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Gimp Selection Tools Tutorial

Selections in Gimp are extremely important, the basics of photo editing.

Italian Web master: Bad English, but good tutorials.

This tutorial concerns the selections with Gimp and its tools, and has been written for beginner users who doesn't know what are the masks or doesn't know how to use the selections.
First of all: What are the selections (also known as masks)?
The selections allows you to work only on a part of the whole picture, so you will be able to make selective corrections, modify only the selected part of an image or create a copy of a portion of the photograph.
To begin, let's discover the different tools that are provided by Gimp.
Each mask must be drawn with left mouse button, generally held down.
To delete a selection you can click anywhere out of the mask, or use Ctrl+A Shortcut.

rectangle select tool

Rectangle Select Tool: The basic selection, the rectangle. It can be used, to create a frame around a photo. To draw a selection, you must press and hold left mouse button, then move the cursor. Release the button when the section is right placed to confirm. You can change the aspect of the selection, by moving the small squares in each corner of the mask.

rectangle mask Gimp
ellipse select tool

Ellipse Select Tool: Is really similar to the Rectangle Selection, but in this case you will draw a circle/ellipse. Is useful to quickly select faces, irises or to create a vignette.
You can use Ctrl and Shift buttons exactly as before, and you can change the aspect by clicking on the squares along the vertices.

ellipse mask
free select tool

Free Select Tool: Allows the creation of complex masks. Hold down mouse button to draw like a pencil, or make consecutive clicks to create "Control points", connected by lines across them. Confirm the selection with a double click, or close the mask clicking the first point you drawn. Use the Ctrl key on your keyboard to "fix the angle".

free mask
fuzzy select tool

Fuzzy Select Tool: Also known as magic wand, allows you to select all the similar colors in a defined area. When you select the fuzzy selection, a Threshold slider will appear in the lower part of the Gimp's toolbox.
You can set an higher Threshold value if you want to select more similar colours.

magic wand of Gimp
select by color tool

Select by Color Tool: is really similar to the fuzzy selection, the only difference is that the Select by Color doesn't consider only the specific area you click, but the whole image. Also in this case there is a Threshold value.

color selection
scissors select tool

Scissors Select Tool: The scissors are an unusual tool, they are really similar to the Free Select Tool, but the shape will try to follow the edge of the subject.
I'm sorry, this explanation sucks :D

Below you can see a Youtube video made by us, that will teach you about selection in a better way. There's nothing more than the thing described above, but in video instead written.

Watch my Gimp's Masks video tutorial:

One you will be confortable with Gimp's Selections, the next step is learn how to add or subtract different selections.
Each time you choose a select tool, in the lower part of the toolbox you will see four icons like the following:

selection modes

I think that the icons are self-explanatory, anyway I will write a couple of lines for a better understanding:
- First one, Replace (No key): When you start a new selection the old one will be deleted.
- Second one, Add (Shift key): The second selection will add to the first.
- Third one, Subtract (Ctrl key): The second selection will be subtracted to the first one.
- Fourth One, Intersect (Ctrl+Shift keys): Simply intersect the first and the second selection.

Continues after the advertising...

Toggle Quick Mask:

This is a fast way to show the aspect of a selection of Gimp. You can also use Ctrl+Q shortcut.
When you toggle in quick mask mode you will see a red area (the unselected part), you can use Paintbrush and Eraser tools to handdraw the selections. You can also apply some filters to the selection (Gaussian Blur, antialisng and many others).

Gimp's Quick mask

Another way to edit the selection is the Select menu, where you can Shrink, grow, distort and so on...

Select window

The next example is a selection of a robot, in order to change the background. As first step, use the Scissors and selected approximately the shape. Click any part of the contour, then continue to click following the border, if the scissors choose a wrong path, add a control point until the selection is complete, but rough. When you have finished (you must click the first point to close the path) click anywhere in the middle of the scissors path to transform it in a selection.

01

Now use Free selection tool to correct the little details, like minor details of the border and all the inner areas. Before start to draw the selection, use Shift and Ctrl keys to add or subtract the new selection to the old one, the result must be similar to the next:

02

Now the selection is complete, to get a smoother result I suggest you to reduce the selection of 1/2 pixels (Select - Reduce) and feather it a little bit, about 1/2 pixels (Selection - Feather).
If your image has a small resolution choose 1 pixel, if the resolution is large (for example 20 megapixels) you can choose even 3 pixels. This last step is necessary to give an "antialiasing" effect to the border.
Now you can copy the robot (Ctrl+C) and paste it in a new background (Ctrl+V). This is the result:

03

I enhanced the contrast with the curves, and I changed the color balance for a better result.

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English version will be posted in English.
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