We reported in 1993 that luminance variations could distort the depicted scene and influence the accuracy of judgment of spatial layout in computer-generated imagery. Since that time, there have been reports that suggest that it is luminance contrast, rather than luminance per se, that functions as a depth cue (based on the argument that a negative luminance-contrast gradient simulates the natural world effect of atmospheric perspective). Since a negative luminance gradient can also simulate atmospheric perspective, and neither a negative luminance gradient or a negative luminance-contrast gradient can be accurately represented with real-tim computer-generated images, we have considered the influence of each of these gradients on observers' perception of scene layouts generated by computer, in order to assess their input on the perceptions of depicted layout. Our findings suggest that both luminance gradients and luminance contrast gradients can affect judgments of spatial layout. However, the nature of their influence depends on the amount and type of other depth information available on the screen. The results are consistent with a model of depth cue combination that weights each cue in relation to the other available cues, with implications for the resolution of conflicting cues. In addition, we suggest that these is a need for convention for luminance contrast measurement of computer monitors, to enable closer comparison between research papers.