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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Select a rectangular region of the image with the Rectangle Select Tool.
Click the Eraser Tool. It looks like a pink eraser.
In the Tool Options pane, change the Opacity to 50%.
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.
In the Tool Options pane, change the Size to make it large enough to completely overlap the entire selected rectangular region.
In the Tool Options pane, change the Dynamics to Basic Simple.
Click the selected rectangular region once to erase it to 50% opacity with the Eraser Tool.
Here is another method to make a selection semi-transparent.
- Choose Layer > Transparency > Add Alpha Channel if not yet done.
- Select your area.
- Choose Select > Float (ShiftCtrlL) for creating a floating selection layer.
In the "Layers" tab from the layers menu (CtrlL) choose the "Floating Selection", leave it Normal and set desired opacity.
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.