How to create the equivalent of an Adjustment Layer in an editor that does not support it?

Question

As a user of GIMP I tried to understand what Photoshop's Adjustment Layers are, without actually using PS. My understanding is that GIMP does not yet support AL (is this correct?).

So, trying to imitate the effect of an AL, is it equivalent to:

1. Duplicate the image to a second layer.
2. Apply the desired effect (filter/adjustment/whatever) to the new layer.
3. Add a layer mask to the 2nd layer and set its transparency areas the same way you'd do with AL, this blending the adjusted image with the original image in the desired areas/amounts.

Am I thinking right?

I realize that there is one limitation to that method - changes to the original image layer will not affect the top layer (AL equivalent). Is there a way to lock the two images in the two layers?

Answer

In short, no. What you're doing is not really related. Layer masks are basically ways of working with the alpha channel of a layer. Adjustment layers aren't really layers at all — they're ways of thinking of filters within the same metaphor. They don't actually accomplish anything you couldn't do simply by applying the filters in the traditional way. However, because the layers model is very powerful, they are a convenient and powerful tool which makes visual experimentation easier.

The problem is that "apply the desired effect" is a destructive operation for the layer you apply it to — if you want to change the parameters of that effect, you have to do something to reverse it. Generally, that means recreating the whole layer. Layer masks let you choose how "strongly" to apply an effect, and limit it to certain parts of the image, but they don't change that basic limitation.

In terms of final results, there's nothing you can do with adjustment layers that you can't do just be restarting the layer every time. The problem is that if you're trying to work with the combination of multiple different adjustments (for example, blur and curves), it becomes tedious.

So, it's basically an ease-of-workflow thing, and since you can combine multiple layers, for complicated operations it can be exponentially easier — nothing to sneeze at.

On the plus side, the Gimp development roadmap has something called "Filter layers (brightness/contrast, blur, etc)" as relatively high priority. Currently, that's slated for Gimp 3.2. That's not the impending future, but it sounds like we'll get it eventually.

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