Removing Gradients with the GIMP

There are times when an astro-photo has a noticeable gradient in some direction across the image. This means that the dark sky is lighter on one side than the other (or top to bottom, or on a diagonal). There are fancy tools for getting rid of gradients but the GIMP has a gradient tool that does the trick nicely.

To use this tool select the Gradient Tool from the toolbox (Figure 157) which will display the options for the Gradient tool.

What we want to do with it is to smoothly remove the gradient from the image without losing any detail, or as little detail as possible. To start with set the options as follows:

Mode

Select Overlay from the drop down list. From the documentation for the GIMP.

Overlay mode inverts the pixel value of the lower layer, multiplies it by two times the pixel value of the upper layer, adds that to the original pixel value of the lower layer, divides by 255, and then multiplies by the pixel value of the original lower layer and divides by 255 again. It darkens the image, but not as much as with “Multiply” mode.

That sounds ominous, but it does the trick very nicely for removing a gradient.

Opacity

Start with the opacity set around 30% and adjust it as needed. We don't want the gradient stomped into the ground but rather gently massaged away.

Gradient

The gradient type we'll be using is 'FG to Transparent' which starts with the foreground colour and fades it away to nothing as the gradient is added. Make sure that the foreground colour is black.

Offset

The offset adjusts how far into the gradient add the darkest part extends. Start with about 30% and adjust as needed.

Fig. 157

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Shape

We are removing a gradient that goes in one direction so we'll use Linear.

All of the rest of the options can be left at defaults unless there is a reason to play with them. To activate the gradient, click on the image where you wish to start and then drag the mouse any direction or distance to where you wish to end. When you let go of the mouse button the gradient will be activated and you can see your work. If you wish to adjust then press Ctrl-Z to undo the previous attempt, adjust the parameters and try again.

The southern Milky Way shot in Chile on a tripod-mounted Nikon DSLR.

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