A test and measurement instrument converts digital data that represents an input signal into a plurality of bitmaps, and then subtracts one of the bitmaps from another one of the bitmaps to produce a difference bitmap. The difference bitmap does not contain density values that are common to the two bitmaps, but instead only contains the density differences between the two, thereby revealing very small density variations in the presence of large density values. In some embodiments, the difference bitmap is displayed on a display device. In other embodiments, the difference bitmap is used to generate a trigger signal.