Best way? Go gradually from 100 to 36% opacity

[Ofnuts]

(11-10-2023, 11:04 AM)T-buch Wrote:

(11-09-2023, 06:22 PM)Ofnuts Wrote:

(11-09-2023, 02:13 PM)T-buch Wrote: Yes the "feathered" selection somehow got the effect I want. The problem is, it is becomes very voluminous to check, - to find out where the opacity starts and where it ends

Use the Pointer dialog and explore the image with the mouse (IRL I keep the Pointer dialog docked in my controls window, because I use it all the time). Keep Sample merged unchecked if you want the opacity of the current layer.

Thanks This was not a "dock" I just had checked out.
And Its better than nothing - But its "follow-up inspection"
There is no way I can tell Gimp "for here to here I want the opacity to go form x-vallue to y-vallue"  for example ?

You create a gradient with the relevant value on a layer mask. But things are made complicated because there can be gamma-corrected values depending on image precision. A good way to do it:

• Set sample point where you want to achieve a given opacity
  
• Add a layer mask (and initialize by transferring the alpha channel)
  
• Make all other layers invisible (because sample points always work on the merged image)
  
• Add a black to white gradient on the mask, going from most transparent to less transparent.
  
• Start the Levels tool
  
• Looking at the sample points, adjust the output levels for black and white.
  
• You can also use the gamma handle in the Inputs area to adjust the value of a middle point
  
• You can also of course use Curves if you want an even more specific opacity shape
  

   

Side note: looking at the required output values to get 25% and 75% opacity, assuming the usual 2.24 gamma value

Code:

(138/225) ^ 2.24 = .253
(225/255) ^ 2.24 = .756

Not a total coicidence