Redo light.cpp.

Remake the light_decode_table.

The table starts out without pre-filled in values since those
are always discarded by the code apparently. We calculate a
pseudo curve with gamma power function, and then apply a new
adjustment table.

The adjustment table is setup to make the default gamma of 2.2
look decent: not too dark at light level 3 or so, but too dark
at 1 and below to be playable. The curve is much smoother than
before and looks reasonable at the whole range, offering a
pleasant decay of light levels away from lights.

The `display_gamma` setting now actually does something logical:
the game is darker at values below 2.2, and brighter at values
above 2.2. At 3.0, the game is very bright, but still has a good
light scale. At 1.1 or so, the bottom 5 light levels are virtually
black, but you can still see enough detail at light levels 7-8,
so the range and spread is adequate.

I must add that my monitor is somewhat dark to begin with, since
I have a `hc` screen that doesn't dynamic range colors or try to
pull up `black` pixels for me (it is tuned for accurate color and
light levels), so this should look even better on more dynamic
display tunings.

4 files changed, 28 insertions, 331 deletions

diff --git a/builtin/settingtypes.txt b/builtin/settingtypes.txt
index 3a740f3ca..8668e1472 100644
--- a/builtin/settingtypes.txt
+++ b/builtin/settingtypes.txt
@@ -442,9 +442,9 @@ fov (Field of view) int 72 30 160
 #    This requires the "zoom" privilege on the server.
 zoom_fov (Field of view for zoom) int 15 15 160
 
-#    Adjust the gamma encoding for the light tables. Lower numbers are brighter.
+#    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 1.8 1.0 3.0
+display_gamma (Gamma) float 2.2 1.0 3.0
 
 #    Path to texture directory. All textures are first searched from here.
 texture_path (Texture path) path
diff --git a/minetest.conf.example b/minetest.conf.example
index b1d6c86bd..262f95115 100644
--- a/minetest.conf.example
+++ b/minetest.conf.example
@@ -508,7 +508,7 @@
 #    Adjust the gamma encoding for the light tables. Lower numbers are brighter.
 #    This setting is for the client only and is ignored by the server.
 #    type: float min: 1 max: 3
-# display_gamma = 1.8
+# display_gamma = 2.2
 
 #    Path to texture directory. All textures are first searched from here.
 #    type: path
diff --git a/src/defaultsettings.cpp b/src/defaultsettings.cpp
index 5bcd7da3d..52f4cfec8 100644
--- a/src/defaultsettings.cpp
+++ b/src/defaultsettings.cpp
@@ -112,7 +112,7 @@ 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", "1.8");
+	settings->setDefault("display_gamma", "2.2");
 	settings->setDefault("texture_path", "");
 	settings->setDefault("shader_path", "");
 	settings->setDefault("video_driver", "opengl");
diff --git a/src/light.cpp b/src/light.cpp
index 5dc01fcf0..4a3ca5a60 100644
--- a/src/light.cpp
+++ b/src/light.cpp
@@ -26,29 +26,9 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 // Length of LIGHT_MAX+1 means LIGHT_MAX is the last value.
 // LIGHT_SUN is read as LIGHT_MAX from here.
 
-u8 light_LUT[LIGHT_MAX+1] =
-{
-	/* Middle-raised variation of a_n+1 = a_n * 0.786
-	 * Length of LIGHT_MAX+1 means LIGHT_MAX is the last value.
-	 * LIGHT_SUN is read as LIGHT_MAX from here.
-	 */
-	8,
-	11+2,
-	14+7,
-	18+10,
-	22+15,
-	29+20,
-	37+20,
-	47+15,
-	60+10,
-	76+7,
-	97+5,
-	123+2,
-	157,
-	200,
-	255,
-};
+u8 light_LUT[LIGHT_MAX+1];
 
+// the const ref to light_LUT is what is actually used in the code.
 const u8 *light_decode_table = light_LUT;
 
 /** Initialize or update the light value tables using the specified \p gamma.
@@ -65,26 +45,26 @@ void set_light_table(float gamma)
 {
 	static const float brightness_step = 255.0f / (LIGHT_MAX + 1);
 
-	/* These are adjustment values that are added to the calculated light value
-	 * after gamma is applied.  Currently they are used so that given a gamma
-	 * of 1.8 the light values set by this function are the same as those
-	 * hardcoded in the initalizer list for the declaration of light_LUT.
-	 */
+	// 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] = {
-		 7,
-		 7,
-		 7,
-		 5,
-		 2,
-		 0,
-		-7,
-		-20,
-		-31,
-		-39,
-		-43,
-		-45,
-		-40,
-		-25,
+		-67,
+		-91,
+		-125,
+		-115,
+		-104,
+		-85,
+		-70,
+		-63,
+		-56,
+		-49,
+		-42,
+		-35,
+		-28,
+		-22,
 		0
 	};
 
@@ -93,296 +73,13 @@ void set_light_table(float gamma)
 	float brightness = brightness_step;
 
 	for (size_t i = 0; i < LIGHT_MAX; i++) {
-		light_LUT[i] = (u8)(255 * powf(brightness / 255.0f,  gamma));
+		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])
-			light_LUT[i] = light_LUT[i-1] + 1;
+		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;
 }
 #endif
 
