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Create transparent areas in still images with Gimp

Create transparent areas in still images with Gimp By saby - Last updated on Saturday, January 14, 2012
45305 Views :: 0 Comments :: Article Rating (Total Votes: 22)
Category: Studio 1-15 Tutorials / Edition  -  Previous tutorials
Published on Monday, May 4, 2009
Created on Sunday, March 15, 2009 6:15 AM
Introduction :

Behind this title hides in fact a whole new world of possibilities for video amateurs like us. Indeed, make some areas transparent, for example, can create a mask, a frame, a button, etc, etc. Everything is a matter of location and size of the areas of transparency within the image.

In short, you will see that this tutorial can be the basis of various tutorials already on the site, such as : Using a frame around your video, Make your own Menu-Buttons, Blend in a still photo with a video that fits in the scene, Thumbnail Buttons Without Borders by TIWA, Custom buttons by TIWA or 3D titling with proDAD Heroglyph.

Here, I propose to use a free and open source software called Gimp. For a description and a link to the official website I invite you to visit our links page- GIMP (EN).

Create a selection :

Before making a transparent area, we have to selecte this area. I'll show you that there are several ways to create this selection. Each method can be more or less adapted to the situation and what you want to do.

You would expect, but above all we need to open the photo in Gimp with File, Open. Then it is necessary to inform Gimp that you want certain areas of the image can become transparent. This additional information in an image is called alpha channel. It is in Layer, Transparency, Add Alpha Channel :

Adding alpha layer

At any time during the creation of your selection feel free to zoom in and/or zoom out on the image if necessary by pressing + or - on the keyboard.

There are seven tools, all present at the top of the Toolbox to create a selection :


1) Free select tool (the lasso) :

With the Free select tool (keyboard shortcut F), draw freely the form that you want. You just make sure to close the loop :

Hand drawing

Small tip if you want to draw a straight line : instead of trying to make a continuous route, simply set one end of the segment, then the other. You can continue to create segments and draw a polygon.

2) Selection of geometric shapes :

With the Rectangle select tool (keyboard shortcut R), or Ellipse select tool ((keyboard shortcut E), guess what ? Trace rectangles, squares, circles or ellipses :

Rectangle selection

Ellptic selection

These selections can be enlarged, distorted or displaced.

3) Smart selection (scissors) :

With Scissors select tool (keyboard shortcut I), this is no longer you who do all the work. You put points on the contour of a shape and Gimp will seek to connect these points by following the line of highest contrast.

For example if you want to select only the left wing of this small plane :


By positioning a little more than twenty points, Gimp does the rest :

Left wing

4) Increasing intelligence :

With Paths tool (keyboard shorcut B), operation is the same as above but the curve between two points can be also corrected with the mouse . There are fewer points to position. The same result as above can be obtained with a dozen points only :

Paths tool

5) Other smart selection (by color) :

Fuzzy select tool (keyboard shortcut U) and Select by color tool (keyboard shortcut Shift + O) allow both to select an area from the color or hue of the pixels that compose it. Either this sort is on the whole image or only on the neighboring pixels to that which was pointed with the mouse.

Pointing the wing left with Fuzzy select tool, Gimp selects only the wing :

Aile couleur

By the same but with the Select by color tool, two wings would have been selected because they have the same color.

6) Combination of selection :

Selection mode

Under the Toolbox, there are four ways to create a selection :

  1. Replace the current selection
    If you have already created a selection, the new selection will replace it.
  2. Add to the current selection
    If you have already created a selection, the new selection will be added.
  3. Subtract from the current selection.
    If you have already created a selection, the new selection will be subtracted.
  4. Intersect with the current selection
    Only the intersection of two selections is maintained.

7) And what about transparency ?

Now that you have become expert in selection, you will see that to make it transparent is a breeze. Just delete it. For this hit Del on the keyboard.

This is what happens in the case of the free selection tool :


All the checkerboard represents the transparent part of the image.

8) Hey, saby, you're sure to have forgotten nothing ?

You are right ! Actually, with the Eraser tool (keyboard shortcut Shift + E) located in the Toolbox :


Everything you trace becomes transparent :


That makes sense: by erasing, pixels were deleted and on the same principle as above, all that is removed becomes transparent.

9) TGA saving

Once transparency created, you must save your image in a format that can keep it. File, Save as, in the new window click Select file type, choose TarGA image, then Save twice.

I will end this by saying that there is no universal tool adapted to all situations. You will need to juggle between each tool depending on the type of area you select.

And yes, cut-out can be an art !
Semi-transparency :

Tiwa, in his tutorial Blend in a still photo with a video that fits in the scene, showed us how to use a mask a few unusual. Indeed, the mask is semi-transparent. We will see how to create this semi-transparency with Gimp.

As I don't own the original used by Tiwa, I chose a similar image :


1) Creation of the layer mask :

In contrast to the method used above, here we will create the gradient directly on the original image. For that, we will create a layer mask. Do not worry, it's absolutely painless.

Open the picture in Gimp with File, Open. If it is not already visible you must display the dockable layers window. For that, Windows, Dockable Dialogs, Layers . Or Ctrl + L on keyboard, it is faster ! Here is what should appear :

Layers window

Right-click the layer and Add Layer mask... :

Mask layer creation

In the opening window choose White (full opacity) and press Add :

Mask layer setup

2) Creation of gradient :

Here you will simply "draw" the transparency area. We will use the Blend tool. Select it in the Toolbox :

Blend tool

Or, easier, press L on keyboard.
In the blend settings zone, choose Mode = Normal, Opacity = 100, Shape = Radial (to draw a circle). Offset fixes the value of the gradient, ie the size of the semi-transparent area. Offset on 0 gives a semi-transparent zone highly diffused, Offset on 100 removes completely the semi-tranparency.

Then it only remains for us to draw the circle by pointing the center and determining its radius. All this directly with the mouse on the image :

Circle drawing

Transparency appears immediately in the form of a checkered zone more or less diffuse :

Transparent area

We finish by applying the mask. Layer, Mask, Apply Layer Mask :

Apply mask

Save in TGA and that's it !

If you understand everything you should be able to achieve this kind of video :

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