With the flag on, canvas row Y was uploaded to texture row (H-1-Y).
Sampling at V=0.58 then returned canvas row 2688 (in the 'south'
half of the corridor), but the shader expected row 3712 (in the
'north' half) for an offset.z=2 sample. So tiles appeared
mirrored across the NS axis — every N pointed south, every S north.
With the flag off, canvas row Y maps to texture row Y, and the
shader's expectation matches the drawing direction. Now N is at
the image's actual north, S at the image's actual south.
Richard Zink
a05951efbd