Russell Croman Astrophotography  

 

 

GradientXTerminator

Tutorial

Click here for detailed instructions on installing the GradientXTerminator plug-in.

Don Waid has produced a video tutorial on how to use GradientXTerminator. He has been kind enough to allow a link to it: Click Here to watch Don's excellent tutorial!

The following is a brief tutorial on how to use GradientXTerminator. You can use these same steps on your own image, or download the file (~6MB) used for this tutorial. For complete instructions on how to use GradientXTerminator, see the online manual. Note: these steps are shown using a Windows computer, but they should be nearly identical for the Macintosh version of the plug-in.

Open the image in Photoshop:

The first step is to do a rough correction of the large-scale gradients in the color channels. Start by using the Lasso tool to select the two galaxies in the image. At this stage, don't worry too much about making sure no parts of the galaxies are selected; just make sure you get most of them. Use the Shift and Alt keys to add or subtract areas to the selection.

We want the background selected, not the galaxies. So invert the selection you just made:

At this point you should have something like this:

The selected portion of the image is what GradientXTerminator will look at to develop a model of the background gradients. Anything not selected will be ignored as far as this model is concerned, but these areas will be corrected based on the surrounding areas. There is no need to feather the selection.

Now invoke the GradientXTerminator filter:

For this first gross correction, set the Detail to Medium, and the Aggressiveness to Low. Check "Balance background color" option if it is not checked. (This can normally be left on always.)

Run the filter. In a few seconds, you should see something like the following:

Note that the gradients have mostly been corrected, but there is still some residual color in the background. The point of the above initial correction was to make the background more easy to select for a subsequent, more detailed correction. Setting the Aggressiveness to Low ensures that only very large-scale gradients will be corrected, reducing the chance that a local feature, such as a faint arm of the galaxy, will be flattened by the filter.

Now we'll do a more detailed correction to fully flatten the background. Choose the Magic Wand tool in Photoshop. Set the tolerance to between 5 and 10, and make sure the "Contiguous" check box is selected.

Click on an area of the background. This may select the whole background nicely, or you may end up with just a part of the background selected.

If only part of the background was selected, you can either increase the Tolerance setting and try again, or hold down the Shift key while clicking on another part of the background with the Magic Wand. This will add to the first selection you made. Note the small "+" that appears next to the Magic Wand when holding down the Shift key, signifying that areas are being added to the selection.

When you are finished selecting the background, you should see something like this:

Note that the faint edges of the main galaxy were selected. We don't want GradientXTerminator to flatten these parts, so we need to deselect them. The easiest way to do this is using the Lasso tool again. Choose it, and hold down the Alt key to remove the edges of the galaxy from selection. Note the small "-" that appears next to the lasso to indicate areas are being removed from selection.

You should now see something about like this:

Now we're ready to run GradientXTerminator again. Invoke it, and set the Detail to Fine, and the Aggressiveness to High. You should get the following:

That's it! The gradients are gone, and the background is a perfect neutral tone. You are now ready to do any final layering and adjustments to the image.

For more complete instructions on running GradientXTerminator, see the online manual.