Lubomir, Regarding your comment:
I haven't thought much about this, but this is k-order statistic (in an array of numbers, find the K-th smallest one). There is an algorithm to do this in O(N). This would be rather useful, perhaps someone should put it in Boost?
It is part of the stl already: std::nth_element in the <algorithm> header. Also,
My suggestion is to ignore the fact that there are multiple channels in a pixel and just white the algorithm to operate on a grayscale image. It is then trivial to extend it by using nth_channel_view.
But nth_channel_view does not work for heterogeneous pixels, runtime specification of the channel requires that all channels be of the same type. As you said in an earlier post 'n' means runtime, 'k' means compile time. This is exactly the type of issue I have been having when I try to write code that is generic enough to apply to heterogeneous pixels. Unfortunately I have started writing code that simply precludes heterogeneous pixels (using nth_channel_view or operator[] to loop through channels) because I was spending too much time trying to write functors for every operation I needed to do on an image. I posted a message on the sourceforge discussion and I will summarize it here: I saw the recent posting on fixed point (or scaled value) types in the works by Phil Endecott, and apparently another version by Maurizio Vitale, and I was wondering whether GIL folks thought this could be useful for describing individual samples in gil. Instead of using bits8, one could define a pixel in terms of scaled<uint8_t, 255>, or scaled<uint16_t, 255>, etc. Operations on the pixels can deduce the return type for each channel using the methods applied by the fixed point authors. This way each channel, regardless of its type, should produce expected results for an expression without all the tweaking required when dealing with integer types directly. Any operation on a pixel could be applied to each of its channels, like in valarray, and code that uses operations on pixels is automatically generic wrt heterogeneous layouts as long as all channels have similar semantic meanings. John