ReplaceBackground

Replacing a Background with The GIMP

Before	After

The Procedure

My original image, loaded into The GIMP.
You will need to make a layer mask for the part(s) of the image that you want to replace with a new background.Here is the mask that I created for this image in my simulating shallow depth of field tutorial. You might want to go off and read that now if you need help creating one.
Open the Layers dialog and make sure that the original image is listed in the drop-down box. Right-click on the Background layer and select New Layer (there is also a button for this in the bottom button bar of the Layers dialog ().Here I named the new layer “New Bg”. It doesn’t matter what you choose for the fill. Now right-click on the New Bg layer and select “Add Layer Mask”. In the Add Mask Options dialog, select white (Full Opacity) and click OK.
Go to the image window containing your prepared layer mask. Select All and Copy (Ctrl+A then Ctrl+C). Now back to the working image window and Paste (Ctrl+V). In the Layers dialog, anchor the pasted image into the layer mask.If your mask is like mine, you’ll need to invert it (Image/Colors/Invert) so that the part corresponding to the subject is black. Also, if you haven’t blurred the mask slightly yet, apply a gaussian blur (Filters/Blur/IIR Gaussian Blur) with a radius of a few pixels. In your image window you should now see only the isolated subject, surrounded by the fill of the New Bg layer.
Now it is simply a matter of pasting your new background into the New Bg layer, or creating a new one. Here I’ll show you how to mimic a background you see in a lot of portraits.In the GIMP toolbox, set the foreground and background color swatches to the light and dark end colors for a gradient background. You set the color by clicking on a swatch icon and using the color editor to select a color. Alternatively, you can open the Palette dialog, choose a color palette and pick two colors from that.
In the GIMP toolbox, double-click the Blend (gradient) tool () to bring up the Tool Options dialog. Choose Gradient type “Radial”.In the Layers dialog, click on the image thumbnail for the New Bg layer to select it instead of its layer mask. Go back to the image window, click down in the center of the subject and drag out towards an edge. A gradient should fill the background. If you don’t like the effect, Undo (Ctrl+Z) and try again or change your colors.
Consider adding some fog.Here I duplicated the New Bg layer so I have the same layer mask and the fog only shows up in the background. For fog I simply ran a Filter/Render/Plasma, desaturated it, put the layer in Overlay mode and reduced the layer transparency to about half.

Tips

• The length of the line that you draw with the gradient fill tool determines how “graduated” the background will be. If you want a sharp delineation, draw a short line; if you want a smooth, graduated change, draw a long line. You will need to experiment to get a feel for it.
• Experiment with the gradient fill tool options, such as Opacity and/or Mode. You can also choose gradient fills other than Radial to get different patterns (e.g. Linear, etc.).
• You can combine this technique with others on this site, e.g. sepia toning (note: different gradient, different fog).
  

  Sepia toned version.