The separable Gaussian kernel of a blur of radius source texels as the
blur pass samples it: pair_count pairs on each side, total_weight the
kernel’s sum with the centre tap’s one, computed once per draw.
One pair of kernel taps on one side of the pixel: the Gaussian weights of
the inner and outer tap, and the one bilinear fetch that stands for both,
its offset in taps from the pixel and its weight their sum. The outer
weight is zero past an odd tap count, which leaves the fetch on the inner
tap alone.
How far, in device pixels, a blur of radius_px carries a source pixel:
the kernel’s taps at the scratch grid, the block each scratch texel
averages on the way down and interpolates on the way back, and the
source’s own antialiased pixel, rounded up to whole blocks so the
scratch grid sits on the source the same way whatever the margin. Past
this distance the blur is exactly zero, so nothing reads or draws
beyond it.