GetElementPosFast_step took the snake parity from GetElementPosSlow_step,
which is the DISPLAY row: it is the acquisition row r = image / n_fast,
flipped to (n_slow-1) - r when the slow step is negative. So for a negative
slow step the parity it hands back is parity(n_slow-1) XOR parity(r), and
with an even n_slow that is inverted on every row - the whole raster comes
out mirrored along the fast axis. An odd n_slow leaves it correct, so the
same scan collected with 20 or 25 images disagreed about where image 0 sat:
n_fast=5, fast +1.5 um, slow -2.5 um, snake on gives images 0..4 at fast
index 4,3,2,1,0 with 4 rows and 0,1,2,3,4 with 5 rows. A positive slow step
was correct at both counts, and so was every non-snake configuration.
Snake means the stage reverses direction on alternate rows in acquisition
order, so the parity has to come from the acquisition row. Taking it from
image_number / n_fast directly makes the fast index independent of the slow
axis and of the row count, and drops the call into the display-row function
that caused the coupling. vertical_scan only relabels which axis is fast, so
it was wrong in exactly the same way and is fixed by the same line.
Affected files: written by an affected build, with snake on, a negative
grid slow step (step_y for a horizontal scan, step_x for a vertical one),
and an even number of rows. Their /entry/sample/transformations/grid_scan_x
or _y is mirrored along the fast axis, as was the grid map in the frontend
and the viewer - both mirrored together, which is why neither showed it.
Tests: the interaction of snake with the step signs was never asserted, only
each in isolation, so add a table over snake x {+,- fast step} x {+,- slow
step} x {even, odd row count} x {horizontal, vertical} asserting positions,
plus a case running one affected configuration through GetXContainer_m /
GetYContainer_m and Rearrange. Every pre-existing assertion is unchanged and
still passes; only the four negative-slow, even-row cells of the product
move.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EFEJG6WBQv8th4UJFNe53N
leonarski_fandClaude Opus 5
92a615085d