It’s very famous issue. That’s why using floating point operation is difficult.
Especially, round-off can make up to 0.5f erratum.
In calculation for drawing on pixels, round-off is frequently used – Do not consider blending and anti-aliasing. And even sum of two errata can make 1 pixel erratum. So, we should always keep this in mind when implement pixel-relative-calculation.
Here is example.
Line L passes point P0 and P1.
Drawing two lines those are orthogonal to L, L-symmetric, passes point P2, P3 respectively and length is R.
As you know, there two lines should be parallel.
But, in this case, we should calculate two end points – the results may be float type. To draw this we should make those as an integer value (usually by using round-off).
For each line, up to 0.5f erratum can exist. So, for two lines, up to 1 pixel erratum can be raised. So, when these two lines are drawn on the canvas, they may not be parallel.