fadeMargin

3D-projection-only soft-edge fade margin in normalised UV units (range 0..0.5). The fragment shader ramps interior alpha 0..1 across the inner 0..fadeMargin strip on each side, blending the projection's frustum boundary into the surrounding terrain instead of guillotining at the unit-square edge.

0 (default) renders a hard cutoff and is free at runtime: the shader's uniform-controlled branch skips the fade math entirely. The 2D-homography path ignores this value.