What does the subtract background command do?
Removes smooth continuous backgrounds from gels and other images. Based on the a “rolling ball” algorithm described in Stanley Sternberg's article, “Biomedical Image Processing”, IEEE Computer, January 1983. Imagine a 3D surface with the pixel values of the image being the height, then a ball rolling over the back side of the surface creates the background. The current algorithm (since version 1.39f) uses an approximation of a paraboloid of rotation instead of a ball.