Selection Tools

Done using GIMP 2.8 on Kubuntu Linux 18.04 LTS

Now that we have looked at the Canvas, the next logical place to go is to look at the Tools we have available. They are on the left hand side of the screen, and the default should look similar to this:

GIMP Tools Sidebar
GIMP Tools Sidebar

We have briefly mentioned them previously, but now it is time to dig a little more deeply. First thing to note is that these are not the only tools available. GIMP has a Toolbox located at Edit–>Preferences–>Toolbox where you add or remove tools as needed.

GIMP Toolbox
GIMP Toolbox

Any tool with the Eyeball icon next to it is displayed on the Sidebar, and you can add or remove just by clicking the tool. You can also access any tool at any time by going to the Tools menu, which is useful for tools that are less often used. The Sidebar is just so you can have frequently used tools available with minimum effort, so customize this is you wish. For my purposes I will stay with the defaults for these tutorials since that is what you will see.

Tools are of course not used in isolation, they are used with other features like Layers, Paths, and Brushes, but we need to build up our knowledge one piece at a time. And a good way to practice these is to create a simple image to practice on. I recommend one that is colored, but lightly, so that you can see what is going on. To do this, go to File–>New, and you will get this dialog box:

GIMP Create New Image
GIMP Create New Image

Note that to see the Advanced Options you would need to click the dropdown. We’ll be using this kind of new image a lot so get used to this. I set my image size at 1920×1080. The resolution is set for 72 pixels per inch, which is fine for computer screens. If you were going to do something to be printed, I would change from pixels per inch to dots per inch, and increase the resolution to 300 for both X and Y. Different media require different choices, in other words. You can access this by using the Templates to get different Print sizes. The color space can be left at RGB (Red-Green-Blue). You have four choices for the fill: Foreground Color, Background Color, White, and Transparency. This is the canvas on top of which you will be working, and I am going to suggest White to make it easy to see as we go to work

Selection Tools

There are three Selection tools at the top, and they are tools you use very frequently. They are Rectangle, Ellipse, and Free Select. You use them frequently for a variety of reasons. First, you can use them to then crop an image. Second, you can use selection to constrain other operations. For instance, you can change the hue of an area,, or apply a brush stroke, or whatever, and it will only apply within the selected area. So you can see that being able to select is very powerful.

Rectangle Select

To use the Rectangle Select, just click and drag your cursor. It will draw a rectangle on the screen, and at the four corners there will be rectangles that function as handles to adjust the sizing of your rectangle. If you recall from our previous discussion, there is also a Tool Options menu that appears on the bottom of the left-hand sidebar, and displays different options depending on the tool that is selected. For the Rectangle Select, this begins with the Mode selection, which can be Replace, Add, Subtract, and Intersect. If you have Replace selected and you draw a second rectangle, the first one disappears. If you have Add selected, drawing a second one means you have two rectangles on the image, and if you need to you can add more as well. They will all stay selected, and you will see the “marching ants” outline on the screen.

Subtract means that any area that overlaps between the the first rectangle and the second one will be removed from the first rectangle for your selection, and Intersect will only leave the overlap between two rectangles as the selection. All of this is pretty straightforward, but you need to do this carefully. Draw a first rectangle on the screen, then use the keyboard to help. With your first rectangle on the screen, hold down the Shift key while you draw the second one and you will get an Added rectangle. Similarly, the Control key can be used to Subtract, and Shift+Control can be used to get an Intersect. And if you hover the mouse cursor over the mode selection buttons you will get a tool tip that reminds you what the keyboard keys are. Just remember to draw the first rectangle normally, then hold down a keyboard key while drawing the second one. If you practice this a bit you will pick it up quickly.

Without going into every option available, let’s cover a few more that are useful. Normally when you click and drag, the point where you click becomes one corner of the rectangle, and as you drag your cursor is the opposite corner. But you can choose Expand from center, and in this case, the point where you click is the center of the rectangle, and all four sides move out at once. Rounded Corners can be useful if you are creating an image such as a button on a Web site. Normally those are rounded, and this will let you do that. And if you want a square, simply hold down the Shift key while drawing a rectangle, or select Fixed and enter an aspect ratio of 1:1. Now you may look at this and wonder about the Shift key, because we already said it would be keyboard key for Add to the current selection. Well, if you are drawing the first rectangle it makes it a square, but once that is in place it turns to being Add. So what if you want to add a perfect square (or Subtract or Intersect for that matter) ? That is where using the box for Fixed comes through for you. You use that to make it a square, and use the keyboard for Add, Subtract, etc.

Ellipse Select

Much of the use of this is similar to the Rectangle Select. You have the same four modes (Replace, Add, Subtract, Intersect), and the same keyboard keys work here. Other options are similar. You can Expand from center just as with the rectangle, and if you either hold own the Shift key while drawing the ellipse, or select Fixed and have a 1:1 aspect, you will get a circle. Once you have mastered the Rectangle Select, the Ellipse Select is something you pick up very quickly.

Free Select

This one has very few options. You should probably use a photo to see how this works. Load the photo, then pick an object in the photo. Click and drag your mouse around the object like you are trying to outline it. When you have made it all the way around and back to your starting point, a little circle will appear to tell you that it is complete. Doing this with a mouse is not easy, it works better with tablet and stylus, but if you are very patient and finicky it can work. The other thing you can do is to click on the edges to set control points, which will connect as a series of straight lines. This is different from the Scissors Select (below) because you will be setting straight lines for your edges.

Fuzzy Select

This tool lets you select a contiguous region of an image based on color similarity. It can even jump small distances, controlled by the Threshhold setting, to select additional areas. You can jump larger gaps in this by increasing the Threshhold. This is something that seems wonderful at first (it even has a Magic Wand icon), but becomes annoying when you try to use it.

Select by Color Tool

This is similar to the Fuzzy Select, the main difference being that this tool does not care about regions being contiguous. It simply selects based on color similarity. This has all of the annoyance of the Fuzzy Select tool. For example, in the photo I picked out I selected the jacket I was wearing, and this tool also selected my wife’s hair and some objects on the table. I cannot imagine offhand what kind of operation I would want to do that would involve all of those simultaneously.

Scissors Select

This can be very useful if there are well-defined borders between regions. It lets you click around an object to set control points, and looks for a path containing sharp boundaries to connect those control points. It is used most frequently to cut out something from an image, hence the “Scissors”. This s different from the Free Form Select (above) because when you have the control points in place, you do not end up with straight lines for your borders, but instead you borders that follow a boundary determined by an algorithm.

Foreground Select

Foreground Select lets you separate foreground and background objects very efficiently. You click-and-drag just like you would with the Free Select tool, but you don’t need to be too precise. Just go around the object roughly, then let the algorithm take over. We will probably go into this tool in more detail later.

Summary

The Select tools are an important part of your toolkit. When you select a region in an image, you can cut it out, crop it, add colors, create masks, and so on. By themselves, the Select tools don’t do anything useful, but they are necessary to use other features of GIMP. As I said in the Introduction to this series, my main interest is in working with old photos and making them better, and that can often mean selecting an area and making some change in it.

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