To simulate the real world we must reproduce the quality of light visible to the observer. The visible spectrum has a much higher dynamic range than the display devices we use to view our simulation. That means we have to start thinking about exposure and how to change it dynamically.
Colour Pass: Indexed Maps rendered Fulbright.
Ambient Pass: Currently baked in as vertex colours.
Diffuse Pass: Light and shadow casting on Active objects only. Balance currently in Vertex colours with cut-in shadows
Depth Pass: Pre-existing channel for depthcue and focus.
Exposure for Shade: If both Ambient and Diffuse light contributions make full use of their dynamic ranges, when they are added, they total greater than our displayable range. The brightest areas overexpose, but details in shadow areas are clearly visible.
Exposure for Sun: Fading down the ambient contribution reduces visible detail in the darkest areas, but the sunlit portion of the image gains detail as its exposure becomes more balanced.
Auto Exposure: With these two extremes available, the balance of each can be driven by the average intensity of the Diffuse pass (for s single crude metering approach).