least square solution of b = A * x with a very huge A matrix

[andrea console]

Hi to all,

I'm not a mathematician, but I have to solve this A*x=b problem. Since I'm working with high resolution "fit" files (images), the related matrices are very huge but sparse. I tried x = A\b, but it  gives the error: "SparseQR: sparse matrix QR factorization filled" that I don't understand.

Additional informations: 

"A" matrix is a matrix of square matrices, n*3 matrices: for each row, the first is a diagonal one, the second is simply sparse and the third is an eye matrix. However, since I have to eliminate saturated pixel, I delete some rows before computation.

I'm working with Octave 3.6.4