mathr / blog / #

After I posted topographic form to the #diagrams IRC channel, I got a comment:

< luite_> ClaudiusMaximus: have you tried making an animation of that?

So that's what I did today, with this Haskell code:

import Diagrams.Prelude
import Diagrams.Backend.SVG.CmdLine (defaultMain)

import System.Environment (withArgs)
import Control.Monad (forM_)

main = forM_ [0..49] $ \f ->
  withArgs ["-w", "788", "-o", drop 1 $ show (f + 100) ++ ".svg"] $
    defaultMain (diagram
      (2 * fromIntegral f / 50)
      (fromIntegral (if f >= 25 then 50 - f else f) / 25))

diagram r s
  = fromVertices (zigzag r s)
  # withEnvelope' (fromVertices [p2 (0, 3), p2 (135, 78)])
  # bg white

withEnvelope' a b = withEnvelope (a `asTypeOf` b) b

zigzag r s = concat $ zipWith (line r (pi/2 * s)) ys (cycle [xs, reverse xs])

line r s y = map (point s (r + y))

point s y x = p2 (x, y + heightMap s x y)

heightMap s x y = maximum
  [ arcH 36   (\t -> t < 0 || pi/2 < t - s) x y
  , arcH 67.5 (\t -> True)                  x y
  , arcH 99   (\t -> -pi/2 < t - s)         x y
  ]

arcH x0 f x y
  | f t && ri < r && r < ro = h0 * cos (pi * (r - r0) / (ro - ri))
  | otherwise = 0
  where
    t = atan2 y' x'
    r = sqrt $ x'^2 + y'^2
    x' = x - x0
    y' = y - y0

y0 = 40.5
r0 = 15.75
ri = r0 * 0.75
ro = r0 * 1.25
h0 = (ro - ri) / 2
xs = [0 .. 135]
ys = [0 .. 81]

To postprocess the output into an animgif I did this in bash:

for i in ??.svg
do
  rsvg $i ${i%svg}png
  convert ${i%svg}png ${i%svg}gif
done
gifsicle --delay 4 --loop --optimize ??.gif > out.gif