var shader_build = function(){
  var fn_str = document.getElementById('shader').value
  if (!fn_str.length) fn_str = ""
  try {
    var fn = new Function('x','y','t','d', fn_str)
    shader = fn
    shade = shade_error_handling
  } catch (e) {
    throw Error ("Shader compilation error")
  }
}

var r,g,b,a;

function shade_no_error_handling(frame, t){
  if (! t || isNaN(t)) throw Error ("No time specified")
  if (! frame) throw Error ("No frame specified")

  var imgData = frame.ctx.getImageData(0,0,w,h)
  var data = imgData.data
  var clone = frame.cloneData.data  
  var realw = w, realh = h, v, index

  for (var x = 0; x < w; x++) {
    for (var y = 0; y < h; y++) {
      q = 4*(y*w+x)
      r = data[q], g = data[q+1], b = data[q+2], a = data[q+3]
      result = shader(x,y,t,clone)
      data[q] = r
      data[q+1] = g
      data[q+2] = b
      data[q+3] = a
      w = realw, h = realh
    }
  }
  cc.putImageData(imgData,0,0)
}

function shade_error_handling(frame, t){
  if (! t || isNaN(t)) throw Error ("No time specified")
  if (! frame) throw Error ("No frame specified")

  var imgData = frame.ctx.getImageData(0,0,w,h)
  var data = imgData.data
  var clone = frame.cloneData.data

  try {
    var realw = w, realh = h, v, index

    for (var x = 0; x < w; x++) {
      for (var y = 0; y < h; y++) {
        q = 4*(y*w+x)
        r = data[q], g = data[q+1], b = data[q+2], a = data[q+3]
        result = shader(x,y,t,clone)
        data[q] = r
        data[q+1] = g
        data[q+2] = b
        data[q+3] = a
        w = realw, h = realh
      }
    }
  }
  catch(e){
    // console.log(e.stack)
    throw Error ("Shader execution error")
  }
  cc.putImageData(imgData,0,0)
  shade = shade_no_error_handling
}

shade = shade_error_handling