to look at pattern matching. Techniques using the Fourier-Mellin Transform are invariant to scale and rotation, so they do what you're looking for, but computing said transform can be quite expensive (more than the Fourier transform of the whole image). Alternatively, since the pattern you look for is quite simple, you could look for the half-circle and the lines separately, and then put it all together using the positions.
Since your question is not completely specific to OpenCV, you might want to ask on Stack Exchange as well: http://dsp.stackexchange.com/
Hope this helps.
