exposal - get an exposal image

Calling Sequence:

	img = exposal(im)
	
Parmeters:

	im: matrix of an input image
	img: output image matrix

Description:

	if a color value of a pixel < 128,then set this value be (255-thisValue),else no change.
	Then we can get an effect as we see a negative in a studio

Example:
	im = imread("lena.png");
	img = exposal(im);