From what I've seen almost all things use coordinates where (0, 0) is at the top left and the positive Y-axis goes in the downwards direction of your screen.
Why is it like this? Why not the conventional positive Y-axis going upwards like shown in graphs in simple math classes?
Answer
This is caused in the history. Early computers had Cathode Ray Tubes (CRTs) which "draw" the image with a cathode ray from the upper left corner to the lower right.
To ease the interface between the graphics card memory and the CRT the memory was read from the beginning and the image was drawn from the top left (with the lowest memory address) to the lower right (with the highest memory address).
Extension (based on the comments)
The CRTs are based on analog TV sets that were available at that time.
The TV sets create the image line by line first from left to right and then from top to bottom. The reason for this can only be assumed to be based on the writing style in western contries.
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