||Intel or AMD
||Intel or AMD
|Intel or PowerPC
||XP SP2 and later
|Tiger (10.4) or later
||Snow Leopard (10.6) or later
||5.5 or later
||CS4 or later
||CS5 or later
||5.0 or later
Installing the Plug-In
Click here for detailed instructions on installing the GradientXTerminator plug-in.
GradientXTerminator works on RGB and Grayscale
images, in 8-bit or 16-bit mode, although results are best if it is used
on 16-bit images. To remove gradients in other images,
first convert them to 16-bit RGB or Grayscale mode. After removing gradients,
you can then convert back to the desired image mode.
GradientXTerminator works by using the selected portions
of an image to estimate the background gradients. A mathematical model
known is constructed based on these
portions of the image. Think of this process as bending a "rubber sheet" so that it follows the brightness gradients in the background, but not the brightness of the objects in the image. This model allows the plug-in to estimate the
pixel values of the background gradient in the unselected portions of the image,
including parts of the image with brighter foreground objects such as
galaxies, nebulas, and star clusters.
How well GradientXTerminator corrects for the background gradients in an
image depends mainly on the accuracy of the input you give it.
Fortunately, the rich set of selection tools in Photoshop make selecting
the background easy.
There are at least three approaches that work well for
selecting the background of an image:
Using Photoshop's Lasso tool, you can approach background selection in a
sort of a backwards fashion. Rather than trying to lasso all of the
background areas of an image, you can first select all of the foreground
(object) areas, and then invert the selection. See the
tutorial for an example of this.
The Magic Wand tool, if set up
properly, is very effective at selecting background regions while
including very little if any object areas. The key setting for the Magic
Wand is "Tolerance." This controls how wide a range of pixel values the
tool will consider equivalent to the part of the image you click on. A
larger tolerance means more of the image will be included. A small
tolerance means less will be included.
Generally, a value
of between 5 and 10 works well for the Magic Wand's tolerance. If you
find that the current setting tends to select too much object in
addition to background, try lowering the tolerance value. If only part
of the background gets selected, try raising the value.
As shown in the tutorial, you can also use
the Shift and Alt keys with the Magic Wand. Holding down Shift while
clicking on the image with the Magic Wand will cause the area you click
on to be added to the selection. Holding down Alt, on the other hand,
will cause the area to be removed from the selection.
Select Color Range
This function is accessed in
Photoshop using the menu: Select->Color Range. If you have invoked this,
you will notice that the cursor turns into a little eyedropper if you
pass it over the image. Click on an area of the background. You will
notice that much of the image background area in the Color Range dialog
box turns white. Adjust the "fuzziness" slider to include more or less
of the background as desired. This is analogous to the Tolerance setting
of the Magic Wand tool. Try clicking on other parts of the image
background with the eyedropper to get the selection you want. Then click
OK to select it.
Sometimes, a combination of selection
methods is appropriate. See the tutorial for an example of using the
Magic Wand in combination with the Lasso tool. With a little practice,
you should be able to select the appropriate parts of an image very
To extract the background gradient information,
GradientXTerminator divides the image into a grid. The fineness of
this grid is controlled by the Detail setting. A higher level of detail
results in a finer grid.
Which setting to use depends
upon the gradients present in your image. A large-scale, slowly varying
gradient, such as a linear gradient from light pollution, is easily
modeled mathematically and so only needs a coarse grid setting. A more
complex gradient, such as that caused by vignetting in the optical
system (when not corrected by a flat field calibration), might need a
medium detail setting to properly correct. The fine setting would be
used for gradients that vary over short distances in the image. Slight
imperfections in flat-field calibrations can sometimes cause subtle
gradients that are well-corrected using the fine setting.
One trade-off with the detail setting is that the filter will take
longer to run with a finer setting; the more grid points there are, the
harder it is mathematically to figure out how the rubber sheet bends to
conform to them. Also, during the background extraction process, the
plug-in has a much easier time of rejecting stars or faint background
objects that might be inadvertently selected when the detail setting is
on a coarser value.