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Photography tips and tutorials

Selection in Digital Image Editing with GIMP Lasso Tool

June 21 2011

by Philip Northeast

Selecting objects or parts of an image are important facets of photographic editing. In the digital era photo editing software such as the GIMP makes this more accessible to photographers as a specially fitted out darkroom is not required, just an everyday computer.

The GIMP is free photo editing program offering a number of selecting tools, but one of the most useful is the Free Select, or Lasso tool. One advantage of the Free Select tool is the degree of control the photographer has in precisely positioning the edges of the selection.

Selecting Objects

Selecting a particular object, or person, is useful in a number of areas including constructing composite images or montages, and placing the object on a different background.

Selecting Areas

Applying adjustments to part of an image, or selective editing, opens up a number of possibilities. Selective editing allows the full array of digital image editing tools to be applied to only parts of the image.

Lasso Tool Advantages

There are a range of selection tools available in the GIMP and other photo editing programs that offer photographers more assistance in making selections. Conversely, the Lasso tool offers photographers total control over placing the anchor points for the selection outline, and a predictable path for the joining lines that create the selection outline.

corvette outlined

Corvette with completed selection shown by marching ants line

For example, in the Corvette cut out some of the shiny lower bodywork on its front left corner reflects the colour of the grass, making a green and yellow Corvette at this point. The boundary between the grass and the car becomes slightly indistinct and this confuses automatic colour contrast tools, such as the intelligent scissors. The approach in this case is to ignore the colours and use the Lasso tool to anchor the selection outline to the overall shape of the car.

Creating the selection does not alter the image, it only marks the part of the image to use in another operation.  Some techniques for isolating of the subject advocate erasing around the image, but this is hard to undo or modify whereas a selection is easier to modify and fine tune.

Using the Lasso Tool

First thing to do is to save a copy of the image in GIMP’s file format (.XCF) and use this as the working image. It protects the original and allows selections to be saved for reuse or editing.

The basic operation is the photographer marks the edge of the target object or area with a series of anchor points and the Free Select tool completes the outline by drawing a straight line between the anchor points. Once the selection is complete saving the image in the GIMPS own file format .XCF saves the selection as well. To complete the selection place an anchor point on top of the starting anchor point. The outline then changes to dashed, or marching ants, line.

For curves and complex shapes placing the anchor points closer together to accurately follow the shape of the object.

lasoo in action

Adding a new area to the exisiting selection

This is not the end, as even though the selection appears finished it can be modified.  In the tool box when the Lasso tool is active there is a graphic mode menu offering  powerful choices.


The GIMP toolbox for the Lasso tool with normal mode chosen

Photographers can edit their selection using the add and subtract modes. One digital editing workflow is to make an initial rough selection and then zoom in and adjust small areas of the selection. By adding or subtracting areas from the original achieves the desired level of accuracy in the selection. After each small section save the image file to preserve the work, it would be a terrible waste to lose the all the fine selection work and have to start again.

Keyboard Shortcuts

Zoom is + or – keys to go in or out on the numeric keypad, but the shift and +  keys on top  row of keys.

Holding down the space bar changes the active tool to a navigation tool, handy when zooming in for precise positioning  and you need to move to the next section of the object you are selecting. There is no need to hold down a mouse button, just move the mouse, or graphics pen above the tablet.

Multiple Selections

Particularly when selective editing there are often a number of related areas rather than one single area to be adjusted. With the mode set to Add these related areas can be selected even though they are not joined to other selected areas.  Typically this approach is used for applying image exposure adjustments to only  parts of the image.

Feather Edges

This controls the abruptness of the selection edges. If the Feather Edge box is not ticked then the edge of the selection has a sharp transition, pixels are either in or out of the selection. When the Feather Edge option is ticked then photographers choose a radius, or width, for the selections edges. This produces a gradual fading from the selection to the surrounding area. This gives a softer look and blends the selection in with its surrounding.

corvette black back

Corvette placed on a black background

The Next Step

The selection process is only the first part of a number of creative possibilities that are explored in future articles.


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