重新映射灰度值，例如可以把灰度值取反

* Invert an image using the proper look-up table
read_image (Image, 'monkey')
dev_update_var ('off')
dev_update_pc ('off')
lut := []
for i := 0 to 255 by 1
    lut := [lut,255 - i]
endfor
dev_update_pc ('on')
lut_trans (Image, ImageInverse, lut)