After extraction you can switch between image and background view in order to check the result. If you are satisfied you can thus apply the correction (subraction or division). The subtraction operation is very different from the simple operation used for the darks and bias/offsets. Indeed, as the level of the background is identical to the one of the image, applying a simple subtraction would result in a image with a lot of black pixels. To avoid any inconvenience, the operation is done with 32bits signed integers and finally converted into native format (16-bits unsigned).