New lighting curve (minetest#5279)

* New lighting curve

* Make polynomial lighting curve

* Update default lighting settings

builtin/settingtypes.txt

@@ -602,7 +602,11 @@ zoom_fov (Field of view for zoom) int 15 7 160
 
 #    Adjust the gamma encoding for the light tables. Higher numbers are brighter.
 #    This setting is for the client only and is ignored by the server.
-display_gamma (Gamma) float 2.2 1.0 3.0
+display_gamma (Gamma) float 1.0 0.5 3.0
+
+lighting_alpha (Darkness sharpness) float 0.0 0.0 4.0
+
+lighting_beta (Lightness sharpness) float 0.0 0.0 4.0
 
 #    Path to texture directory. All textures are first searched from here.
 texture_path (Texture path) path

minetest.conf.example

@@ -702,8 +702,14 @@
 
 #    Adjust the gamma encoding for the light tables. Higher numbers are brighter.
 #    This setting is for the client only and is ignored by the server.
-#    type: float min: 1 max: 3
-# display_gamma = 2.2
+#    type: float min: 0.5 max: 3.0
+# display_gamma = 1.0
+
+#    type: float min: 0.0 max: 4.0
+# lighting_alpha = 0.0
+
+#    type: float min: 0.0 max: 4.0
+# lighting_beta = 0.0
 
 #    Path to texture directory. All textures are first searched from here.
 #    type: path

src/defaultsettings.cpp

@@ -168,7 +168,9 @@ void set_default_settings(Settings *settings)
 	settings->setDefault("leaves_style", "fancy");
 	settings->setDefault("connected_glass", "false");
 	settings->setDefault("smooth_lighting", "true");
-	settings->setDefault("display_gamma", "2.2");
+	settings->setDefault("lighting_alpha", "0.0");
+	settings->setDefault("lighting_beta", "0.0");
+	settings->setDefault("display_gamma", "1.0");
 	settings->setDefault("texture_path", "");
 	settings->setDefault("shader_path", "");
 	settings->setDefault("video_driver", "opengl");

src/light.cpp

@@ -20,6 +20,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "light.h"
 #include <math.h>
 #include "util/numeric.h"
+#include "settings.h"
 
 #ifndef SERVER
 
@@ -32,52 +33,27 @@ u8 light_LUT[LIGHT_MAX+1];
 const u8 *light_decode_table = light_LUT;
 
 /** Initialize or update the light value tables using the specified \p gamma.
- *  If \p gamma == 1.0 then the light table is linear.  Typically values for
- *  gamma range between 1.8 and 2.2.
- *
- *  @note The value for gamma will be restricted to the range 1.1 <= gamma <= 3.0.
- *
- *  @note This function is not, currently, a simple linear to gamma encoding
- *        because adjustments are made so that a gamma of 1.8 gives the same
- *        results as those hardcoded for use by the server.
  */
 void set_light_table(float gamma)
 {
-	static const float brightness_step = 255.0f / (LIGHT_MAX + 1);
-
-	// this table is pure arbitrary values, made so that
-	// at gamma 2.2 the game looks not too dark at light=1,
-	// and mostly linear for the rest of the scale.
-	// we could try to inverse the gamma power function, but this
-	// is simpler and quicker.
-	static const int adjustments[LIGHT_MAX + 1] = {
-		-67,
-		-91,
-		-125,
-		-115,
-		-104,
-		-85,
-		-70,
-		-63,
-		-56,
-		-49,
-		-42,
-		-35,
-		-28,
-		-22,
-		0
-	};
-
-	gamma = rangelim(gamma, 1.0, 3.0);
-
-	float brightness = brightness_step;
+// lighting curve derivatives
+	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;
+	const float b = 3 - 2 * alpha - beta;
+	const float c = alpha;
+// gamma correction
+	gamma = rangelim(gamma, 0.5, 3.0);
 
 	for (size_t i = 0; i < LIGHT_MAX; i++) {
-		light_LUT[i] = (u8)(255 * powf(brightness / 255.0f, 1.0 / gamma));
-		light_LUT[i] = rangelim(light_LUT[i] + adjustments[i], 0, 255);
-		if (i > 1 && light_LUT[i] < light_LUT[i - 1])
+		float x = i;
+		x /= LIGHT_MAX;
+		float brightness = a * x * x * x + b * x * x + c * x;
+		brightness = powf(brightness, 1.0 / gamma);
+		light_LUT[i] = rangelim((u32)(255 * brightness), 0, 255);
+		if (i > 1 && light_LUT[i] <= light_LUT[i - 1])
 			light_LUT[i] = light_LUT[i - 1] + 1;
-		brightness += brightness_step;
 	}
 	light_LUT[LIGHT_MAX] = 255;
 }

src/settings_translation_file.cpp

@@ -245,6 +245,8 @@ fake_function() {
 	gettext("Field of view while zooming in degrees.\nThis requires the \"zoom\" privilege on the server.");
 	gettext("Gamma");
 	gettext("Adjust the gamma encoding for the light tables. Higher numbers are brighter.\nThis setting is for the client only and is ignored by the server.");
+	gettext("Darkness sharpness");
+	gettext("Lightness sharpness");
 	gettext("Texture path");
 	gettext("Path to texture directory. All textures are first searched from here.");
 	gettext("Video driver");