I'm just using a Spots node to simulate a greyscale image map such as might be used as a bump map.

You can see the masking effect. However, because the darker colour in the mask is grey, not black, there is still some specular highlight showing in areas where we don't want to see it, in between the brighter parts.

I'll fix this in the next picture with an extra node.....

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)