GIMP is one of the best known open source software applications and is often called the open source equivalent of Photoshop. While it does lack some of Photoshop’s more advanced features it is, despite the name, a very powerful image editing tool.
When creating a new image or scaling an image (or anywhere else you might want to enter in an image or layer size) GIMP has the ability to use input field math.
This means, when asked for the size you can enter in a math expression and it will evaluate it.
As you can see here, I’ve entered in 1920*2 because I want to change the default new image size to be twice as wide.
As soon as I leave that input field the expression is evaluated and the result takes it’s place. This works not only on new images, but also when scaling images or layers and any other place like this.
When scaling the default is to lock the width and height values together, so an expression in one, will automatically apply itself to the other.
In the image above, the width and height are linked and I’ve entered an expression to divide by 2 to bring it down to half the current size. When I leave that field, because they are linked, it will apply that divide by 2 to the height as well.
As you can see, it’s halved both the width and height. You can of course, un-link them if you want it to apply to only one of those sizes.
Incidentally, did you know you can also do this in the 3D software Blender?