// This macro subtracts the average of the lines in a rectangular 
// selection from each line in an image. The selection must be the
// same width as the image.

  if (selectionType!=0)
      exit("Rectangular selection required");
  getBoundingRect(x, y, width, height);
  if (width!=getWidth)
      exit("Selection width must equal image width");
  if (bitDepth==24)
      exit("This macro does not work with RGB images");
  setupUndo();
  profile = getProfile();
  width = getWidth;
  height = getHeight;
  for (y=0; y<height; y++) {
      for (x=0; x<width; x++) {
          value = getPixel(x,y)-profile[x];
          setPixel(x, y, value);
      }
      showProgress(y, height);
  }