Problem in 'localizeExtremumViaQuadraticFit'.

```
keypoint.octave = octave_index + image_index * (2 ** 8) + int(round((extremum_update[2] + 0.5) * 255)) * (2 ** 16)
```
when it comes to count keypoint.octave,why, image_index need to multiply by （2**8), and I'm confused about the  `int(round((extremum_update[2] + 0.5) * 255)) * (2 ** 16)`。