A method for generating a multi-frame image rendering of a challenge-response test on a display is presented. The method begins by identifying a pattern with graphical elements, and a display region for rendering an entry object of the multi-frame image of the challenge-response test. Then a foreground sampling window having a non-patterned area defined by the entry object is defined. The foreground sampling window captures graphical elements of the pattern along a first path. In addition, a background sampling window that captures graphical elements of the pattern along a second path is defined. The foreground sampling window is overlaid on the background sampling window at the display region of the display, such that the entry object of the challenge-response test is discernable from the pattern during a period of time when multiple frames are rendered.