subdivisions calculation

[facebook]

subdivisions calculation

[evgenykuzyakov]

The idea here is to iterate through sub-pixels of each pixel. We divide each pixel into sub-pixels (e.g. 4x4). So for the top-left pixel from (0.0, 0.0) to (1.0, 1.0) we have top-left sub-pixel from (0.0, 0.0) to (0.25, 0.25). Its center is at (0.125, 0.125). The next sub-pixel on the right has the center at (0.375, 0.125). Here is how it's computed:

(subx + 0.5f) / s->w_subdivisons

for subx = 0: (0 + 0.5) / 4 = 0.125
for subx = 1: (1 + 0.5) / 4 = 0.375

So now to calculate global sub-pixel offset we just need to add left and top offset of the pixel itself.
I hope it helps.