# Sub-pixel coordinate origin

When working with sub-pixel coordinates in OpenCV (e.g. chessboard corners in the camera calibration pipeline), what is the origin of this coordinate system?

I've seen this asked a few times, but the answers are always a bit ambiguous, so just to clarify:

I understand that in the **integer pixel** coordinate system, the origin is (0,0) at the top-left pixel. But for the coordinate system attached to **floating-point sub-pixel** coordinates, does (0,0) sit in the middle of the top-left pixel, or the top-left corner of the top-left pixel?

Many thanks!

I would say that it is at the center of the top-left pixel, because else there would be an imbalance in coordinate systems with negative valued axes ...