How to do SVD back substitution

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I am applying SVD on image using opencv 2.4.9 command SVDecomp. at the time of back substitution it asks rhs array.What is that array and how to get it?

commands:

 1. cv::SVDecomp(im_enc1,w,u,vt);

where w,u,vt are SVD components and im_enc1 is inputimage.

 2. cv::SVBackSubst(w,u,vt,rhs,im_enc2);

where w,u,vt are SVD components and im_enc2 is output image.