How to Fake Some Adjustment Layer Effects in GIMP

One of the common complaints about GIMP is that the application doesn't offer Adjustment Layers. As Photoshop users will know, Adjustment Layers are layers that can be used to edit the appearance of all layers stacked below, without actually editing those layers, meaning an Adjustment Layer can be removed at any point and the layers below will appear as before.
Because there are no GIMP Adjustment Layers, layers have to be edited directly and effects cannot be removed later. However, it is possible to fake some basic non-destructive Adjustment Layers effects in GIMP using blending modes.

Don't Expect Miracles

The first thing to say is that this isn't a miracle solution to the GIMP Adjustment Layers issue. It doesn't offer the fine control that you can get using true Adjustment Layers, and most advanced users looking to process their images to produce the finest results will probably consider this a non-starter. However, for less advanced users looking to achieve quick and easy results, these tips may be useful additions to an existing workflow, using the Mode drop down and Opacity slider located in the top of the layers palette.
These tips may not be effective with every image, but in the next few steps, we​'ll show you some quick and easy ways to fake basic GIMP adjustment layers to achieve simple non-destructive editing in GIMP.

Use Screen Mode

If you've got an image that's looking a little dark or under-exposed, a really simple trick to lighten it up is to duplicate the background layer and then change the Mode to Screen.

If you find that the image has gone too bright and some areas have burned out or become pure white, you can reduce the effect by sliding the Opacity slider to the left so that more of the background layer shows through.

Alternatively, if the image still isn't bright enough, you can duplicate the new layer so that now there are two layers set to Screen. Remember, you can fine tune the effect by adjusting the Opacity of this new layer.

Use Layer Masks

You can use a Layer Mask so that only certain parts of the image are lightened when you duplicate the Screen layer.

We duplicate the Screen layer and then right click on the new layer in the Layers Palette and click Add Layer Mask. We then select Black (full transparency) and click the Add button. With white set as the foreground color, we now paint into the mask with a soft brush so that the t-shirt is unmasked and appears lighter. Alternatively, we could use the Paths Tool to draw around the area you wish to select, make a Selection from Path and fill that with white for a similar result.

Use Soft Light Mode to Lighten

If the area still isn't light enough following the last step, we could just duplicate the layer and mask again, but another option would be to use the Soft Light Mode and a new layer with a fill of white that matches the mask applied previously.

To do this, we add a new empty layer on top of the existing layers and now right click on the Layer Mask on the layer below and select Mask to Selection. Now we click on the empty layer and fill the selection with white. After deselecting the selection, we just change the Mode to Soft Light and, if necessary, adjust the Opacity of the layer to fine tune it.

Use Soft Light Mode to Darken

After spending the last few steps lightening the image, this step may seem a bit odd, but it does demonstrate another way to use Soft Light Mode--this time to darken the image. We add another blank layer on top and this time fill the whole layer with black. Now, by changing the Mode to Soft Light, the whole image is darkened. In order to bring some detail back into the area that's been lightened, you can reduce the Opacity a little.

Experiment, Then Experiment Some More

I said at the beginning that this isn't a true alternative to real GIMP Adjustment Layers, but until a version of GIMP is released with Adjustment Layers, then these little tricks may offer GIMP users some simple options for making non-destructive tweaks to their images.
The best advice we can give is to experiment and see what effects you can produce. Sometimes we apply Soft Light Mode to complete duplicated layers (which we haven't shown here). Do remember that there are many other Modes available that you can also experiment with, such as Multiply and Overlay. If you apply a Mode to a duplicated layer that you don't like, you can easily delete or hide the layer, just as you would if using true Adjustment Layers in GIMP.