Image Crop from center - ACS Named Transform Image

I am using ACS Commons Name Transform Image Servlet to resize and crop my images. I am trying to figure out what should be the parameters to crop the image from the center. What should be x,y if the size of the source image are not known and you want to apply a crop.

bounds=[x,y,width,height]

crop:bounds=25%,0,50%,100%