Function skyFactorEstimate

Compute the normalised sky factor for a cell, given how many of its n cosine-weighted hemisphere samples were visible (escaped the scene without hitting an occluder) and the cosine of the zenith angle of each visible sample.

Result is in [0, 1] — 1.0 means the cell sees the full unobstructed sky under the chosen model, 0.0 means the cell is entirely roofed over.

The Monte Carlo estimator is derived from

  E_horiz = ∫_Ω L(ω) · cos(θ) dω
          ≈ (π/n) · Σ L(ω_i)         (cosine-weighted PDF)
  SkyFactor = E_horiz / E_horiz_unobstructed
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The E_horiz_unobstructed analytic for each model is:

• Uniform : π · L → factor reduces to visible / n
• CIE Overcast : (7π/9) · L_z → factor is 3 · Σ(1 + 2·cosθ) / (7n)

Both are azimuth-symmetric so cosTheta is all the information needed per sample.