perlin.js

// Based on code by iron_wallaby posted at http://www.ozoneasylum.com/30982
function perlin_noise (canvas, colorFunc) {
  var canvas_ctx = canvas.getContext ("2d");
  var saved_alpha = canvas_ctx.globalAlpha;
  var offscreen = newCanvas(canvas.width, canvas.height);
  var offscreen_ctx = offscreen.getContext ("2d");
  var offscreen_id = offscreen_ctx.getImageData (0, 0,
                                                 offscreen.width,
                                                 offscreen.height);
  var offscreen_pixels = offscreen_id.data;
  for (var i = 0; i < offscreen_pixels.length; i += 4) {
	var rgba = colorFunc();
    offscreen_pixels[i    ] = rgba[0];
    offscreen_pixels[i + 1] = rgba[1];
    offscreen_pixels[i + 2] = rgba[2];
    offscreen_pixels[i + 3] = rgba[3];
  }

  offscreen_ctx.putImageData (offscreen_id, 0, 0);

  /* Scale random iterations onto the canvas to generate Perlin noise. */
  for (var size = 4; size <= offscreen.width; size *= 2) {
    var x = irand(offscreen.width - size);

		var verticalSize = Math.min(size,offscreen.height);
    var y = irand(offscreen.height - verticalSize);

    canvas_ctx.globalAlpha = 4.0 / size;
    canvas_ctx.drawImage (offscreen, x, y, size, verticalSize,
                                     0, 0, canvas.width, canvas.height);
  }
  canvas_ctx.globalAlpha = saved_alpha;
}