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opencv has a reshape method, that will flatten your image quite cheap(no memory is moved, it's just rearranging the header)

 Mat m = ...
 Mat m_flat = m.reshape(1,1); // flat 1d
 // also note, that it *returns* a new Mat header (common pitfall, folks tend to forget to catch it)

if you really need to seperate the channels, it gets more expensive:

vector<Mat> chans;
split(m, chans);

Mat res; // stack the channels into a new mat:
for (size_t i=0; i<chans.size(); i++)
     res.push_back(chans[i]);

res = res.reshape(1,1); // flat 1d, again.