-
-
-#if 0
-/*
-Made using this and:
-- adding 220 as the second last one
-- replacing the third last one (212) with 195
-
-#!/usr/bin/python
-
-from math import *
-from sys import stdout
-
-# We want 0 at light=0 and 255 at light=LIGHT_MAX
-LIGHT_MAX = 14
-#FACTOR = 0.69
-#FACTOR = 0.75
-FACTOR = 0.83
-START_FROM_ZERO = False
-
-L = []
-if START_FROM_ZERO:
-    for i in range(1,LIGHT_MAX+1):
-        L.append(int(round(255.0 * FACTOR ** (i-1))))
-    L.append(0)
-else:
-    for i in range(1,LIGHT_MAX+1):
-        L.append(int(round(255.0 * FACTOR ** (i-1))))
-    L.append(255)
-
-L.reverse()
-for i in L:
-    stdout.write(str(i)+",\n")
-*/
-u8 light_decode_table[LIGHT_MAX+1] = 
-{
-23,
-27,
-33,
-40,
-48,
-57,
-69,
-83,
-100,
-121,
-146,
-176,
-195,
-220,
-255,
-};
-#endif
-
-#if 0
-// This is good
-// a_n+1 = a_n * 0.786
-// Length of LIGHT_MAX+1 means LIGHT_MAX is the last value.
-// LIGHT_SUN is read as LIGHT_MAX from here.
-u8 light_decode_table[LIGHT_MAX+1] = 
-{
-8,
-11,
-14,
-18,
-22,
-29,
-37,
-47,
-60,
-76,
-97,
-123,
-157,
-200,
-255,
-};
-#endif
-
-#if 0
-// Use for debugging in dark
-u8 light_decode_table[LIGHT_MAX+1] = 
-{
-58,
-64,
-72,
-80,
-88,
-98,
-109,
-121,
-135,
-150,
-167,
-185,
-206,
-229,
-255,
-};
-#endif
-
-// This is reasonable with classic lighting with a light source
-/*u8 light_decode_table[LIGHT_MAX+1] = 
-{
-2,
-3,
-4,
-6,
-9,
-13,
-18,
-25,
-32,
-35,
-45,
-57,
-69,
-79,
-255
-};*/
-
-
-// As in minecraft, a_n+1 = a_n * 0.8
-// NOTE: This doesn't really work that well because this defines
-//       LIGHT_MAX as dimmer than LIGHT_SUN
-// NOTE: Uh, this has had 34 left out; forget this.
-/*u8 light_decode_table[LIGHT_MAX+1] = 
-{
-8,
-11,
-14,
-17,
-21,
-27,
-42,
-53,
-66,
-83,
-104,
-130,
-163,
-204,
-255,
-};*/
-
-// This was a quick try of more light, manually quickly made
-/*u8 light_decode_table[LIGHT_MAX+1] = 
-{
-0,
-7,
-11,
-15,
-21,
-29,
-42,
-53,
-69,
-85,
-109,
-135,
-167,
-205,
-255,
-};*/
-
-// This was used for a long time, manually made
-/*u8 light_decode_table[LIGHT_MAX+1] = 
-{
-0,
-6,
-8,
-11,
-14,
-19,
-26,
-34,
-45,
-61,
-81,
-108,
-143,
-191,
-255,
-};*/
-
-/*u8 light_decode_table[LIGHT_MAX+1] = 
-{
-0,
-3,
-6,
-10,
-18,
-25,
-35,
-50,
-75,
-95,
-120,
-150,
-185,
-215,
-255,
-};*/
-/*u8 light_decode_table[LIGHT_MAX+1] = 
-{
-0,
-5,
-12,
-22,
-35,
-50,
-65,
-85,
-100,
-120,
-140,
-160,
-185,
-215,
-255,
-};*/
-// LIGHT_MAX is 14, 0-14 is 15 values
-/*u8 light_decode_table[LIGHT_MAX+1] = 
-{
-0,
-9,
-12,
-14,
-16,
-20,
-26,
-34,
-45,
-61,
-81,
-108,
-143,
-191,
-255,
-};*/
-
-#if 0
-/*
-#!/usr/bin/python
-
-from math import *
-from sys import stdout
-
-# We want 0 at light=0 and 255 at light=LIGHT_MAX
-LIGHT_MAX = 14
-#FACTOR = 0.69
-FACTOR = 0.75
-
-L = []
-for i in range(1,LIGHT_MAX+1):
-    L.append(int(round(255.0 * FACTOR ** (i-1))))
-L.append(0)
-
-L.reverse()
-for i in L:
-    stdout.write(str(i)+",\n")
-*/
-u8 light_decode_table[LIGHT_MAX+1] = 
-{
-0,
-6,
-8,
-11,
-14,
-19,
-26,
-34,
-45,
-61,
-81,
-108,
-143,
-191,
-255,
-};
-#endif
-
-