Update __main__.py

[xyperias]

extract motion vectors at h264 quarter-pixel precision

Currently the code uses only the integer precision end-point of motion vectors. However, AVMotionVector contains motion_x, motion_y and motion_scale values which allow for sub-pixel motion vector lengths.

This should be hardly visible in the visualization, but will show how to get more accurate motion vectors.

[LukasBommes]

Thanks a lot for your contribution, @xyperias. I will include this in the next release.

Just for my own understanding: I get the purpose of the factor after reading the "Detailled description" section on this page. But what's the use of the constant offset by 0.5? Is this suppoed to act as a ceil operation?