Question about parameter of cv::calcBackProject( )

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I am doing it as a part of mean-shift tracking.

calcBackProject( const Mat* images, int nimages, const int* channels, const SparseMat& hist, OutputArray backProject, const float** ranges, double scale=1, bool uniform=true );

The 3rd parameter is: int* channels.

if do it in 1 channel, I know it is define as: int ch[1]={0}, now I want to do in 3 channels (input a RGB 3 channel color image), how should I define it?

int ch[3]={2} ? or int ch[3]={0,1,2}? or int ch[3]={2,1,0}? or int ch[3]={2,2,2}?

Does anyone know it and Could you give me some tips? Thank you.