hpr-knowledge-base/hpr_transcripts/hpr3140.txt

Episode: 3140
Title: HPR3140: GIMP: Selection Tools
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3140/hpr3140.mp3
Transcribed: 2025-10-24 17:40:23

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This is Hacker Public Radio Episode 3140 for Friday, 14 August 2020. Today's show is entitled,
Gimp Selection Tools. It is hosted by Ahuka and is about 16 minutes long
and carries a clean flag. The summary is,
the key to using Gimp effectively is to select the area you want to work on.
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Hello, this is Ahuka, welcoming you to Hacker Public Radio and another exciting episode
in my new series on the Gimp. What I want to do today is talk about the selection tools that are available.
We looked at the canvas last time. The canvas is generally in the center of the screen if you have it as a single large window.
On the left hand side is the tools menu or the sidebar, tools sidebar.
There are a number of selection tools and that's what I want to talk about right now.
We kind of mentioned them in our briefly in our overview, but now let's start digging a little more deeply.
Now, when you take a look at the sidebar, there's a number of different icons for various tools.
But one of the things that you might wonder is, are these the only tools available?
Well, they're most of the tools, they're not all of them. Gimp has a toolbox and if you go to edit,
the edit menu, then to preferences, then to toolbox, you can add or remove tools as needed.
So you can customize the Gimp setup to fit how you do things.
Any tool that has an eyeball icon next to it is displayed in the sidebar.
And if you click on the tool, if it already has the eyeball icon, it'll remove it and take it off of the sidebar.
And if it doesn't, when you click on it, it'll add the eyeball and then it will be on the sidebar.
Now, you can also access any tool at any time by going to the tools menu.
And that's true for all of the tools that we're going to look at.
And that's probably makes sense for tools that you don't use all that often.
If you try and have everything displayed as icons on the desktop, it can get a little bit difficult to navigate.
So you can customize this to suit your purposes.
Now, for what I'm going to talk about, I'm sticking with the defaults.
So I'm going to cover what you would see if you just installed Gimp from the website or from your repository.
Now, tools, of course, are 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.
So a good thing to do that is to start practicing and learning each of these things one area at a time.
So what I suggest is take a simple image that you can practice on.
So I recommend one that has a background color, probably white, is going to be easiest to work with all of these things.
And to do that, go to the File menu and select New, and you'll get a dialog box for create a new image.
Now, the first thing you're going to see there is the image size.
So if you have a large monitor like I do, I created one that was 1920 by 1080.
And then next to that is a little drop down, you can select what units you're using.
And for this, I'm using pixels.
So it's 1920 pixels width and 1080 pixels height.
Then if you take a look at the advanced options, this little drop down, you can click and opens that up.
And you would probably want to do that.
So now the resolution, I have mindset at 72 pixels per inch, which is fine for computer screens.
Now, if you're going to do something to be printed, I would probably change from pixels per inch to dots per inch and increase the resolution to 300 for both the X and Y.
In different media, require different choices, in other words.
Now, you can access this by using templates to get different print sizes.
And that might be handy in certain situations.
The color space can be left at RGB, red, green, blue.
And I think it's going to be fine for any image you're going to be using on a computer screen, where you've got all that color space.
Again, right now does not have CMYK color.
But if you're going to be printing in black and white, then you might as well just go to grayscale.
Now, you have four choices for the fill, foreground color, background color, white and transparency.
Now, this is the canvas on top of which you'll be working.
So I'm going to suggest white, because we want to make it easy to see what we do as we go to work here.
Now, there are selection tools right across the top.
And the first one we see is rectangle.
And then it ellipse and then free select.
Now, they're not the only selection tools, but they're probably the ones you use most often.
And you use them frequently for a variety of reasons.
First, cropping an image.
Second, you can use a 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.
Now, let's start with the rectangle.
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 upon 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.
So you're replacing one rectangle with another, kind of makes sense.
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 what is referred to as the marching ants outline.
And that is kind of a dashed line surrounding the selection, but the dashes are moving slowly.
So you can sort of see what they call marching ants. It kind of looks like that.
Now, if you go to subtract, that means any area that overlaps between the first rectangle and the second one will be removed from the first rectangle for your selection.
And if you select intersect, then only the areas that overlap will stay there 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'll get an added rectangle.
Similarly, the control key can be used to subtract, and shift plus control gives you 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 the keyboard key while drawing the second one.
If you practice this a bit, you'll pick it up quickly.
Now, without going into every option available, let's cover a few more that are useful, and many of these are applicable to all of the selection tools.
Normally, when you click and drag, the point where you click becomes one corner of the rectangle, and as you drag your cursor, it becomes 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're correcting an image, such as a button on a website.
Normally, these are slightly rounded, and this will let you do that.
Now, if you want a square, simply hold down the shift key while drawing a rectangle, or go to the options area and select fixed, and enter an aspect ratio of one to one.
Now, you may look at this and wonder about the shift key, because we already said it would be the keyboard key for add to the current selection.
Well, if you're drawing the first rectangle, it makes it a square, but once it 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?
Well, that's where using the box for fixed comes through for you. You can use it to make it a square, and use the keyboard for add, subtract, and so on.
Now, ellipse select. Very similar to rectangle select. You have the same four modes, replace, add, subtract, and 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 hold down the shift key while drawing the ellipse, or select fixed and have a one to one aspect, you will get a perfect circle.
Once you've mastered, the rectangle select, the ellipse select is something you pick up very quickly.
Now, 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 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 does work somewhat better if you have a tablet and stylus, but if you're very patient and finicky, you can make it work.
A fuzzy select lets you select a contiguous region of an image. Contiguous means these are regions that are touching each other. They're connected.
And it does it based on color similarity. It can even jump small distances in the color spectrum, controlled by the threshold setting to select additional areas.
So, an example of how that might work is you might be selecting a shirt that a person is wearing.
Well, you know, shirts tend to have folds and small differences in the intensity of the color because of shadows and what have you.
So, you can set the threshold select threshold setting on this to the point where you can select the entire shirt.
Now, it does have some drawbacks. Even though it is a magic wand icon, it's not really a magic tool.
One that a little easier is, but similar as the select by color tool, the main difference being it does not care whether the regions are contiguous.
It simply is based on color similarity. Now, this also has all of the annoyance of the fuzzy select tool.
For example, in the photo I picked out when I was experimenting, I selected the jacket I was wearing, which happened to be a brown jacket.
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 these things simultaneously.
Now, scissors select is very useful if there are well-defined borders between regions.
And this is probably the one you would use most often. It lets you click around an object to set control points.
And every time you click the mouse, you add a control point and it connects back to the ones you've already created.
So you go around the object, outlining it with these control points until you get all the way back to the beginning.
And what it will do is the software will look for the boundary. It uses algorithms to take a look at the various pixels.
And that's why it says if there's well-defined borders, this works well because if the borders are very fuzzy, it's going to be very hard to figure out what's going on.
Now, this is used very frequently to cut something out of an image, hence the scissors. You select and then you cut.
Now, another one is foreground select. Forground 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.
I will probably get into this tool in more detail later. It can be handy with some sort of photographic things that you might want to work on.
So, in summary, select tools are an important part of your tool kit. When you select a region in an image, you can cut it out, crop it, add colors, create masks, and so on.
Now, by themselves, the select tools don't do anything useful, but they are a great help in making use of other features of GIMP.
Now, as I said in the introduction to the 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.
So, with that, this is Huka for Hacker Public Radio, and I'm going to sign off, and as always, wish you to support Free Software. Bye-bye!
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