rainbow.lua

-- A note about faking toruses:
-- Spheres under projection become circles.
-- If we treat a torus like a chain of overlapping spheres, then we can draw
-- it using just circfill :)

::_::
cls(1)

-- The camera rotation angle.
q=t()/6

-- Two passes: first for the clouds, second for the rainbow.
for m=0,1 do
  
  -- We'll use use m and c to 'remove' certail parts of expressions on each pass.
  -- (Since multiplying by 0 will 'remove' them)
  c=1-m
  
  -- The rainbow requires two passes to make sure the rendering order is correct.
  -- We want to render back to front. For the back half we want to do the outer
  -- rings first, and for the front half we want to do the inner rings first.
  for n=-1,1,2 do
    
    -- Iterate the five rings, changing direction depending on the pass.
    for j=2-n*2,2+n*2,n do
      
      -- The radius of the ring.
      -- The factor '7' was chosen since we don't want the rings to overlap
      -- (there's a subtle render order issue if you choose less than 7).
      r=22+j*7
      
      -- Iterate across the arc of the ring (a is an angle).
      for a=0,.5,.005 do
        -- The position in the Z axis.
        z=r*cos(a)*cos(q)

        -- Depth scaling! We'll divide X and Y by this factor to get perspective.
        -- Basic perspective explanation:
        -- https://twitter.com/2DArray/status/1063509842592182273
        k=1+z/150

        -- This circfill is used for drawing both the clouds and the rainbow.
        -- The clouds and the rainbow share X and Z positions.
        -- But the Y position varies for the rainbow.
        --
        -- [sgn(z)==-n]:
        -- For each pass we only want to draw one half of the arc.
        -- If z is less than zero than we're in the back half of the arc.
        -- So we can compare the sign of z to the n (-1 or +1) to do the limitting.
        --
        -- [sin(a)*c>-.4]:
        -- For the cloud, we only want to draw the outer parts.
        --
        -- This line has been expanded from a single-line-if.
        if sin(a)*c>-.4 and sgn(z)==-n then
          circfill(
            64+r*cos(a)*sin(q)/k,
            64+(r*sin(a)*m+14+c*8)/k,
            (3.5+c*4)/k,
            7*c+m*(12-j)
          )
        end
      end
    end
  end
end
flip()goto _