Renders the image in shades of black, white, and gray.

For example, if you have the following image:
You can use the Grayscale transformation to create a black, white, and gray image:

If you add the IMQuery transformation to a policy, use the im variable, and select Grayscale, you can use a query string to create a black, white and gray version of any image using this policy. See Syntax and Examples for the syntax for the query string parameter.