[Bf-blender-cvs] [2af62e27e2c] soc-2018-hair-shader: Fix improper optimization in the trimmed logistic function

[L. E. Segovia]

Commit: 2af62e27e2c894217b5d7daa880434447ea6ed35
Author: L. E. Segovia
Date:   Sat May 26 16:05:47 2018 +0000
Branches: soc-2018-hair-shader
https://developer.blender.org/rB2af62e27e2c894217b5d7daa880434447ea6ed35

Fix improper optimization in the trimmed logistic function

This was the source of the noise that Ronan mentioned.

Ref T54796

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M	intern/cycles/kernel/closure/bsdf_hair_principled.h

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diff --git a/intern/cycles/kernel/closure/bsdf_hair_principled.h b/intern/cycles/kernel/closure/bsdf_hair_principled.h
index 36d2325a397..8237c13251e 100644
--- a/intern/cycles/kernel/closure/bsdf_hair_principled.h
+++ b/intern/cycles/kernel/closure/bsdf_hair_principled.h
@@ -100,7 +100,7 @@ ccl_device_inline float trimmed_logistic(float x, float s)
 	/* The logistic distribution is symmetric and centered around zero,
 	 * so logistic_cdf(x, s) = 1 - logistic_cdf(-x, s).
 	 * Therefore, logistic_cdf(x, s)-logistic_cdf(-x, s) = 1 - 2*logistic_cdf(-x, s) */
-	float scaling_fac = 1.0f - 2.0f*logistic_cdf(-x, s);
+	float scaling_fac = 1.0f - 2.0f*logistic_cdf(-M_PI_F, s);
 	float val = logistic(x, s);
 	return val / scaling_fac;
 }