I was looking at the 'UNDER THE BRUSH' thread by hanzz where he demonstrated his hand at caricatures and to quote ' studying the values. '

http://www2.ambientdesign.com/forums...pic.php?t=7966

I thought that being able to apply various filters to the reference image would be a real study aid. A digital squint!!

E.g. colour correction, adjust contrast, gamma, make BW, find edges etc