The designers of Gimp are at pains to tell us it is not a drawing tool. Yet it can convert a selection into an outline. So we can for example draw a rectangular selection and make an outline from it. Outlines are useful in the NZ Rail Maps aerial mosaics, where a lot of pixels can be saved by replacing base layer tiles with outlines, resulting in a smaller file size. A 4800×7200 tile drawn as an outline 3 pixels wide around the edge will only have 72000 pixels filled instead of 34.5 million, saving a lot of disk space if there are a lot of these layers in the image. The purpose of the outline is to give it the name of the base layer tile it replaces, which means when a mosaic section is exported, the sidecar files from the original base tile can be looked up and copied to the folder where the mosaic extract is stored so it can be loaded into the GIS for editing.
The steps to create an outline are simple:
- Create a new layer in an image.
- Create the selection using the selection tools
- Use the Stroke Selection command on the Edit menu to draw the outline around the edges of the selection.
A square can be drawn in a few different ways; one is to press Shift and Alt when drawing the selection.
A circle can be drawn using the Ellipse select tool. Hold down the Shift and Alt keys while dragging with the mouse and a circle will be drawn. All selections can be adjusted prior to stroking using the Tool Options tab, which displays the selection size and position in the layer canvas. The Freehand selection tool can also be used.
With any outline that has a thin border, getting the mouse pointer right over that border to be able to drag it around the canvas can be a little tricky. I draw an extra square in the top left corner (make a selection and then fill it completely with the Bucket Fill tool) to make it easier to drag.
To actually put the outline into an image, just drag the layer from a template file to the image you want it to be in.