Most Photoshop techniques make heavy use of "layer effects" when it comes to creating glassy buttons. I modified one of those techniques so that it could be applied to the Gimp environment that lacks this feature. The following tutorial shows you how to create a glassy button in 5 easy steps.
The result:
1) First, open a new document in the Gimp and create a basic button shape on a new layer. One way of creating a button shape is shown in the following image. When it is done fill it with medium gray.
2) Duplicate the button shape layer 2 times. Name the layers as shown below.
3) Create a selection similar to the one in the following picture; while still selected make the top layer as your active layer and clear the selection (Edit>Clear).
4) Next, fill the top and the bottom layers with gradient to look similar to the image below. If you want you can blur the top layer a bit (1 or 2 pixels) to make the gloss line smoother.
5) And lastly, load the selection of the base shape, go to Select>Shrink and shrink the selection about 2 pixels; invert the selection, activate the bottom and the top layers in subsequent steps and clear the selection. This creates an edge around the button.
And this basically completes the tutorial. If you want you can add text to the button as I did but this is not absolutely necessary.
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