Romero and Carmack spent the early stage of development focusing on engine features instead of the game concept. "Wolfenstein" had required levels to be a flat plane, with walls at the same height and at right angles; while the "Doom" world was still a variation on a flat plane, in that two traversable areas could not be on top of each other, it could have walls and floors at any angle or height, allowing greater level design variety. The fading visibility in "ShadowCaster" was improved by adjusting the color palette by distance, darkening far surfaces and creating a grimmer, more realistic appearance. This concept was also used for the lighting system: rather than calculating how light traveled from light sources to surfaces using ray tracing, the engine calculates the "light level" of a section of a level, which can be as small as a single stair step, based on its distance from light sources. It then darkens the color palette of that section's surface textures accordingly. Romero used the map editing tool he developed to build grandiose areas with these new possibilities, and came up with new ways to use Carmack's lighting engine such as strobe lights. He also programmed engine features such as switches and movable stairs and platforms.