From 7f81f26889b84542b0b35685b0e80383256cdc47 Mon Sep 17 00:00:00 2001
From: Mikko Rasa <tdb@tdb.fi>
Date: Sat, 8 May 2021 20:31:42 +0300
Subject: [PATCH] Use non-random sequences to initialize ambient occlusion data

This allows selecting the formulas to obtain a better distribution of
values.
---
 source/effects/ambientocclusion.cpp | 21 +++++++++++----------
 source/effects/ambientocclusion.h   |  2 +-
 2 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/source/effects/ambientocclusion.cpp b/source/effects/ambientocclusion.cpp
index b4050aef..a407c2d9 100644
--- a/source/effects/ambientocclusion.cpp
+++ b/source/effects/ambientocclusion.cpp
@@ -41,11 +41,10 @@ const Texture2D &AmbientOcclusion::get_or_create_rotate_lookup()
 	rotate_lookup->storage(RGBA8, 4, 4, 1);
 	resources.add(name, rotate_lookup);
 
-	unsigned seed = 1;
 	unsigned char data[64];
 	for(unsigned i=0; i<16; ++i)
 	{
-		Geometry::Angle<float> a = Geometry::Angle<float>::from_turns(random(seed));
+		Geometry::Angle<float> a = Geometry::Angle<float>::from_turns(i*7/16.0f);
 		unsigned char c = (cos(a)*0.5f+0.5f)*255;
 		unsigned char s = (sin(a)*0.5f+0.5f)*255;
 		data[i*4  ] = c;
@@ -58,11 +57,12 @@ const Texture2D &AmbientOcclusion::get_or_create_rotate_lookup()
 	return *rotate_lookup;
 }
 
-float AmbientOcclusion::random(unsigned &seed)
+float AmbientOcclusion::radical_inverse(unsigned n)
 {
-	static const unsigned modulus = (1U<<31)-1;
-	seed = (static_cast<UInt64>(seed)*48271)%modulus;  // minstd
-	return static_cast<float>(seed)/(modulus-1);
+	unsigned inv = ((n&0x55)<<1) | ((n&0xAA)>>1);
+	inv = ((inv&0x33)<<2) | ((inv&0xCC)>>2);
+	inv = ((inv&0x0F)<<4) | ((inv&0xF0)>>4);
+	return inv/256.0f;
 }
 
 void AmbientOcclusion::set_n_samples(unsigned n)
@@ -70,13 +70,14 @@ void AmbientOcclusion::set_n_samples(unsigned n)
 	if(n<1 || n>32)
 		throw out_of_range("AmbientOcclusion::set_n_samples");
 
-	unsigned seed = 1;
-	float radius_divisor = (n-1)*(n-1);
 	Vector3 sample_points[32];
 	for(unsigned i=0; i<n; ++i)
 	{
-		Vector3 v(random(seed)-0.5f, random(seed)-0.5f, random(seed)-0.5f);
-		sample_points[i] = normalize(v)*(0.1f+0.9f*i*i/radius_divisor);
+		float z = static_cast<float>(i)/n;
+		float r = sqrt(1.0f-z*z);
+		float d = radical_inverse(i);
+		Geometry::Angle<float> a = Geometry::Angle<float>::from_turns(d);
+		sample_points[i] = Vector3(cos(a)*r, sin(a)*r, z)*(0.1f+0.9f*d*d);
 	}
 	shdata.uniform3_array("sample_points", n, &sample_points[0].x);
 	shdata.uniform("n_samples", static_cast<int>(n));
diff --git a/source/effects/ambientocclusion.h b/source/effects/ambientocclusion.h
index 61d4d4f3..f186db3d 100644
--- a/source/effects/ambientocclusion.h
+++ b/source/effects/ambientocclusion.h
@@ -54,7 +54,7 @@ public:
 
 private:
 	static const Texture2D &get_or_create_rotate_lookup();
-	static float random(unsigned &);
+	static float radical_inverse(unsigned);
 
 public:
 	void set_n_samples(unsigned);
-- 
2.43.0