openslide/openslide — comment on commit 5e3f1f836adca1b42c1fa63361a71619155719a8
File:  line 0
By bgilbert:

It turns out that some of those old slides have some X coordinates which are negative. (I know this is correct because their distance from the next position is consistent with other inter-position distances.) The negative X coordinate coincides with a Y low-order byte equal to 255.

Fixed-point interpretation:

.x x x x .y y y y z3
00 e0 ff ff ff 64 36 00 00

Padding-byte interpretation:

z1 x x x z2 y y y z3
00 e0 ff ff ff 64 36 00 00

Eureka!

z1 x x x x y y y y
00 e0 ff ff ff 64 36 00 00

I have seen {0, 1} in z1, {0} in z3 and now {0, 255} in z2. In f1412ba5, Marco reported seeing z3 == 255, so that also fits.