I learn something new every day!

>It is traditional in computer graphics to have the origin of the graphics buffer in the upper left corner. This makes buffer arithmetic easier in that the address of a pixel = address of buffer + (Y * width of X Axis) + X
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>Y Axis
>(0,0)
>________________ X Axis
>|\
>| \
>|  \ <<- Positive slope
>|   \
>Y Axis
>