Folks:
We've been stumbling over slices of certain BRIKs (notably mask or segmentation BRIKs) being invisible in the AFNI viewer, and finally composed some test BRIKs to track this down.

It appears that if an Anatomy slice contains less then 1312 non-zero voxels, then the viewer window shows blank. You can see that the data is present by looking at other orientations, or by poking around with the mouse and reading the Anat value in the Define Function window.

Simple test case:
An inverted pyramid, where each successive axial slice contains a square of dimensions (2S+1) on each side (where S = slice number, so starting with a single point on slice zero). Intensity = 128, though exact number doesn't seem to matter.

Slices with squares of 35 x 35 or less don't show in AFNI viewer.

Finer test case:
Square of 35x35 on every slice, plus S additional pixels, where S is slice number. This allowed us to zero in on 1312 as the magic number.

"Workaround"
Just for viewing purposes, one can "cure" this invisibility by adding in some random background noise values throughout the volume (instead of zero). Needless to say this isn't very useful for actual use.

Just in case this is actually a feature instead of a bug, we'd sure appreciate learning how to operate it

Regards,

Graham