Directions to Chroma key an Image with GIMP:

If you take a picture with a solid colored background (blue or green works best), then you can have a computer program pull out the blue or green pixels and replace them with another color or a transparent layer. This process is called "chroma key" and is used in weather forecasts and other special effects. The terms "green screen" and "blue screen" also refer to the chroma key process.

These directions describe how to use GIMP to add a transparent layer and remove blue or green pixels from an image. This allows you to layer images on different backgrounds or use pictures as Sprites in Scratch or Greenfoot.

Brief outline of process:

1. Start GIMP

2. Open the picture file.

3. Create an "Alpha Channel" (Transparent Pixels)

4. Select "Like Colored Pixels" (Click on the Green or Blue background)

5. Control->K to delete the pixels and reveal the Transparent background.

6. Save the picture as a .gif file. (Not .jpg!)


Detailed Directions:

1. Start GIMP: (The "Gnu Image Manipulation Program")

a. Click the "K" menu

b. Select "Graphics"

c. Select "GIMP Image Editor"

Note about the GIMP:

Most computer applications run in one window. GIMP will run different windows for each function or object you are working with. (Separate windows for paintbrush menus, images, backgrounds, etc . . .)


2. Select "File-> Open"

3. Navigate to your File Area and select a chroma key picture

4. Once your picture opens, add the "Alpha Channel"
a. Select "Layer" from the picture menu bar
b. Select “Transparency”
c. Select "Add Alpha Channel"



5. Click the "Select Region by Color" icon.

6. Click once on the green or blue color in the background.



7. Press "Control -> K" on the keyboard to delete the colors. The background will appear as a gray checkerboard pattern.



8. Repeat the click and "Control-K" process to remove the background colors.

9. Once the background colors are removed, save the picture:
a. Select "File-Save As" from the menu bar.
b. Type "LastnamePicture.gif" as the filename.

c. Select "Convert to Indexed" - Click Export

d. Click “OK”

e. the picture should save.

10. You can now use this picture in Scratch for a sprite, or in other graphics projects to combine with backgrounds.