Metuchen High School: GIMP

13. Changing a Background

6 points

You can use GIMP to change a photo's background.  Here we'll use a Layer Mask to change the background. 

Example

Here are two photos.  The one at left is of Spiderman.  The one at right is of a seashore. Our goal is to replace the background in the Spiderman image with the seashore.

• Use Open as Layers to open both images. Each should reside in its own layer. Windows > Layers will open the Layers panel.

• Use the up and down arrows at the bottom of the Layers panel to move Spiderman to the top.

• Right-click on each layer and choose Layer Boundary Size. Make sure that the layer holding the new background is at least as large as the layer holding the main image. If it isn't, use Scale Layer to increase its size.

• Right-click on the 'main' image in the top layer (here, Spiderman) and choose Add Layer Mask. Add a White mask (full opacity). The white Layer Mask is opaque -- it doesn't allow the layer below to show through.

• If you paint on a white layer mask with a black paintbrush, you remove the opacity where you paint, allowing the layer below to show through.  Click on the Paintbrush tool in the Toolbox, change the foreground color to black, and set the size of brush you want to use.  One approach would be to begin with a small brush size.  Paint, slowly and carefully, around the edge of Spiderman. Wherever you paint, the other layers shows through.

• Once you have the boundary of Spiderman painted, switch to a larger brush size and paint the rest of the background.  Paint with a series of small strokes, rather than one long stroke, so that if you make a mistake it is easy to hit CTRL-Z and undo the mistake. 

  Here is the result:

  

   
  

  Exercises

  Find an image whose background you'd like to replace.  Find a second image that you'd like to use as the new background for the first image. 

  Follow the procedure above to replace the background of the image.
   

  GIMP Links

  • docs.gimp.org/en/gimp-layer-mask-add.html

  • docs.gimp.org/en/gimp-image-combining.html

  • docs.gimp.org/en/gimp-tool-paintbrush.html

     

  Videos

  www.youtube.com/watch?v=sdAagXJ_VCo (has sound)

  www.youtube.com/watch?v=nC-5Bjykg4U (has sound, starts at around 0:20)