Improve light curve parameter limits and documentation (#9054)

Revert gamma upper limit to 3.0 because that was raised based on
a misunderstanding and had no benefit. A sane upper limit is
needed as players on a competitive server tend to use the maximum.
Set gamma lower limit to 0.33 for consistency with 3.0.
Set sane limits on alpha, beta, boost and enforce these in code
to limit values entered in minetest.conf and to avoid easy cheating
by editing settingtypes.txt.
Improve documentation and 'readable' setting names.
Clarify that gamma does not significantly affect natural night light.
light.cpp: Various codestyle and comment improvements.

1 files changed, 20 insertions, 16 deletions

diff --git a/src/light.cpp b/src/light.cpp
index 9b6f5c210..8196fedff 100644
--- a/src/light.cpp
+++ b/src/light.cpp
@@ -29,44 +29,48 @@ 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;
+	float a, b, c; // Lighting curve polynomial coefficients
+	float boost, center, sigma; // Lighting curve parametric boost
+	float gamma; // Lighting curve gamma correction
 };
 
 static LightingParams params;
 
+
 float decode_light_f(float x)
 {
-	if (x >= 1.0f) // x is equal to 1.0f half the time
+	if (x >= 1.0f) // x is often 1.0f
 		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
+	brightness += params.boost *
+		std::exp(-0.5f * sqr((x - params.center) / params.sigma));
+	if (brightness <= 0.0f) // May happen if parameters are extreme
 		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)
 {
-// Lighting curve derivatives
-	const float alpha = g_settings->getFloat("lighting_alpha");
-	const float beta  = g_settings->getFloat("lighting_beta");
-// Lighting curve coefficients
+// Lighting curve bounding gradients
+	const float alpha = rangelim(g_settings->getFloat("lighting_alpha"), 0.0f, 3.0f);
+	const float beta  = rangelim(g_settings->getFloat("lighting_beta"), 0.0f, 3.0f);
+// Lighting curve polynomial coefficients
 	params.a = alpha + beta - 2.0f;
 	params.b = 3.0f - 2.0f * alpha - beta;
 	params.c = alpha;
-// Mid boost
-	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
-	params.gamma = rangelim(gamma, 0.5f, 10.0f);
+// Lighting curve parametric boost
+	params.boost = rangelim(g_settings->getFloat("lighting_boost"), 0.0f, 0.4f);
+	params.center = rangelim(g_settings->getFloat("lighting_boost_center"), 0.0f, 1.0f);
+	params.sigma = rangelim(g_settings->getFloat("lighting_boost_spread"), 0.0f, 0.4f);
+// Lighting curve gamma correction
+	params.gamma = rangelim(gamma, 0.33f, 3.0f);
 
 // Boundary values should be fixed
 	light_LUT[0] = 0;