From 79e393cf4b5027eb9071df23f4834aac3f3a689f Mon Sep 17 00:00:00 2001
From: Vitaliy <numzer0@yandex.ru>
Date: Sun, 16 Sep 2018 19:59:42 +0300
Subject: [PATCH] Light curve: Simplify and improve code, fix darkened daytime
 sky (#7693)

---
 src/light.cpp      | 55 +++++++++++++++++++++++++++++++++-------------
 src/light.h        | 16 +-------------
 src/util/numeric.h |  5 +++++
 3 files changed, 46 insertions(+), 30 deletions(-)

diff --git a/src/light.cpp b/src/light.cpp
index c63ffc3bd..c60f8eaca 100644
--- a/src/light.cpp
+++ b/src/light.cpp
@@ -29,6 +29,28 @@ static u8 light_LUT[LIGHT_SUN + 1];
 // The const ref to light_LUT is what is actually used in the code
 const u8 *light_decode_table = light_LUT;
 
+struct LightingParams {
+	float a, b, c; // polynomial coefficients
+	float boost, center, sigma; // normal boost parameters
+	float gamma;
+};
+
+static LightingParams params;
+
+float decode_light_f(float x)
+{
+	if (x >= 1.0f) // x is equal to 1.0f half the time
+		return 1.0f;
+	x = std::fmax(x, 0.0f);
+	float brightness = ((params.a * x + params.b) * x + params.c) * x;
+	brightness += params.boost * std::exp(-0.5f * sqr((x - params.center) / params.sigma));
+	if (brightness <= 0.0f) // may happen if parameters are insane
+		return 0.0f;
+	if (brightness >= 1.0f)
+		return 1.0f;
+	return powf(brightness, 1.0f / params.gamma);
+}
+
 // Initialize or update the light value tables using the specified gamma
 void set_light_table(float gamma)
 {
@@ -36,26 +58,29 @@ void set_light_table(float gamma)
 	const float alpha = g_settings->getFloat("lighting_alpha");
 	const float beta  = g_settings->getFloat("lighting_beta");
 // Lighting curve coefficients
-	const float a = alpha + beta - 2.0f;
-	const float b = 3.0f - 2.0f * alpha - beta;
-	const float c = alpha;
+	params.a = alpha + beta - 2.0f;
+	params.b = 3.0f - 2.0f * alpha - beta;
+	params.c = alpha;
 // Mid boost
-	const float d = g_settings->getFloat("lighting_boost");
-	const float e = g_settings->getFloat("lighting_boost_center");
-	const float f = g_settings->getFloat("lighting_boost_spread");
+	params.boost = g_settings->getFloat("lighting_boost");
+	params.center = g_settings->getFloat("lighting_boost_center");
+	params.sigma = g_settings->getFloat("lighting_boost_spread");
 // Gamma correction
-	gamma = rangelim(gamma, 0.5f, 3.0f);
+	params.gamma = rangelim(gamma, 0.5f, 3.0f);
 
-	for (size_t i = 0; i < LIGHT_SUN; i++) {
-		float x = i;
-		x /= LIGHT_SUN;
-		float brightness = a * x * x * x + b * x * x + c * x;
-		float boost = d * std::exp(-((x - e) * (x - e)) / (2.0f * f * f));
-		brightness = powf(brightness + boost, 1.0f / gamma);
-		light_LUT[i] = rangelim((u32)(255.0f * brightness), 0, 255);
+// Boundary values should be fixed
+	light_LUT[0] = 0;
+	light_LUT[LIGHT_SUN] = 255;
+
+	for (size_t i = 1; i < LIGHT_SUN; i++) {
+		float brightness = decode_light_f((float)i / LIGHT_SUN);
+		// Strictly speaking, rangelim is not necessary here—if the implementation
+		// is conforming. But we don’t want problems in any case.
+		light_LUT[i] = rangelim((s32)(255.0f * brightness), 0, 255);
+		// Ensure light brightens with each level
 		if (i > 1 && light_LUT[i] <= light_LUT[i - 1])
 			light_LUT[i] = light_LUT[i - 1] + 1;
 	}
-	light_LUT[LIGHT_SUN] = 255;
 }
+
 #endif
diff --git a/src/light.h b/src/light.h
index 203bd851e..44082a163 100644
--- a/src/light.h
+++ b/src/light.h
@@ -63,21 +63,7 @@ inline u8 decode_light(u8 light)
 
 // 0.0 <= light <= 1.0
 // 0.0 <= return value <= 1.0
-inline float decode_light_f(float light_f)
-{
-	s32 i = (u32)(light_f * LIGHT_MAX + 0.5);
-
-	if (i <= 0)
-		return (float)light_decode_table[0] / 255.0;
-	if (i >= LIGHT_SUN)
-		return (float)light_decode_table[LIGHT_SUN] / 255.0;
-
-	float v1 = (float)light_decode_table[i - 1] / 255.0;
-	float v2 = (float)light_decode_table[i] / 255.0;
-	float f0 = (float)i - 0.5;
-	float f = light_f * LIGHT_MAX - f0;
-	return f * v2 + (1.0 - f) * v1;
-}
+float decode_light_f(float light_f);
 
 void set_light_table(float gamma);
 
diff --git a/src/util/numeric.h b/src/util/numeric.h
index 61370a3f4..b1e2845b8 100644
--- a/src/util/numeric.h
+++ b/src/util/numeric.h
@@ -243,6 +243,11 @@ inline s32 myround(f32 f)
 	return (s32)(f < 0.f ? (f - 0.5f) : (f + 0.5f));
 }
 
+inline constexpr f32 sqr(f32 f)
+{
+	return f * f;
+}
+
 /*
 	Returns integer position of node in given floating point position
 */