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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?

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I am not sure this is exactly like an AL. An AL is a way to apply, in a non-destructive way, stacking edits...such as levels, curves, saturation, contrast, etc. You tweak the settings of those TOOLS in an adjustment layer, and they get applied to the layers below the AL. I don't think you can get the same effect by duplicating and blending layers manually...unless I am misunderstanding your process. –  jrista Jun 23 '11 at 0:59
    
@mattdm - not mentioning GIMP in the question title was intentional, as the described method applies to whatever editor that support layers and layer masks - including PS. –  ysap Jun 23 '11 at 2:23
    
@ysap: sorry. Please roll back or otherwise correct. –  mattdm Jun 23 '11 at 11:24
    
But, could you put the title in the form of a question of some sort? I'm actually, now, a little unclear on exactly what you're asking. Is it "Does this layer mask technique approximate what adjustment layers do?" –  mattdm Jun 23 '11 at 14:13
1  
As of now, this question has one vote for closure. I can see why, but reading the faq, I think it is in the grey zone, as the faq permits "photographic processing or retouching". Then it avoids "Image Manipulation (outside of the Photography context)". I think that this becomes an important technique in the post process for many photographers, and not necessarily only for graphic designers. –  ysap Jun 23 '11 at 14:26
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1 Answer

up vote 3 down vote accepted

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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As I mentioned at the end of the question, I realize that this is not 100% similar operation. You are absolutely correct, re-doing the "AL"(s) every time an image is modified is a tedious task. However, the graphical effect (end result) is the same, isn't it? –  ysap Jun 23 '11 at 2:32
    
You might be missing the general point of an AL...its not so much the end result...but the fact that they are non-destructive. You can get the same result by directly applying a specific tool (such as curves) to a copied layer, but thats destructive to the layer. With an AL, you don't have to touch your base layer(s)...you just apply the tools in the AL, and the combined effect gets "rendered" to the image without any actual edits. –  jrista Jun 23 '11 at 2:59
    
@jrista - obviously, you are right. The emphasis in AL is at the process itself, rather than the result. That said, the AL let you achieve results that are impossible using a layer mask (on the base layer), b/c it adds the localization factor to the game - whereas applying the adjustment (or filter) to the layer is a global operation. The method I presented (of course I was not the 1st one to come up with this, though) let you achieve the localization effect. –  ysap Jun 23 '11 at 14:07
    
I'm really not sure where you're going with the layer mask thing; that's basically an orthogonal concept. –  mattdm Jun 23 '11 at 14:11
    
OK, let me try and clarify a little bit. Let's say that you want to gradually convert a color image to B&W. With an AL, you would fill the AL with a gradient, where the adjustment operation itself is "Desaturate". I can't see a way of doing this with a single image layer and an associated layer mask in GIMP. The mask merely controls the transparency of the image layer. With the method I specified, I duplicate the image, desaturate the whole new layer and add a layer mask with a gradient fill. –  ysap Jun 23 '11 at 21:26
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