Short answer: rounding down.
The ray in the image above might be represented as r = u + tv where u = <0,-3/4> and v = <1, 8/9>, i.e. r follows the line y = (3/4)+ (9/8)x.
Note: that is a typo, it should read y = (3/4)+ (8/9)x.
Thus, ray.origin.x = 0 and ray.origin.y = -3/4.
We can start from these terms: y = y₀ + mx. The ray ...