Knowing how to make selections is an absolutely crucial skill when working with the GIMP. One major advantage of working with selections is that it allows us to modify parts of the image while leaving other parts unchanged - only the selected area is affected by any changes that we make, while unselected areas are protected.
Sometimes when working with the GIMP you will try to make some change and nothing will happen - perhaps now you can guess the probable cause. If part of the picture is selected and you try to change an unselected part then nothing will happen... press Shift+Ctrl+A to deselect the selection.
There are several ways of making selections in the GIMP, some easier than others. The easiest selection technique of them all is Ctrl-A, used to select All the image (as long as none of it spreads beyond the edges of the canvas). Similarly, Ctrl+Shift+A or Select>None can be used to deselect everything. Some tools are available for making selections including:
- Fuzzy Select and Select-by-color tools
- Rectangle Select and Ellipse Select tools
- Free Select, Scissors Select and Paths tools
Before making a selection you should always first decide how much 'feathering' should surround the selection. This is a little difficult to explain, for the feathering setting won't make any obvious difference to the way the selected area appears on your screen. However, when you do anything to the selected area, such as lightening it, moving it, copying it or whatever, the feathering will make a difference, for the feathering blurs the edges of the selection. Since the selection no longer has a sharp edge, parts of the picture that appear to be just outside the selected area will also be slightly affected by what you do. Ultimately, you must learn not to be misled by the 'marching ants' - they do not simply indicate the limit of the 'selected area' - rather, they indicate which parts of the image will be affected strongly by what you do while the selection is in effect.
You can apply feathering to a selection either before or after making the selection - the main difference is that if you set up feathering BEFORE making the selection then the settings will be remembered and applied to future selections made with the same tool. Feathering adjustments applied to an existing selection will not be remembered.
To set feathering before making the selection, choose a selection tool (such as the free selection tool) turn on the feathering option in the selection tool's options window then set the feathering radius. A radius of 1 or 2 pixels would give a subtle effect while a radius of 10 or more would give a very obvious effect.
To increase the feathering of an existing selection choose Select>Feather... then set the feather radius.
To remove feathering from an existing selection choose Select>Sharpen.
The best way to understand feathering is to try it for yourself. The above photograph shows a young boy chained naked to the gates of the Paris town hall (!). Open the picture in the Gimp and then choose the ellipse selection tool (see image at left). In the tool's option dialog, make sure that feathering is turned off, then make a circular selection around the boy's face. You can constrain the selection to be circular (as opposed to elliptical) by holding down the Shift key as you use the tool. If the circle is not quite in the right place then Alt+drag it (if you drag the circle you will move the boy's face with it, leaving behind a circle filled with the current background color).
Now let's make a copy of the selection to see what we've got. Sometimes you can duplicate a selection by Ctrl+Alt+dragging it - try it if you like - a surer way is to copy then paste then drag the selection. Either way, you should see that the selection has hard edges. If you look at the layers dialog (Ctrl-L) you will see that the duplicated selection is in a layer called 'floating selection' this is a temporary layer which you may lose unless you click the new layer button (see image at left) in the layer dialog (do it!).
Now let's see whether it is possible to apply feathering after a selection has been made. Make sure that the background layer is active (i.e. dark) in the layers dialog then make another circular selection just as before, without any feathering. After having made the selection, choose Select>Feather>5. Then duplicate and move the selection as before - you should see that the duplicated selection now has a soft edge.
Finally, let's try setting the feathering before we make the selection. Turn on the feathering option of the ellipse selection tool and set the feathering to 25 pixels. Make sure that the background layer is active (i.e. dark) in the layers dialog then make another circular selection just as before. Then duplicate and move the selection as before - you should see that the duplicated selection now has a very soft edge. You may now have a picture something like this:
In this image, the three duplicated heads have featherings of (from top to bottom) 0, 5 and 25.
This is about the same as using feathering set to 1 i.e. it blurs just very slightly the edge of the selection, making it less jagged. Since only a narrow edge is blurred, there is no loss of detail.
Adding to and subtracting from selections
If you ever want to add to an existing selection, hold down the Shift key while you use your selection tool (rectangle selection tool, free selection tool or whatever) - a small '+' sign will appear to confirm that you are adding. You may realise that there is a possible problem here because the Shift key is also used to constrain some selection tools e.g. the ellipse tool is constrained to make circles. But what if you want to add an ellipse, not a circle? The Gimp programmers have thought of that - just hold down Shift as you begin making the selection but release it before you release the mouse button. Or, if that sounds difficult, use the 'add to selection' option (see image at left) instead of using Shift.
To subtract from the existing selection, hold down the Alt key while you select the area to be subtracted (look for the small '-' sign) or choose the 'subtract from the current selection' option (see image at left).
The fact that you can easily add to or subtract from a selected area should be reassuring for you, for it means that if your first attempt at selecting an area is not quite right then you don't need to start all over again - just make corrections to the selection by adding or subtracting as necessary.
Now you are ready to begin learning about the Fuzzy Select and Select-by-Color tools - follow the corresponding link near the top of this page.