A display (101) is controlled by a display drive signal (S2) generated based on an input signal (S1) encoding a safety-critical quantity. A checksum (S4) is computed based on the display drive signal and is used to verify the rendering process by which the display drive signal has been produced. In order for the checksum to depend on the safety-critical quantity only, the checksum is computed based on a filtered display drive signal (S3) in which pixels with a certain value have been excluded. In embodiments of the invention, safety-noncritical quantities are represented using colors that are due to be excluded. Similarly, a checksum for verifying a given quantity can be made independent of other quantities represented in adjacent screen areas by representing the latter using excluded colors. In other embodiments, pixel values corresponding to particular pixel positions may be excluded from contributing.