Tuesday, September 11, 2012

pyMVPA searchlight shapes

Michael Hanke listed the number of voxels for different searchlight radii in pyMVPA. He also wrote that pyMVPA considers neighboring voxels to be those that share two edges (i.e., a face).

Here's a diagram of following the faces-touching rule. The center voxel is black, the one-voxel radius surrounds red, the two-voxel radius surrounds purple, and the three-voxel radius surrounds blue.
This gives 1 + 9 + 21 + 24 + 21 + 9 + 1 = 86 surrounding voxels for the three-voxel radius.

But Michael wrote that a three-voxel radius searchlight in pyMVPA has 123 voxels (counting the center). If so, at left is how I could come by that count: 13 + 28 + 40 + 28 + 13 = 122.

This shape is not quite symmetrical: there are four voxels to the left, right, front, and back of the center voxel but only two above and below.

Anyone know if this is what pyMVPA does?

UPDATE: Neither of these sketches is correct, see pyMVPA searchlight: solved.

No comments:

Post a Comment