6. The Tools in the Toolbox / 6.3 Tools for Image Layers
The Copy Brush is a quite interesting tool. It allows copying an arbitrary part of a picture to another place or to another picture window.
The Tool Settings allow choosing, whether the distance between source and destination will stay the same, even if the mouse button is not pressed. If Preview Copy is turned on, you will see a preview of the copy result while moving the mouse.
Source controls whether the source data are fetched from the source layer, from all layers up to the active one or from all visible layers.
Additional you can scale and rotate the copy data. A click on Reset will clear this transformation.
To use the copy brush, you have to do the following:
Tip: In the channel settings of the Copy Brush, there is the blend mode "Replace". This copies all color channels, including transparency. Note, however, that it is only available when Source is set to Active Layer.