Best way? Go gradually from 100 to 36% opacity

[T-buch]

What is the most controllable and precise way to mark ("free select") an area on a layer and gradually reduce it from 100% opacity in the edges to a defined area where there should be 36% opacity?

Please see the accompanying image (which is certainly not the ultimate solution for this task)

   

[rich2005]

A bit pushed for time, it being the Friday meeting (for a couple of beers)

I might go this way.

1. The brain layer at reduced opacity

   

2. The brain layer duplicated, Opacity up to 100, a layer mask (white) added

   

3. A radial gradient, black-to-white applied. Center is solid / edge is reduced

   

Edit:

A better possibility, same as before but with a different gradient option.
First make a selection of the brain Maybe Layer -> Transparency -> Alpha to Selection
Then in the layer mask, a shaped gradient which follows the selection boundary Then adjust the mid-point.

   

[T-buch]

Thanks
I am very sorry to tell. that I was inaccurate in my explanation of what I wanted to do. In fact, my explanation turned out to be the exact opposite of what I actually wanted to do
It's out to the edges that I want low opacity and in the middle: 100% opacity
This example gives a better impression. Thanks for your help rich2005, it actually got me on the right track
I still think the best way to do it is with the Free select tool. But I might be more family with the gradient tool

   

[rich2005]

Oh dear... I had a tidy up and deleted my example images,   ...however The problem is your specification of 36 % opacity

You can use a selection and feather the edges . Make the selection then apply Select -> Feather which allows for a large value when required. You can then cut the selection which goes from 100 % to some amount at the edges.

For me that means those two layers and a layer mask to get that 36 layer opacity.

Best way, well another way. Two layers 100 and 36  White layer mask and a large fuzzy brush. In the layer mask, carefully paint around the edges with black. Black = transparent The fuzzy brush applies partial transparency. Center white is solid.

   

[T-buch]

Sorry to keep you busy.
36% is not a holy number to me. The idea is to find a way to be able to control the transparency in the best way between levels of opacities - which gradually merge into each other. That means having the best control and not leaving too much to chance

So it is:
- Out in a user-DEFINED edge area: Almost full transparency
- In a USER-DEFINED middle area where the opacity is at... let's stick with 36%
- and again a user-DEFINED solid area in the middle

The problem with the "Free Select" tool is that I don't really know where it starts to turn down the opacity. when I click delete

It could be good if it first started at the edge that I define with it

...
I don't answer right away - I have a son who nags about a game of chess and I can't very well ignore that

[Ofnuts]

(11-05-2023, 12:37 PM)T-buch Wrote: The problem with the "Free Select" tool is that I don't really know where it starts to turn down the opacity. when I click delete

• With the standard anti-aliased selection, there is a is one-pixel band where the opacity can be partial after you delete (this is what makes a smooth edge)
  
• Without anti-aliasing (or if you "sharpen" the selection) all pixels completely inside the selection are completely deleted, and all those outside are left unchanged.
  
• With a "feathered" selection you get a wide band of pixels (as define by the feathering) that are progressively more transparent after deletion.
  

[T-buch]

(11-05-2023, 08:56 AM)rich2005 Wrote: ...You can then cut the selection which goes from 100 % to some amount at the edges.....

I'm not quite sure I fully understand this. Would you mind trying to rephrase this sentence?

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(11-05-2023, 11:26 PM)Ofnuts Wrote:

(11-05-2023, 12:37 PM)T-buch Wrote: The problem with the "Free Select" tool is that I don't really know where it starts to turn down the opacity. when I click delete

• With a "feathered" selection you get a wide band of pixels (as define by the feathering) that are progressively more transparent after deletion.
  

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

[Ofnuts]

(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.

   

[T-buch]

(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 ?

[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