Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 267

You've probably noticed that the image is still missing the left and top
sides of the television along with the monitor's shadow. Begin by creating
three new layers: The top layer will naturally be on the top of the stack in the
Layers dialog. Insert the shadow and side layers directly above the Background
layer.
You can use paths in Design mode with the Polygonal option set in the
tool options to produce the necessary objects on these layers.
• Start with the side layer. Create a square with a closed path that attaches
to the left side of the front part of the monitor. Create a selection from
this path. Then fill it with the same gray tone you used for the front layer.
• Duplicate the side layer (using the right-click menu in the Layers dialog)
and call it side shadow . Fill the selection with black this time. Reduce the
layer's opacity to about 50%.
• Create the cabinet top on the previously prepared layer. To create the
path, just follow the corner points of the surfaces. Use a very light gray
to fill the area.
Figure 3.82
Using a path and a
selection from that
path to create a cabi-
net side