Sobel Edge Detection

http://bitecode.co.uk/category/image-processing/

def sobel(img):
  if img.mode != "RGB":
      img = img.convert("RGB")
  out_image = Image.new(img.mode, img.size, None)
  imgdata = img.load()
  outdata = out_image.load()

  gx = [[-1, 0, 1], [-2, 0, 2], [-1, 0, 1]]
  gy = [[-1, -2, -1], [0, 0, 0], [1, 2, 1]]

  for row in xrange(1, img.size[0]-1):
      for col in xrange(1, img.size[1]-1):
          pixel_gx = pixel_gy = 0
          pxval = sum(imgdata[row,col])/3
          for i in range(-1, 2):
              for j in range(-1, 2):
                  val = sum(imgdata[row+i,col+j])/3
                  pixel_gx += gx[i+1][j+1] * val
                  pixel_gy += gy[i+1][j+1] * val
          newpixel = math.sqrt(pixel_gx * pixel_gx + pixel_gy * pixel_gy)
          newpixel = 255 - int(newpixel)
          outdata[row, col] = (newpixel, newpixel, newpixel)
  return out_image