Sector_SetPlaneReflection
From ZDoom Wiki
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159:Sector_SetPlaneReflection(tag, floor, ceiling)
tag: Tag of affected sectors floor: Amount of relectiveness of the floor. 0 means none and 255 means fully reflective. ceiling: Amount of relectiveness of the ceiling. 0 means none and 255 means fully reflective.
Makes the tagged sector floors and/or ceilings reflective like mirrors. If used improperly this can reduce the frame rate quite drastically. You should keep the amount of reflective planes as small as possible.

