Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 197

Figure 3.23
Selecting image data and depositing it precisely. The selection point is marked
yellow and the first spot to place the cloned spot is enhanced in red.
In this example, place it on the corner of the perspective surface. This should
be at an intersection of the façade's raster. The mouse pointer is a circle with a
white arrow depicting the center point, which helps you spot more precisely. Just
as with the Clone tool , you select a point by holding the Ctrl key and clicking the
left mouse button at the same time. After selecting a spot, let go of the Ctrl key.
Next, search for a corresponding spot in the façade's grid where you want to
insert your data. Click and hold your left mouse button and paint over the surface
where you want to insert the image data. The brush will paint it into the image.
If the perspective setting was precise, you can simply paint over the
entire surface. However, if you seem to be getting an irregular pattern while