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I want to give the middle full opacity but let the border effect that is already existing.

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Each layer in GIMP can have the transparency for each pixel specified in two ways, that add to each other:

If you imported a PNG file with transparency, you background layer already has an alpha channel - you can keep that untouched - Just right click on the layerś thumb on the Layers Dialog ctrl + L and pick Add Layer Mask - that way a mask is created. It behaves as a grayscale image, where black means transparency and white means opacity - you can select if you are painting on the actual layer or on its maks by clicking on the respective thumbnails - Just select the mask and paint it gray (by dragging and dropping a shade of gray on the canvas, or using the bucket fill) - and you have extra, prortional transparency for the whole image, without touching the original transparency levels.

Kitten with layer mask active

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One option might be to convert a color in your image to transparency:

enter image description here

Go to Layer -> Transparency -> Color to Alpha

This way you can choose exactly what you want to be transparent by making it a certain color and then converting that color to transparency.

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Not sure what you mean with "border effect" but here is how to have a transparent senter with a feathered border:

  1. Layer > Transparency > Add Alpha Channel adds a transparent layer to the image if not yet present.

  2. Select area with border effect (here Feather).

  3. Press Del or select Edit > Clear to remove selected area leaving the transparent layer in the background.

    enter image description here

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I mean the .png I oopen in Gimp already has a good anti-aliasing effect (it's an Android star icon that I try to colorize, never though it would be that complex). I try your solution, brb – Poutrathor Nov 29 '13 at 11:55
As far as I understand, your idea is to remove the picture and to keep only the alpha channel layer? Can I add a layer over an image to increase opacity unifomly? – Poutrathor Nov 29 '13 at 12:01
Its the other way round: I removed the selection from the image to leave a transparent hole. Anything you paint on that transparent area will become the opacity of the paint tool used. So yes, you can add a layer to that image. – Takkat Nov 29 '13 at 12:09

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