L0 Gradient Projection

Minimizing L-0 gradient, the number of the nonzero gradients of an image, together with a quadratic data-fidelity to an input image has been recognized as a powerful edge-preserving filtering method. However, the L-0 gradient minimization has an inherent difficulty: a user-given parameter controlling the degree of flatness does not have a physical meaning since the parameter just balances the relative importance of the L-0 gradient term to the quadratic data-fidelity term. As a result, the setting of the parameter is a troublesome work in the L-0 gradient minimization. To circumvent the difficulty, we propose a new edge-preserving filtering method with a novel use of the L-0 gradient. Our method is formulated as the minimization of the quadratic data-fidelity subject to the hard constraint that the L-0 gradient is less than a user-given parameter a. This strategy is much more intuitive than the L-0 gradient minimization because the parameter a has a clear meaning: the L-0 gradient value of the output image itself, so that one can directly impose a desired degree of flatness by a. We also provide an efficient algorithm based on the so-called alternating direction method of multipliers for computing an approximate solution of the nonconvex problem, where we decompose it into two subproblems and derive closed-form solutions to them. The advantages of our method are demonstrated through extensive experiments.