The RRU Gimp Tutorial

Selections

A selection is a piece of an image you have somehow selected. You can have more than one selection simultaneously; this is known as multiple selection. Selections work on all image types.

If one or more selections are in effect, GIMP operations should operate only on the selected areas. (Poorly written plug-ins may not do this correctly, but those shipped as part of the GIMP distribution should all work properly.)

TIP: Undo works on selections as it does other actions. If you make a mistake with a selection, simply press CTRL-Z or us the Edit menu to undo the last selection[s].

Selection Tools

The top two rows of buttons on the main toolbar are all selection buttons. Selecting any of these turns the cursor into a selection tool while over an image.

From left to right, then top to bottom, here are the selection functions available on the toolbar.

Select rectangular regions
Select elliptical regions
Select hand-drawn regions
Select contiguous regions
Select regions using Bezier curves
Select shapes from image

Common Options

Double clicking on a selection tool pops up the tool option menu (also available via the Dialogs->Tool Options... menu). All of the selection tools include a ``Feather'' option, which feathers, or blurs, the selection boundaries. The effects of this include adding a radius to corners and shrinking the area selected. This option always includes a slider to select the eatherings radius. Larger values produce a more radiused corner and (generally) a smaller selection.

All selection tools other than the rectangular selection tool also include an ``AntiAliasing'' option. If antialiasing is turned on, the edges of the selected region will contain intermediate values which give the appearance of a sharper, less pixelized edge.

A few of the tools have other options specific to that tool.

How Modifier Keys Affect Selection

Ordinarily, making a new selection replaces the current selection. However, modifier keys can let you add to or delete from the current selection. They can also affect the selections on other ways. Here are how the modifier keys generally affect selections:

Key	Effect
Alt	Move the selection The cursor becomes a drag tool. Press the Select Mouse Button and move it; the selection will move as the cursor does. Releasing the button anchors the selection at its new location.
Shift	Add to the selection When you make a new selection, previous selections are not replaced. The selection now consists of all previous selections plus the new one.
Control	Subtract from the selection The new selection area is subtracted from the previous selection area. If the new selection area does not overlap in any way with the previous selection area, there is no change in the selection.
Shift & Control	Take interesction of the selections If the new selection area intersects with the previous selection area, the result when the mouse button is released is that the current selection area is only that intersecting area. If there is no intersection, all selections are removed.

Some modifier buttons have further effects for certain selection types. These effects are noted under the affected selection type.

In all cases, pressing the Select Mouse Button starts the selection; releasing the button finalizes the selection. During the interim, the selection is highlighted by drawing the appropriate shape in contrasting colors. Once the selection is finalized, it is indicated by moving dashed lines, or ``marching ants''.

Using The Tools

Select rectangular regions

Pressing the Select Mouse Button anchors one corner of the rectangle. The current cursor position is used for the opposite corner of the rectangle; it follows the cursor until the Select Button is released.

The Control key modifies this procedure; the point at which the Selection Button is depressed becomes the center of the rectangular area to be selected.

The Shift key causes the selection area to be a square. In this case the cursor always indicates some point on an edge of the square, but not necessarily a corner point.

Select elliptical regions

Pressing the Select Mouse Button anchors one corner of a rectangle whose sides' center points correspond to the ellipse used to select an area. The current cursor position is used for the opposite corner of that rectangle; it follows the cursor until the Select Button is released. You never actually see a rectangle; you see the elliptical outline of the area being selected.

The Control key modifies this procedure; the point at which the Selection Button is depressed becomes the center of the elliptical area to be selected.

The Shift key causes the selection area to be a circle. In this case the cursor always indicates some point on an edge of the square containing the circle, but not necessarily a corner point.

Select hand-drawn regions

This is a free-hand method for making selections. Pressing the Select Mouse Button anchors a point, from which a line is drawn followwing the cursor. Releasing the cursor finishes the selection by drawing a straight line from the current cursor location to the starting point. This allows you to select virtually any shape, including complex shapes, figure eights, or anything else.

