Skybox

Sky gradient

The game uses the fraction of a day (6 to 18 hours) in 0..255 range to copy 8 consecutive values from DREARY/DAYTIME.COL into indices 1..8 of the VGA palette, and uses DITHER/DITHER2.IMG to draw the sky gradient. The dither textures are 8x100 and are columns of pixels to fill the upper half of the screen.

Mountains and clouds

placeSkyboxStatics(num, template, pos, var, randomht) <-
   for i <- 0, num
      x <- rnd() mod var
      if x==0 then x<-var
      < format x as 2 decimal digits and insert them into template at pos >
      <load image>
      if randomht then
         yPos <- rnd() mod 64
         < set Y image parameter to yPos >
      endif
      angle <- rnd() mod 512 # Arena angle units
      < place the image at angle >

if currentCity.tileset==TEMPERATE then
   template <- "temp00.img"
   pos <- 4
   var <- 10
else if currentCity.tileset==DESERT then
   template <- "desert0.img"
   pos <- 6
   var <- 4
else # MOUNTAIN
   template <- "mount000.img"
   pos <- 6
   var <- 11
endif
seed <- currentCity.seed
srand(seed)
nMount <- (rnd() mod 4) + 2
placeSkyboxStatics(nMount, template, pos, var, false)

if weather==0 then # clear
   seed <- getRandomSeed() + (currentDay mod 32)
   srand(seed)
   placeSkyboxStatics(7, "cloud00.img", 5, 17, true)
endif

Morrowind

In Morrowind, the skybox contain the volcano image. It is positioned as if it was located at 132, 52 in the province coordinates. The filename is chosen from the list located @46f59, if the ArenaDistance between the settlement and the volcano is less than 150, the second element is used, otherwise third. Probably, the first element was intended to be used when the distance is less than 80, but it doesn't, probably due to the bug.

Stars

Arena draws the sky by projecting stars onto the screen, depending on diurnal sky rotation, player angle, and the latitude.

getRndCoord() <-
   d <- 0x800 + rnd() & 0x0FFF
   if (d & 2) then d <- -(d)

stars <- []
planets <- [false, false, false]

srand(0x12345679)
for i <- 0, 39
   star <- new Star
   star.x <- getRndCoord()
   star.y <- getRndCoord()
   star.z <- getRndCoord()
   s <- rnd() mod 4
   if s then # constellation
      starlist <- []
      n <- 2 + rnd() mod 4
      for j <- 0, n-1
         ss <- new SubStar
         ss.dx <- rnd() sar 9 # -64 .. 63
         ss.dy <- rnd() sar 9
         ss.color <- rnd() mod 10 + 64
         starlist.append(ss)
      star.sublist <- starlist
   else # large star
      do
         var <- rnd() mod 8
      while var>=5 and planets[var-5]
      if var>=5 then planets[var-5] <- true
      star.type <- var
   endif
   stars.append(star)

Stars are drawn either by drawing STARx.IMG at projected screen coordinates, or putting a single pixel around that point. If any color component of that point in the sky is larger than 1/4, (during sunrise/sunset?) no star is drawn there.