SetViewport `sets a viewport`. A viewport is NOT a constrained area within which graphics are drawn. A viewport is a matrix transformation from world coordinates to window coordinates. It has nothing to do with cropping the graphics operations. That's why you also have to use a cropping feature such as a scissor window. In BlitzMax SetViewport does both - it sets up a viewport transformation matrix and switches on the scissor window to clip graphics. You must have both to prevent drawing outside the area.