pfstmo_fattal02 - Online in the Cloud

PROGRAM:

NAME

pfstmo_fattal02 - Gradient domain high dynamic range compression

SYNOPSIS

pfstmo_fattal02 [--alpha <val>] [--beta <val>] [--gamma <val>] [--saturation <val>]
[--noise <val>] [--detail-level <val>] [--black-point <val>] [--white-point <val>]
[--multigrid] [--verbose] [--help]

DESCRIPTION

This command implements a tone mapping operator as described in:

Gradient Domain High Dynamic Range Compression R. Fattal, D. Lischinski, and M. Werman In
ACM Transactions on Graphics, 31(3), p. 249, 2002.

With respect to the original paper, this program provides additional parameter which
limits the amplification of noise. The noise is often starkly amplified because of
division by zero in one of the equations in the paper. Extension contributed by Przemyslaw
Bazarnik.

At the core of the programme is a Poisson PDE which as suggested in the original paper is
solved using a Full Multigrid Algorithm. However, this is an iterative solver which seems
to lose accuracy when applied to higher resolution images resulting in halo effects and
surreal looking images. For that reason a second solver has been implemented using the
discrete cosine transform as the underlying method and is considerably more accurate
mainly because it is a direct solver. This solver is the preferred method and is used by
default. The old multigrid solver can be selected with the --multigrid (-m) option.

OPTIONS

--alpha <val>, -a <val>

Set alpha parameter. This parameter is depreciated as setting a <val> other than
1.0 has only the effect of a global gamma adjustment of the luminance channel which
can be directly specified using the --gamma option. See the paper for the
definition of alpha. It can be shown, although not mentioned in the paper, that
setting alpha other than 1.0 has the same effect as setting gamma =
alpha^(k*(1-beta)), where beta is the value as specified by --beta and k is the
number of levels of the Gaussian Pyramid (see paper for details), which depends on
the image pixel size (smallest k so that 2^(k+detail_level) >=
min(width,height)/MSIZE, MSIZE see source code, e.g. 8 or 32).

--beta <val>, -b <val>

Set beta parameter. <val> sets the strength of gradient (local contrast)
modification. Suggested range is 0.8 to 0.96, default is 0.9 (see paper for
details). Value of 1 does not change contrasts, values above 1 reverse the effect:
local contrast is stretched and details are attenuated. Values below 0.5 lead to
very strong amplification of small contrast, so consider using --noise parameter to
prevent noise.

--gamma <val>, -g <val>

Set luminance gamma adjustment. This can be described as a global contrast
enhancement and is applied after the local enhancement as specified by the
parameter --beta is performed. Gamma adjustment or correction is defined by a
power-law, in this case
L_out(x,y) = L_in(x,y)^gamma, where L_in(x,y)=exp(I(x,y)) is the luminance value
after the local contrast enhancement (I is the solution of the Poisson PDE). The
suggested range for <val> is 0.6 to 1.0, default is 0.8.

--saturation <val>, -s <val>

Amount of color saturation. Suggested range is 0.4 to 0.8. Default value: 0.8.

--noise <val>, -n <val>

Reduces the gradient amplification value for gradients close to 0 and reduces noise
as a result. <val> defines gradient value (luminance difference of adjacent pixels)
which is treated as noise. Suggested range is 0.0 to the value of alpha. Default
value calculated based on alpha: 0.001*alpha.

--detail-level <val>, -d <val>

Specifies up to which detail level the local contrast enhancement should be
performed. It basically means that local contrast levels within small squares of
pixel size 2^<val> are not changed. In the implementation this corresponds to
removing the <val> finest levels of the Gaussian Pyramid as described in the paper,
i.e. the paper only considers <val>=0. Suggested values are 1, 2 or 3; 3 for high
resolution images. The default is 3 for --fftsolver, and 0 if the original multi-
level solver is used (to be consistent with the paper).

--white-point <val>, -w <val>

Specifies the percentage of pixels which are allowed to be overexposed and
therefore blown out. This can be useful for example when there is a very bright
object in the image like the sun and details of it do not need to be resolved. As
a result the overall image will look brighter the greater <val> is. Default is
0.5.

--black-point <val>, -k <val>

Same as --white-point but for under-exposed pixels. Default is 0.1.

--multigrid, -m

Enable the use of the multigrid solver as suggested by the original paper. For
accuracy the default fft solver is generally recommended especially when using high
resolution images. The user will benefit by obtaining photo-realistic rather than
surreal looking images. The fft solver is also faster despite the fact it is only
O(n*log n) with n=width*height, as compared to O(n) for the multigrid solver. The
speed improvement is thanks to the very efficient fftw3 library which is used to
calculate the discrete cosine transform.

--verbose

Print additional information during program execution.

--help

Print list of command line options.

EXAMPLES

pfsin memorial.hdr | pfstmo_fattal02 -v -t | pfsout memorial.png

Tone map image (using fft solver) and save it in png format.

pfsin memorial.hdr | pfstmo_fattal02 -v -t -b 0.85 -g 0.7 -w 2.0 \
| pfsout memorial.png

Tone map image (using fft solver) with stronger contrast modification than default,
i.e. beta=0.85, gamma=0.7 and white point 2.0%.

pfsin memorial.hdr | pfstmo_fattal02 -v | pfsout memorial.png

Tone map image (old style) and save it in png format.

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