Radial transparency fade

[Ofnuts]

(07-17-2020, 05:30 AM)SparkyNZ Wrote: I would like to create a layer from the below image that goes from full opacity in the centre of the clock to transparent at a certain radius. I guess this is a sort of "radial transparency fade selection". So I would like a circular image which is 100% opaque in the centre, and completely transparent around the cirumference.

How would I go about doing this in GIMP? I've tried using the Gradient tool on a white transparency mask, a FG to Transparent gradient but I am getting absolutely nowhere.

Can somebody please help me or point me to a good tutorial that does this?

Use a layer mask:

• Layer>Mask>Add layer mask, initialize to white. From that point on you are editing the mask
  
• Draw a white(center)-to-black radial gradient
  

Note that the darkness of the gradient is really a measure of how far you are from the center. Using Curves/Levels on that gradient on the mask you can alter the gradient and define the radius-opacity relationship:

   

In the screenshot above (red layer added as a background), the vertical line at x=177 is obtained by clicking on the rim of the clock. Anything to its right is the clock, and anything to its left is outside the clock (the further to the left, the farther from the clock). With  Curves, it is easy to change the gradient to hold the opacity while being in the clock dial  and fall off more quickly outside the dial.