Is the MacBook Pro retina HDMI port an input, output, or both?

I am trying to find documentation on the HDMI port on the MacBook Pro Retina 15" early 2013 (A1398). Is it a video output only port, or is it also an input port? I have seen a hint that the headphone (audio out) port also can function as an audio input port, so that makes me wonder if I can also use the HDMI as an input, specifically, for using the computer as a field preview device for my Nikon D800--to preview images and delete rejects before I download them to the computer. Also, it might make the Mac a useful video capture device to capture realtime uncompressed video from the D800. Before I hook up the D800 and try it, I'd like to know if this will fry the computer.