Linedef type 221, Intangible, Translucent, is a type of translucent floor over floor platform which is not solid. It can be passed through as though it doesn't exist. This type of FOF is good for creating intangible clouds or thin layers of mist which hug the ground.
Setup
The flats, floor height, and ceiling height of the FOF are determined by the flats, floor height, and ceiling height of the control sector, and the sidedefs of the FOF's walls are determined by the sidedef of the control linedef.
The alpha value of the FOF's translucency can be changed by setting the control linedef's front side upper texture to the number symbol (#), followed by a three-digit decimal number, from 000 to 255. #000 is completely invisible, whereas #255 is completely opaque. Note that in the Software renderer, there are actually only eleven different values: #000–#011, #012–#037, #038–#063, #064–#088, #089–#114, #115–#139, #140–#165, #166–#191, #192–#216, #217–#242, #243–#255, where #000-#011 is invisible, and #243–#255 is opaque. The advantage of using the completely opaque values is the ability to properly render flats which use transparent pixels. If no value is set in the front side upper texture, however, the default alpha value of #128 will be used.
If Flag [6] / Not Climbable is checked on the control linedef, the FOF will cast a shadow. In this situation, the light level of the control sector will determine the brightness of the shadow cast by the FOF. If the light level of the control sector is lower than that of the target sector, then it will cast a dark shadow. If the light level of the control sector is higher than that of the target sector, then the FOF will illuminate the area below it. If the light level of the control sector is equal to that of the target sector, then the FOF will not affect the light of the area below it.
Linedef special info
Floor height
Height of bottom of FOF
Ceiling height
Height of top of FOF
Sector light level
Brightness of shadow cast (If [6] Not Climbable is checked)
Front upper texture
Alpha value of FOF's translucency, from #000 to #255 (#000 is completely invisible, #255 is completely opaque, #128 is default)