The initial stages of editing a photograph usually are intended to affect the entire image, e.g. brightness, contrast, colour balance, rotation or cropping.
Once those tasks are complete, some photographs require further changes to just part of the image. In these cases the relevant part of the image must be selected before further adjustments are made. With a selection in place any further edits will apply only to the part within the selected area(s).
The GIMP has a range of selection tools. Four of them, with their set of adjustment facilities, are particularly useful for photographers. Together they will allow you to select just the area you need.
The four main tools for photographers are listed below. As with all the editing tools it will be important to set the tool options as required before using the tool. Some of the options are available only to a single tool. Others are shared with more than one, and some are common to all the selection tools.
Holding the CTRL, SHIFT and ALT keys, either singly or in various combinations, often provides a quick way of selecting the tool options. Watch the status line on the Image window for brief hints. In some cases a different effect will be obtained if the key is held after a drag is commenced.
Shown on the left is the button for the Elliptical Select Tool and its associated pointer. This selection type is useful for framing a subject where the rest of the image is to be deleted or merged with a background colour. It is commonly used with Feather edges, one of the options common to all selection tools.
On dragging with the pointer a rectangular box is drawn containing the ellipse. When dragging is complete the pointer can be moved to any of a set of boxes in the corners and along the edges of the containing rectangle. Dragging on these will resize the containing box and ellipse. Dragging in the centre will move the ellipse. Clicking in the centre will fix the selection.
In addition to the options common to all the selection tools are:
"Expand from Centre" changes the way the selection appears. Unchecked, dragging starts at the corner of a box that contains the ellipse. With the option checked, dragging starts from the centre of the ellipse. Once dragging has started pressing CTRL toggles the option.
If the "Fixed" option is set to Aspect Ratio then the ellipse can be constrained to a circle if "1:1" (without the quotes) is entered in the associated field. Other ratios can also be entered. The small portrait and landscape buttons only become active if "Size" is chosen as the fixed option. Once dragging has started pressing SHIFT toggles the option.
The "Position" and "Size" options are best used for minor corrections once a selection has been dragged out.
"Highlight" provides a mask to make it easier to see the selected area and can be useful on certain backgrounds.
The final two options are not likely to be of use in a photograph. Auto shrink attempts to find an existing ellipse in the current layer of the image and Shrink merged will search all layers for an ellipse.
Shown on the left is the button for the Free Select Tool and its associated pointer. The tool allows you to delineate any area of the image on which you wish to work. Only the Common Options, discussed below, are available.
On dragging the pointer a path is defined. On lifting the mouse button the area to be defined is completed with a straight line joining the start and end points of the path.
1. Use the Zoom tool, or magnification tool on the image window, to get in close to check that the path of the selection is accurate.
2. Don't try to complete a large selection in one go. It is much easier to tackle it bit by bit, using the add and subtract modes to complete and correct the intended selection.
3. Free select can be very effective in limiting the impact of a Colour Select. First do a rough Free Select then following it with a Colour Select in Intersect mode.
Shown on the left is the button for the Fuzzy Select Tool and its associated pointer, often known as the "Magic Wand" because of its pointer shape.
Click on the image to select a colour and all contiguous matching colours will be selected.
The most important option for this tool is "Threshold", used to control how close a colour match there has to be to for selection to take place.
Depending on circumstances, it may be worth using "Feather" edges, but generally photographers will leave the remaining options at their default settings. Select transparent areas and Sample merged only have an effect on images with several layers.
TIPS:1. If an unexpected area becomes selected, immediately use CTRL-Z to undo it and try again, clicking in a different position. If it still does not produce the desired result, adjust the threshold.
2. Areas of subtly changing colour, such as foliage in hedges, the fringes of trees or fluffy clouds, can produce a mass of small "holes" in the selection. Rather than attempt a re-select, use the grow and shrink technique.
Shown on the left is the button for the Colour Select Tool and its associated pointer. It is similar to Fuzzy Select, with all the same tool options, but this time all matching colours become selected, anywhere on the image.
TIP: It is unlikely you will want to select a colour from anywhere on a photograph, so do a Free Select first to constrain the area that the Colour Select will affect, and use Intersect mode when applying the Colour Select.
Each of the selection options includes the following three tool options:
Controls how a additional selections are handled.
NOTE: The pointer shows an additional marker to indicate which mode is in operation:
- Replace any existing selection with the new selection.
- Adds the new selection to the existing selection. (The additional selection does not need to overlap the first, as the icon might suggest.)
Hold SHIFT before dragging for a temporary switch to Add mode.
- Subtracts any overlap in the new selection from the existing selection.
Hold CTRL before dragging for a temporary switch to Subtract mode.
- Only the new area that overlaps the existing area remains selected.
Hold CTRL+SHIFT before dragging for a temporary switch to Intersect mode.
Smooths the edge of a ragged selection. By default it is turned on and should normally be left this way. Its purpose is to reduce the inevitable stepped appearance of lines that are not vertical or horizontal. (It works by applying single pixel feathering to non-vertical and non-horizontal edges.)
For photographic work you will normally want this option turned on. It smoothly blends the edge of a selection with its surrounding area. When active a further control appears. Either drag the Radius slider or use the spinwheel control to control how wide the blended area is.
The blending takes place to either side of the selection border creating a graduated change to any edit made rather than a sharp cut off exactly at the border. This dramatically reduces the chance of being able to spot where an edit has been made.
TIP: To see the effect of feathering use the "Quick Mask". (Use SHIFT-Q to toggle it on/off or click the rectangle beneath the vertical ruler on the Image window.) The Quick mask prevents editing so turn it off to make changes to the selection.
There are a number of options on the Selection menu of the Image window that can prove useful in refining a selection.
Problem: The area you wish to select will be difficult to isolate, but a surrounding area would be far simpler.
Solution: Select the simple area then then invert the selection, so what was selected is de-selected and vice versa.
This is an alternative to the Selection Tools Feather edges option. Use it in the same way.
Problem: You make a Colour or Fuzzy selection that is almost perfect, but has a large number of "holes" in it, indicated by a large area of flickering black and white "marching ants". You try increasing the threshold but this has over expanded the selection.
Solution: Grow the selection by a couple of pixels and then shrink it by the same amount. If the grown selection touches the image border ensure that the "Shrink from image border" checkbox is clear.
Growing the selection will "fill" the holes and the internal borders will disappear in the expanded selection. Shrinking the expanded selection back to its original size will return the outer border to its original position without re-introducing the holes.