convert single channel to 3 channel

edit

Hi, as given in the document of opencv, convertTo converts an array to another data type with optional scaling. And it is clearly mentioned that the number of channels of output and input must be same. rtype – desired output matrix type or, rather, the depth since the number of channels are the same as the input has; if rtype is negative, the output matrix will have the same type as the input.

use cvtColor(src,dst,COLOR_GRAY2BGR)