4.10. Separating an Object From Its Background

4.10. Separating an Object From Its Background
	4. Common Tasks

Figura 3.39. Object with Background

Sometimes you need to separate the subject of an image from its background. You may want to have the subject on a flat color, or keep the background transparent so you can use it on an existing background, or any other thing you have in mind. To do this, you must first use GIMP's selection tools to draw a selection around your subject. This is not an easy task, and selecting the correct tool is crucial. You have several tools to accomplish this.

The “Free Select Tool” allows you to draw a border using either freehand or straight lines. Use this when the subject has a relatively simple shape. Read more about this tool here: Secció 2.4, “Free Selection (Lasso)”

Figura 3.40. Free Select Tool

The “Intelligent Scissors Select Tool” lets you select a freehand border and uses edge-recognition algorithms to better fit the border around the object. Use this when the subject is complex but distinct enough against its current background. Read more about this tool here: Secció 2.7, “Intelligent Scissors”

Figura 3.41. Intelligent Scissors Select Tool

The “Foreground Select Tool” lets you mark areas as “Foreground” or “Background” and refines the selection automatically. Read more about this tool here: Secció 2.8, “Selecció del primer pla”

Figura 3.42. Foreground Select Tool

Once you have selected your subject successfully, use Select → Invert. Now, instead of the subject, the background is selected. What you do now depends on what you intended to do with the background.

To fill the background with a single color:

Click the foreground color swatch (the top left of the two overlapping colored rectangles) in the toolbox and select the desired color. Next, use Secció 3.4, “Cubell de pintura” to replace the background with your chosen color.

Figura 3.43. Result of Adding a Plain Color Background
To make a transparent background:

Use Layer → Transparence → Add Alpha Channel to add an alpha channel. Next, use Edit Clear or hit the Del key on the keyboard to remove the background. Please note that only a small subset of file formats support transparent areas. Your best bet is to save your image as PNG.

Figura 3.44. Result of Adding a Transparent Background
To make a black-and-white background while keeping the subject in color:

Use Colors → Desaturate. In the dialog that opens, cycle between the modes and select the best-looking one, then click OK.

Figura 3.45. Result of Desaturating the Background