Transmittance LUT
Direct render of the 256×64 RGBA16F LUT to the screen. UV.x = view zenith cos (horizontal), UV.y = altitude (vertical).
Bruneton parameterization: U is non-linear in ray-path length (U=0 = shortest path to atmosphere top, U=1 = longest / near-tangent). V is view-height (V=0 = ground, V=1 = top of atmosphere). Expected: bright top-left (short path from the top), darkest bottom-right (grazing ray at ground), warm tint where extinction is heavy.
Linear-light, no tone mapping (LUT values are in [0,1] directly).