The Rectangle Select Tool in GIMP is a lot more powerful than it first appears.
We will start by selecting the Rectangle Select Tool, which is conveniently the first icon in the toolbox.
Most options must be set before selecting a rectangle.
First, we will feather the edges.
The smaller the radius, the sharper the edges will look. For this example, I am using a radius of 10 pixels.
We will also be applying rounded corners to aid in the look and feel.
Once again, the smaller the radius, the more the corners will look like, well, corners. A radius of zero will give sharp corners. This can be adjusted after making the selection.
It is now time to create the selection. Prior to making the selection, I’ve created a new, layer with a transparent background.
Create the selection by starting at one corner and dragging to the corner diagonal from the start point.
At this point, you will see the rounded corners, and if you’ve selected a huge feathering radius, you will also see that.
Now that we have the selection, grab the Blend Tool.
Be sure to select both a foreground and a background color, as both will be used in the blend. Also, while not needed, I’ve applied a bit of transparency to the blend. This can be useful if you plan on overlaying the final result on top of something else. Everything else is set to the default values.
This next part I wasn’t able to get a screenshot of, but to make the blend, click in the upper region of the selection, and drag to the lower region, it may take a time or two to get it just how you like it. In order to keep the blend perfectly straight, hold the Crtl key down while dragging. This is what it will look like just after the blend is applied:
To see the final product of your hard work, deselect (Ctrl+Shift+a).
If you remembered to put it on it’s own transparent layer, you can turn off the background layer (which is currently why the background is white).
With a little bit of tweaking to this technique, you can do quite a bit: