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27 November 2012

GIMP 2.8 Tutorial: Which Tool Should I Use to Crop a Photo? — Part 3

GIMP Casual User Series — The Ellipse Select Tool

Do you have minimal experience with GIMP, but would like to know more about the best tools to use for trimming photos? Are you interested in learning about the various options that make these tools easier to use? Would you like a method for making a decision about which tool is best for which type of job? If so, read on for an exploration of the third tool in this series— the Ellipse Select Tool.

Giraffes Cropping Grass
Giraffes Cropping Grass

For this article, as in the previous ones, I'll be using GIMP 2.8 and a photo of some giraffes, which you can download by clicking on the picture. (When you get to the full-sized image, drag it and drop it on your desktop.) It is assumed that you have some basic knowledge about GIMP, such as how to invoke it and load an image, how to navigate the menus, and how to save a file. By the end of this article you should have a good feel for using the Ellipse Select Tool, along with all of its options that are applicable to cropping photographs.

The Reference Picture

The rest of this article will refer to this image when discussing various aspects of the Ellipse Select Tool.

GIMP's Ellipse Select Tool
GIMP's Ellipse Select Tool

The Basics

Let's say you want to cut out the adult and baby in the center of the image with an oval. Select the Ellipse Select Tool [1] (shortcut E). Left-click in the image above and to the left of the center giraffe, then drag down and to the right to create a rectangle. The selection box that you created will have a "marching ants" outline in the shape of an oval and handles [2] along the edges and in the corners of the rectangular selection. If you left-click inside of a side or corner handle, you can drag it to change the size of the selection box. You can move the selection box without changing its size by left-clicking in the central area and holding the mouse button down while dragging the box around. Take note of how the cursor changes when you move the mouse into each of the various zones of the selection box. If you don't see any of the special cursors when you have the Ellipse Select Tool selected, then left-click inside the selection box to make them appear. (Left-clicking inside the selection box toggles the selection zones on and off.) When you're satisfied with your selection, choose Image > Crop to Selection [3] to crop the entire image and make the new image size the size of the area you selected. Alternatively, you can choose Layer > Crop to Selection, which will crop the image, but leave a transparent frame around it the original size of the image.

You will see that the cropped image is still a rectangle and that the oval selection is still there. Images must always be rectangles, but you can remove the part of the picture that's outside of the oval, leaving a transparent background. Choose Layer > Transparency > Add Alpha Channel [4] to make transparency possible. Then choose Select > Invert (shortcut CTRL+I) [5] to select the area outside of the oval. Press the Delete key. The result is an oval-shaped image on a transparent background. Export the image to a .png file (File > Export) [6] and look at it in a picture viewer. You should see something like the following:

The Options

Now let's take a look at the options for the Ellipse Select Tool, which are displayed in the Tool Options pane, which is usually found in the Toolbox pane. If you don't see the Toolbox pane, select Windows > New Toolbox (shortcut CTRL+B) and resize the Toolbox pane to something useful.  If you don't see the Tool Options pane, then select Windows > Dockable Dialogs > Tool Options. This will open a separate Tool Options pane. You can dock it in the Toolbox pane by grabbing the Tool Options tab inside the pane and dragging it to the lower section of the Toolbox pane.

Use CTRL+Z a few times to undo the crop you did previously, then use the Ellipse Select Tool to make a selection box again. Grab a selection handle and resize the box. Notice how the numbers change in the Size fields [7] of the Ellipse Select Tool Options. Grab the selection box in the middle and slide it around. Notice how the numbers change in the Position fields [8]. If you want to make very small adjustments to the size and position of the selection box, you may find it easier to change the numbers in the Size and Position fields rather than trying to drag the selection box with the mouse. Or, if you have specific dimensions that you want to use for cropping, enter them in the Size boxes, then grab the selection box and move it around until you like the composition. The drop-down boxes next to the Size and Position fields let you choose from among a number of units; for example, pixels, percent, or inches.

Another way to affect how the selection box size changes is to check the Expand from center option [9]. When this option is on, grabbing a handle on the side and moving it will cause the opposite side to move the same amount in the opposite direction. Grabbing a corner will cause all four sides to move in unison.

Now grab the bottom handle and slide it downward. Notice that it is not constrained by the edge of the image and will move freely outside of the image boundary. If you move any edge of the selection outside the edge of the  image, GIMP will assume the edge of the image is the boundary for cropping. This makes it easy to crop to the edge of an image, because you don't have to worry about matching the edge of the selection exactly to the edge of the image. Simply drag a little past the edge to cause the image edge to be the limit.

You can add even more control to how the selection box changes by turning on the Fixed option [10]. The drop-down list next to it lets you select Aspect ratio, Width, Height, or Size. Aspect ratio keeps the width and height of the selection box in the same proportions no matter which handle you use to change the size. You can specify a specific aspect ratio by entering one in the associated number box; for example, "3:4". Clicking on the portrait or landscape button will change to the opposite aspect ratio and flip the numbers. Note that usually the selection box over the image will not change size until you select a handle and move it.

The Fixed Width, Height, and Size options are also controlled by the number box. Width and Height are specified as single decimal numbers in any of a number of units that you can select from the drop-down list next to it. Size is specified as width x height (for example, 1000x500), is always in pixels, and can be changed to portrait or landscape mode just like the Aspect ratio option.

If you want to cut down on the distraction of the surrounding image while you're adjusting your selection, tick the Highlight box [11] to darken the area outside of your selection.

If you need some guidelines to help you line things up in your selection, you can choose from among several guides in the Guides drop-down list [12]. The Center lines option gives you a crosshair to help you center the selection box. The Rule of thirds option gives you two horizontal and two vertical guidelines that divide the selection box up into a 3x3 grid. The Rule of fifths option divides the selection box into a 5x5 grid. The Golden sections option is similar to the Rule of thirds, but the central sections are smaller than the outer sections. The Diagonal lines option shows diagonal lines coming from each corner of the selection box. If your selection box is a square, they'll form an X crosshair; otherwise, you'll see two parallel lines going along each angle.

Center Lines
Center Lines

Rule of Thirds
Rule of Thirds

Rule of Fifths
Rule of Fifths

Golden Sections
Golden Sections

Diagonal Lines
Diagonal Lines

The Antialiasing option [13], which is on by default, causes there to be a small amount of fade along the curved edges of the cropped image. With this option off, the edge transition is sharp along the curves.

To remove all selections at any time, choose Select > None (shortcut SHIFT+CTRL+A).

It is unlikely you'll be interested in using the Mode buttons, Feather edges, or Auto Shrink with simple photos, but I'll describe them here for completeness.

Along the top of the Ellipse Select Tool Options are four Mode buttons [14]: Replace the current selection, Add to the current selection, Subtract from the current selection, and Intersect with the current selection. For cropping a photo, you would normally use Replace current selection, where each time you left-click and drag, the old selection box is removed and a new one is created. Choosing Add to the current selection lets you create irregular elliptical shapes or unconnected selected areas. If you then choose Image or Layer > Crop to Selection, GIMP cuts the image down to a rectangle that encloses the outermost edges of all the selected areas. If you want to remove the parts of the image that are not selected, choose Select > Invert and hit the Delete key. The Subtract from the current selection option lets you remove part of an existing selection by drawing a new selection box over that part. The Intersect with the current selection option lets you draw a new selection over part of an existing selection leaving only the part common to both.

The Feather edges option [15] is generally intended for use when filling the selected area with a color, not for when cropping an image. Feathering the edges means to soften the edges of the  image so that it fades into the background instead of having an abrupt change. If this option is on when cropping, it simply adds a border around the selection which is not feathered. The size of the border is based on the selected Radius.

The Auto Shrink button [16] causes GIMP to attempt to locate a border in the active layer inside the current selection box to which it can shrink the selection box. This option works well only with isolated objects that contrast sharply with the background. Turning on the Shrink merged option causes GIMP to take into account all visible layers instead of just the currently active layer.

Continue experimenting with the Ellipse Select Tool until you feel confident in your use of it. Don't forget that you can zoom in (CTRL+Mouse Wheel Forward) to make it easier to make small adjustments to the size of the selection box. If you have any questions about this tool, please leave a comment or send me a message.

Related GIMP Articles

GIMP 2.8 Tutorial: Which Tool Should I Use to Crop a Photo? -- Part 1
GIMP 2.8 Tutorial: Which Tool Should I Use to Crop a Photo? -- Part 2
GIMP 2.8 Tutorial: Which Tool Should I Use to Crop a Photo? -- Conclusion

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