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I have a picture and I want to have one specific rectangle of this picture appear 50% opaque. The rest remains as it was. How do I do this in GIMP?

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closed as off-topic by Johannes, go-junta, Vincent Mar 9 at 12:44

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "If you're asking for help with implementation, please include what you've tried and why it didn't work with screenshots. Please edit your post with what your desired results are, what resources you referenced and why those didn't work. See this meta post for discussion and see this post on how to ask a good question." – Johannes, go-junta, Vincent
If this question can be reworded to fit the rules in the help center, please edit the question.

up vote 6 down vote accepted
  1. Select a rectangular region of the image with the Rectangle Select Tool.

  2. Click the Eraser Tool. It looks like a pink eraser.

  3. In the Tool Options pane, change the Opacity to 50%.

  4. In the Tool Options pane, change the Brush to the brush that looks like a sharp-edged black circle. The name of the brush in the Brush field will change to 2. Hardness 100.

  5. In the Tool Options pane, change the Size to make it large enough to completely overlap the entire selected rectangular region.

  6. In the Tool Options pane, change the Dynamics to Basic Simple.

  7. Click the selected rectangular region once to erase it to 50% opacity with the Eraser Tool.

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Great, it worked! How come I could not find such a clear tutorial on the web? ;-) Thanks! – tw-S Feb 27 at 11:52
    
I can't always find answers to my own questions about GIMP on the web either. That's why I signed up for Graphic Design Q&A. My first question was successfully answered by a graphics software expert less than two hours after I posted it. – karel Feb 27 at 12:21

Here is another method to make a selection semi-transparent.

  1. Choose Layer > Transparency > Add Alpha Channel if not yet done.
  2. Select your area.
  3. Choose Select > Float (ShiftCtrlL) for creating a floating selection layer.
  4. In the "Layers" tab from the layers menu (CtrlL) choose the "Floating Selection", leave it Normal and set desired opacity.

    enter image description here

Note that if you need the selected area to be 100% transparent you just have to delete the selected area after step 2. above (Edit > clear or Del. No need to create a floating selection.

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