Image Editing with GIMP

Image-editing programs can be so frustrating, and it seems that often the most frustrating things are the simplest.  When I can’t figure out how to do something or other, I look it up and it turns out that usually the solution was simple and I was trying too hard.  Anyway here are a few handy things to remember about GIMP.

To draw a straight line using a tool like the pencil, first click the starting point, then hold the shift key, and click the end point.

To draw a circle, use the oval-select tool.  Draw a selection, then click Edit, Stroke Selection.

To select shapes/regions from an image, use the Path tool.  Click on the border of the region you want to select and it’ll make a "node;" click again and it’ll make another connected to the first.  Do this until you have the shape/region fully enclosed -- if you want to connect the last node to the first, hold the control key and click the first node.  Then you can click Select, From Path to turn your path into a selection.  You can also save ("export") your path to a file.  And here’s one of those simple-but-nonobvious things: when you switch to another tool and your path’s nodes disappear, the way to get them to show up again is to switch back to the Path tool and then click around somewhere that you know a node is at (approximately).  This will make them all show up again.

To make the background transparent, you have to add a new layer, an "alpha" layer.  Just click Layer, Transparency, Add Alpha Channel.  Now when you use the eraser tool (or select an area then press Ctrl-k), it will erase to "transparent" instead of to the background color.  (Of course, only PNG and GIF images support transparency, or at least, they are the only popular formats that do.  JPEG and BMP do not.)

To remove red-eye from your photos, download the script from the bottom of this page and save it to your /home/user/.gimp-2.0/scripts/ directory.  Then it’ll show up in GIMP under Script-Fu, Selection, Red Eye.  All you need to do is select a small part of the red area inside the eye, then click that menu item, and it’ll do all the work for you.

After changing an image’s canvas size, you often need to click Layer, Layer to Image Size.  And if after doing some manipulation you find you can no longer draw on your image, you may need to do Select -> None, and/or Image -> Flatten Image.

There are a few more that aren’t coming to me right now, but if and when they do I will post them here.