Add dithering

On some displays, banding is easily noticeable in Minetest.
Dithering before the quantisation to 8 bits can prevent banding artifacts
and an implementation requires only a few lines of code.

Dithering is disabled by default and can be enabled with a setting.
It is applied in the second-stage shader and uses the algorithm supplied by Calinou.

builtin/settingtypes.txt

@@ -580,6 +580,15 @@ enable_auto_exposure (Enable Automatic Exposure) bool false
 #    Requires: shaders, enable_auto_exposure
 exposure_compensation (Exposure compensation) float 0.0 -1.0 1.0
 
+#    Apply dithering to reduce colour banding artifacts.
+#    Dithering significantly increases the size of losslessly-compressed
+#    screenshots and it works incorrectly if the display or operating system
+#    performs additional dithering or if the colour channels are not quantized
+#    to 8 bits. Therefore it is disabled by default.
+#
+#    Requires: shaders
+dithering (Enable Dithering) bool false
+
 [**Bloom]
 
 #    Set to true to enable bloom effect.

client/shaders/second_stage/opengl_fragment.glsl

@@ -79,6 +79,20 @@ vec3 applySaturation(vec3 color, float factor)
 }
 #endif
 
+#ifdef ENABLE_DITHERING
+// From http://alex.vlachos.com/graphics/Alex_Vlachos_Advanced_VR_Rendering_GDC2015.pdf
+// and https://www.shadertoy.com/view/MslGR8 (5th one starting from the bottom)
+// NOTE: `frag_coord` is in pixels (i.e. not normalized UV).
+vec3 screen_space_dither(vec2 frag_coord) {
+	// Iestyn's RGB dither (7 asm instructions) from Portal 2 X360, slightly modified for VR.
+	vec3 dither = vec3(dot(vec2(171.0, 231.0), frag_coord));
+	dither.rgb = fract(dither.rgb / vec3(103.0, 71.0, 97.0));
+
+	// Subtract 0.5 to avoid slightly brightening the whole viewport.
+	return (dither.rgb - 0.5) / 255.0;
+}
+#endif
+
 void main(void)
 {
 	vec2 uv = varTexCoord.st;
@@ -125,5 +139,10 @@ void main(void)
 	// return to sRGB colorspace (approximate)
 	color.rgb = pow(color.rgb, vec3(1.0 / 2.2));
 
+#ifdef ENABLE_DITHERING
+	// Apply dithering just before quantisation
+	color.rgb += screen_space_dither(gl_FragCoord.xy);
+#endif
+
 	gl_FragColor = vec4(color.rgb, 1.0); // force full alpha to avoid holes in the image.
 }

minetest.conf.example

@@ -508,6 +508,15 @@
 #    type: float min: -1 max: 1
 # exposure_compensation = 0.0
 
+#    Apply dithering to reduce colour banding artifacts.
+#    Dithering significantly increases the size of losslessly-compressed
+#    screenshots and it works incorrectly if the display or operating system
+#    performs additional dithering or if the colour channels are not quantized
+#    to 8 bits. Therefore it is disabled by default.
+#    type: bool
+# dithering = false
+
+
 ### Bloom
 
 #    Set to true to enable bloom effect.

src/client/shader.cpp

@@ -767,6 +767,9 @@ ShaderInfo ShaderSource::generateShader(const std::string &name,
 		shaders_header << "#define SSAA_SCALE " << ssaa_scale << ".\n";
 	}
 
+	if (g_settings->getBool("dithering"))
+		shaders_header << "#define ENABLE_DITHERING 1\n";
+
 	shaders_header << "#line 0\n"; // reset the line counter for meaningful diagnostics
 
 	std::string common_header = shaders_header.str();