This program tone matches or equalizes the brightness and/or contrast of a list of
input cubes prior to mosaicking. All the input cubes, including mosaics, must be in identical
map projection since equalizer uses the mapping information to gather statistics in
overlapping areas.
Statistic gathering is done on a band-by-band basis. The statistics
are used in a least squares solution to determine multiplicative (GAIN or MULT) and additive
(OFFSET or BASE) corrections for each image. In addition to simply calculating corrective
factors and applying said factors to each image (BOTH), the user has the option to only
calculate statistics (CALCULATE) and then examine these gathered statistics in a text file
(OUTSTATS). Then, equalizer can be run again using that statistics file as input (INSTATS)
to apply correction to any or all of the images without the need to calculate corrective
factors again.
Corrective factors will be undetermined if there are insufficient valid overlaps or if there
are any images that have no overlaps. If an output statistics file is provided (OUTSTATS),
images that do not overlap will appear in the file.
When corrective factors are undetermined, the user can use the RECALCULATE process option
to try to solve them. To do this, images can be added to the input list to try to create
more overlaps or images without overlaps can be removed. An input statistics file (INSTATS)
containing previously calculated image statistics must be provided. These statistics will be
reloaded so overlap statistics will only be recalculated for new images in the modified input
list. If all the images have overlaps and sufficient valid overlaps exist, corrective factors
will be determined. Statistics can be recalculated as many times as desired by repeating the
above steps.
Additionally, the RETRYBOTH process option may be used to recalculate statistics and then
apply corrective factors. This requires the user to provide an INSTATS file containing
previously calculated statistics. It is also suggested that the user provide an OUTSTATS file
in case corrective factors cannot be determined. If the corrective factors are determined,
they will then be applied to the input images.
The addition of these RETRYBOTH and RECALCULATE parameters has modified the content of
the output statistics. Additional information is included in the output statistics
so that previously calculated overlap statistics can be reloaded when using these parameters.
Note that statistics files created with previous versions of ISIS 3 can still be used as
input statistics.
The actual equation to be used for equalization, for ADJUST=BRIGHTNESS, CONTRAST or BOTH, on
each band in each cube is as follows:
newdn(s,l,b) = (olddn(s,l,b) - avg(b)) * GAIN(b) + (avg(b) + OFFSET(b))
where
s = sample index
l = line index
b = band index
The actual equation to be used for equalization, for ADJUST=GAIN, on
each band in each cube is
newdn(s,l,b) = olddn(s,l,b) * GAIN(b)
where
s = sample index
l = line index
b = band index
Prior to equalizing, the user can choose whether to adjust the
brightness and/or contrast of the cubes. The default is to adjust both;
however, if the brightness (average) of all the cubes is the same,
then simply adjusting the contrast may suffice. Likewise, if the
standard deviation of all the cubes is similar, then a contrast
adjustment is not necessary. Adjusting for contrast only implies
the OFFSET values will be held to zero. Similarly, adjusting for
brightness implies the GAIN values will be held to one.
The OFFSET and GAIN values are computed independently for each image,
therefore we have two least squares computations with N unknowns,
where N is the number of cubes to be equalized. The overlaps, M,
between all the cubes are computed, and in some cases M < N.
This implies an underdetermined system, and the program will report
an error if this occurs. You can hold one or more images to alleviate
this problem. Holding an image forces GAIN and OFFSET to 1.0 and 0.0
for that image, respectively.
If the user chooses to apply correction to the images, then a list
of output file names can be specified with the TOLIST parameter. If no
TOLIST is specified, the equalized cubes will be named the same as the
input cubes with the addition of '.equ' prior to the '.cub'
extension, and placed in the same directories as their input files.
For the sake of efficiency, the user may choose to set the "sampling
percent" to be less than its default value of 100.0. By doing so, the
program will likely perform its statistic gathering noticeably faster,
but at the risk of losing accuracy in the results. It should
be noted that the user also runs the risk of encountering an error if
decreasing the sampling percent results in the amount of valid data in
the calculated overlaps being less than the minimum set by the
MINCOUNT parameter (default value of 1000). Sampling percent must be
a decimal value between 0.0 (exclusive) and 100.0 (inclusive).
This program defaults to solving the least-squares system using the
SPARSE matrix method. This method is able to find valid solutions, even
when no hold list is provided.
The QRD method is fairly accurate and fast, but does not
produce valid results in all cases, especially if no images are held.
Previous versions of equalizer used this method to solve.