This is similar to the ``lasso'' function on many other programs.

Select contiguous regions

This is an ``intelligent'' selection that attempts to select areas contiguous to the cursor, based on color. From the current pixel, each adjacent pixel is checked against a threshold; if it is deemed "close enough" to the current pixel, it is also selected, and its adjacent pixels are also checked. This continues until no more pixels have been selected.

This may not be effective on busy images, especially in areas with low contrast.

This tool includes a ``Sample Merged'' option. This causes the selection to act as if all visible layers were merged, and select on that data.

Select regions using Bezier curves

This selection tool works rather differently than the others. First you press and release the Select Button where you want to start. This sets the first point. The point appears inside a small circle, which is the point's ``anchor''.

You repeat this action around the region you wish to select; each time you set a new point, the tool draws a line from the previous point to the new point. When you are ready to finish, click within the anchor of the starting point.

You aren't finished yet! So far you have a series of straight lines. Now select one of the anchors and press the Select Button again, holding it down. As you hold it, move the cursor away from the point. You will see two lines move out from the point, each with a square at the end away from the anchor. One of these is beneath the cursor. These are the ``handles'' with which you adjust the curves. As you move the handles, you will see the lines curve. Move the cursor in a circle about the anchor, at various points moving the cursor closer to the handle or farther away (still holding the Selecct Button down). It sounds more complicated than it is.

Pressing Control and the Select Button on an anchor lets you move the anchor. When you release the Select Button, the anchor will again be anchored (sic).

The Shift key allows you to move either handle independently of the other.

Let's try an example. Create a new image of size 256x256. Now select the Bezier tool, and create a triangle by clicking somewhere near the middle of the image, then near the bottom a little to the left of that anchor, then over to the right of the new anchor, then back on the first anchor. You should see a "dunce cap" shape.

Now select the top anchor. This generates handles on this anchor. Drag the mouse to the left - it looks sort of like a stick figure with arms. Notice that what was the top angle of the triangle is now a curve. Drag the handle in a circle about the anchor; the stick figure is dancing! But notice how the curves change as it dances.

Release the Select button, then select the anchor again while the Control key is held down. This moves the top anchor; moving it up and down makes your (now bow-legged) stick figure looks like it's trying to jump with its feet glued to the floor.

Release the Select button and the Control key. Press the Shift key and hold it as you select one of the handles. You can make the stick figure wave - at the same time changing only one side of the curve. Moving this handle near the other should make it look as if your stick figure is bowing.

Release the Select button and the Shift key. You may play around with the other anchors if you like.

If you need to get the handles out of the way to see better, simply click anywhere outside the shape (but not on a handle).

When you are satisfied with the shape of your selection, press the Select button inside the selected area, but not on a handle. The shape becomes a selection, as noted by the fact that the lines turn into ``marching ants''.

Select shapes from image

This is also known as ``intelligent scissors''. The cursor works as in selecting hand-drawn regions, except that after releasing the Select Button, GIMP tries to decide what you really meant to select, based on the surrounding pixel values, somewhat as with selecting contiguous regions. The selected area does not become a selection until you again press and release the Select Button over the image.

This feature is not fully developed; for instance, it does not currently work with layered images. Use this at your own risk!

Intelligent scissors has several extra options that are best understood by playing with them. The notable exception is the ``Convert to Bezier Curve'' button, which converts the selection outline to a bezier curve, and switches you from the current to to the bezier curve selection tool.

The other options are all sliders:

Curve Resolution
Edge Detect Threshold
Elasticity

Be careful; on my system setting any of these (or ``Feather Radius'' to their minimum values for imaginary scissors will crash the GIMP.

Last updated: 11 September 1998

Miles O'Neal, <meo@rru.com>

11501 Johnson Rd / Leander, TX / 78641-5823