Snippet from https://play.nim-lang.org

playground.nim

import complex, sequtils
import threadpool

{.experimental.}

proc mandel(x, y: float, maxIters: int): int =
  var
    c = (re: x, im: y)
    z = (re: 0.0, im: 0.0)
  for i in 0 ..< maxIters:
    z = z*z + c
    if (z.re*z.re + z.im*z.im) >= 4:
      return i
  return 255


proc fractalRow(min_x, pxSizeX, imag: float): array[16000, int] =
  for x in 0 ..< 16000:
    let real = min_x + float(x) * pxSizeX
    result[x] = mandel(real, imag, 20)


proc createFractal(min_x, max_x, min_y, max_y: float, iters: int): array[12000, array[16000, int]] =
  parallel:
    let
      height = result.len
      width = result[0].len
      pxSizeX = (max_x - min_x) / float(width)
      pxSizeY = (max_y - min_y) / float(height)

    for y in 0 ..< height:
      let imag = min_y + float(y) * pxSizeY
      result[y] = spawn fractalRow(min_x, pxSizeX, imag)


let r = createFractal(-2.0, 1.0, -1.0, 1.0, 20)
echo r[51][72]