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You can find some axis system in opencv explanation here

center(0,0) is at upper left of image corner.

x axis is horizontal with positive value on the right

y axis is vertical Positive values down

90° means x axis becomes y axis after rotation

Ellipse is good

You can find some axis system in opencv explanation here

center(0,0) is at upper left of image corner.

x axis is horizontal with positive value on the right

y axis is vertical Positive values down

90° means x axis becomes y axis after rotation

Ellipse is goodgood

You can find some axis system in opencv explanation here or here