scan_mat4

The running product of a run of 4x4 matrices, whole or per segment: the frames of a sweep, each placed relative to the one before it.

A matrix is 16 consecutive floats, column by column. The product runs in run order, so out[i] = m[0] * m[1] * ... * m[i]. params.exclusive picks the product of the matrices before each one (the identity for the first) or the product up to and including it. total holds each group's product, the identity for an empty group. A segmented scan (params.segments above 0, the number of segments) restarts at each start in starts.

One workgroup scans one group at a time, a tile of 128 matrices at a time, with the product of everything before a tile carried into the next. The dispatch size n is the number of matrices; any workgroup count from 1 up scans every group.

Exact: the order of multiplication is fixed, so the same run gives the same bits on every dispatch.