cnn_input_image( normalize between -1 to 1) and cnn_output_image are image tensor and “b” is scalar(constant value like 0.8) . So operation is like
image_tensor (out=1,3,256,256) = image_tensor(cnn_input_image=1,3,256,256) - scalar (b=1,3,1,1)/ image_tensor(cnn_output_image = 1,3,256,256).
cnn_input_image is subtracted from some constant value and after that it will divide by output of CNN which is image tensor.