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[AP] How to apply curves within a mask ?


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Apparently, you can paint in a mask with a brush or apply a gradient to it, or apply filters from the Filters menu, but you can not apply a level or curve adjustment.  Or did I miss something ?

 

It is an important feature for an advanced image processing ; I would like to have it in AP.

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Hi ch22,

Drag the Curves Adjustment over the thumbnail of the mask in the Layers panel, then in the Curves Adjustment dialog change the channel to Alpha (on the bottom). If you then adjust the Adjustment Curve's graph it will affect the mask. See attached sample file.

 

curves_mask.afphoto

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Thanks. This gives me a little light but  I still have a lot of problems — I am aiming at an adaptation of the process described long ago (2004) by Darren Bernhardt in http://www.dbphoto.net/techniques/luminancemask/page2.htm(simulation of the Shadows/Highlights filter with adjustment layers). I shall come back here with more precise questions or remarks.

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Well, I succeeded in achieving a simulation of the Highlights Filter following the idea of Darren Bernhardt. A demonstration file is attached. I first explain what I made and then I state new remarks ; maybe the operation of AP could be improved—or my brain reoriented ?

 

When you open my file, you see the background layer only : a little girl with a hat a bit too luminous, which would be better somewhat darkened. In the Layers Panel, the 2nd line is a group containing the simulated filter ; for completeness, the top layer shows the corrective adjustement I made in Photoshop, namely a curve adjustment with the ad hoc mask. Please do not activate the two corrective layers simultaneously ; also do not pay attention to the strength of the correction, this is only an exercise.

 

The simulated filter is made of three layers :

(i)            a curve adjustment with a luminosity mask within its mask ( “Strength“ layer). The curve applies a very strong contrast to the lighter tones. Normally this would result into a strong darkening of medium and dark tones, but this effect is contained by the dark tones in the luminosity mask.

(ii)           This containing action is not sufficient, it is necessary to put black in the darkest tones of the mask. This is made with a Level ou Curve adjustment  (“Range“ layer) acting on the mask of the first layer  ("Strength")

(iii)         At this point, the lightest tones of the picture are darkeneanxd, as wished, but also somewhat flattened. This is counteracted by blurring the mask of the first layer (“Strength“), through a live filter adjustment in the “Radius“ layer. As a gaussian blur may easily lead to undesirable halos along edges, I used a more sophisticated bilateral  blur.

Obviously, the names Strength, Range and Radius come from the control sliders of the Highlights Filter. And obviously as well, in Photoshop, steps (ii) and (iii) are not reversible.

 

Now the new remarks :

 

(1) when I open the Level or Curve adjustment over a mask, as in step(ii) above, I have to switch to the alpha channel.  Cannot this be done automatically, since it should be possible to recognize that one is working on a mask? More comfortable !

(2) When you apply a Curve/Level adjustment to a mask and switch from RGB channels to the alpha channel, the bottom histogram suddenly disappears, which is a little alarming, especially for a beginner. Would not it be possible to display the histogram of the mask alpha channel?   

(3)  Mere curiosity : would it be possible (simply) to merge the three layers into a single curve adjustment layer, Photoshop-like, with a mask including the action of steps (ii) and (iii) ?

(4) More annoying : I did not manage to run the dodge/burn tools on a mask. Try with the upper layer in my file ; alt-click its thumbnail in order to display the mask. I could paint it with the brush tool but I got nothing with the dodge/burn tools.  Did I miss something again ?

la_bonne_glace.afphoto

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