The latest update to the interactive AFNI image viewer is the ability to checkerboard the underlay and overlay images. This feature is toggled on and off with the '#' key. (The inverse pattern of checks can be invoked with the '3' key.)

Checkerboarding is not fully integrated into the image viewer. In particular, Save-ing won't write out a checkerboarded image -- you'll either have to use Record to save the image to a separate viewer, or use an external snapshot tool.

I've attached a snapshot that shows the popup menu (attached to the grayscale intensity bar in the image viewer) that lets you control the check size. If you set the size to 0, the program will choose for you.

The principal reason for this display mode is to check alignments of 2 T1-weighted volumes; for example, from the same subject on different days. No doubt other uses will evolve. Or be intelligently designed.