Negative values in the intrinsics matrix generated in the MOVI datasets

Hello,

I've been playing around with the Movi dataset, and I found something odd about the intrinsics matrix:
```
K = [[1.09375, 0.0, -0.5],
[0.0, -1.09375, -0.5],
[0.0, 0.0, -1.0]]
```
AFAIK the intrinsics matrix should have the form of
```
K = [[fx, 0.0, -cx],
[0.0, fy, -cy],
[0.0, 0.0, 1.0]]
```
where fx, fy, cx, cy > 0. What does it mean when the intrinsics matrix has negative values i.e. entries K_11 = -1.09375 and K_22 = -1.0?

Thank you in advance!