Screen-space safety margin, in framebuffer pixels, added to each half
extent of the tightened quad. Covers vertex-vs-fragment float disagreement
on the ellipse (eigen extents vs the fragment's Cholesky solve and its
sub-1e-6 px² floors) and multisampled coverage: every sample of a pixel
lies within √2/2 px of the pixel centre at which the flat-varying fragment
test is evaluated.
Screen-space safety margin, in framebuffer pixels, added to each half extent of the tightened quad. Covers vertex-vs-fragment float disagreement on the ellipse (eigen extents vs the fragment's Cholesky solve and its sub-1e-6 px² floors) and multisampled coverage: every sample of a pixel lies within √2/2 px of the pixel centre at which the flat-varying fragment test is evaluated.