J. Mat
´
ıas Di Martino, Gabriele Facciolo, Enric Meinhardt-Llopis
to similarity crite ri a and they used both L
1
and L
2
norms between patches or their gradients to
synthesize the inpainted image. More recently Sadek et al. [
27] also proposed a variationa l model
for gradient-based video editing. In this work optical flow was used to gu i d e the propagation of the
information in the gradient dom a i n . The authors showed that this method was tempora l l y consistent
and able to handle fast and abrupt illumination changes in tim e as well as smooth illuminatio n
changes in space. Their experimental results showed that the proposed model was able to handle a
large number of frames and relatively complex sequences.
Bhat et al. stud i ed a wide variety of applications of Poisson Equation and proposed a set of
techniques and filters using modified versions of i t . In 2008 [7] they proposed a varia t i on a l formulation
that combines a data term with a gradient term, the combination of these two kinds of terms allowed
them to derive several kinds of filters such as Sharpening, Flattening and de-blocking. For example, a
sharpening filter is proposed as a minimization problem where the objective function trades off two
terms: a fidelity term to the original image against a fidel i ty term to the ampli fi e d gr a d i ents. The
image that mi n i m i zes this objective function presents boosted high frequencies while its intensity
values keeps close to the input image. In this work the authors a l so present an analy si s of the
proposed functionals in the Fourier domain, and show that the minimization can be performed in a
direct, exa ct and efficient way in this domain. The work previ o u sl y described was extend ed by th e
same authors in [
8, 9]. In th ese works the authors d escr i bed in a more general way their proposed
framework which combines zero order and first order (i.e. first derivative) fid el i ty terms. In addition
they introduced spatially varying weights over the constraints to approximate the L
1
norm and
extend some of their previous ideas to video se qu e n ces by addi n g temporal constraints.
More recently Morel et al. [
22] proposed a Fourier i m pl em entation of Poisson ima ge editing
method, which all ows an exact and fast solution of the Poisson equation even when the region of
interest has a complex shape. In addi t i on th e aut h o r s presented a n au t o m at i c meth od for selecting
the region of interest and applied it t o a classic l ocal contrast enhancement principl e . The main
idea behind this work is to solve the Poisson equation over the entire domain using the modified
gradient map i n si d e the region o f interest, an d the gradient of the background i m age outside it. In
addition Dirichlet boundary conditions are replaced by Neumann conditions (at the edges o f the entire
image). Even though the p r o posed (Fourier) implementation solves the Poisson e qu a t i on over the
entire domain of the image (instead of the selected su b -d o m ai n ) , Morel et al. empirically showed that
the solutions of both (the exact and the Fourier based) i m p l em e ntations are perceptually equivalent
outside the sub-domain for a varied set of im ages. In a similar direction, Limare et al. [
18] proposed
an implementation in the Fourier domain of the Retinex
3
algorithm as a Poisson Equation with
Neumann boundary conditions. The method Limare et al. p r oposed consists in sett i n g to zero those
gradients whose modu l u s is below certain thr esh o l d , and then compute th e image that correspon d s
to the modified gradient map by solving the Poisson equation. Thus, in the new image the small
details and shades have been removed, which, according to Retinex t h eo ry models t h e property of
our perception of constant col o r s regardless of their shading. This m et h od is very similar to one of
the ap p l i ca t i on s presented in P´erez et al. work [24] as Texture flattening, i n which only the gradients
at edg e locations are kept before integrating with the Poisson solver. Finally, in a related work by
Morel et a l . [21] a functional containing a first order term and a zero order term was u s ed with the
aim of removing the effect of nonuniform i l l u m i n at i o n whi l e pre ser v i n g the fin e deta i l s of the image .
The proposed appr o ach is similar to the one presented by Bhat et al. in [
7], the main difference lies
in the data (zero order) term, while Bhat et al. adjust the solution to an input image, Morel et al.
adjust the solution to be close to it’s own mean value.
3
Retinex theory was formulated in 1964 by Edwin H. Land, and was the first quantitative attempt to simulate how
the human visual system perceives color.
304