Add dithering (#9014)

builtin/settingtypes.txt

@@ -580,6 +580,17 @@ 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 color 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 color channels are not quantized
+#    to 8 bits.
+#    With OpenGL ES, dithering only works if the shader supports high
+#    floating-point precision and it may have a higher performance impact.
+#
+#    Requires: shaders
+debanding (Enable Debanding) bool true
+
 [**Bloom]
 
 #    Set to true to enable bloom effect.

client/shaders/second_stage/opengl_fragment.glsl

@@ -1,6 +1,13 @@
 #define rendered texture0
 #define bloom texture1
 
+#ifdef GL_ES
+// Dithering requires sufficient floating-point precision
+#ifndef GL_FRAGMENT_PRECISION_HIGH
+#undef ENABLE_DITHERING
+#endif
+#endif
+
 struct ExposureParams {
 	float compensationFactor;
 };
@@ -79,6 +86,20 @@ 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(highp vec2 frag_coord) {
+	// Iestyn's RGB dither (7 asm instructions) from Portal 2 X360, slightly modified for VR.
+	highp 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 +146,10 @@ 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.
 }

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("debanding"))
+		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();

src/defaultsettings.cpp

@@ -264,6 +264,7 @@ void set_default_settings()
 	settings->setDefault("enable_waving_plants", "false");
 	settings->setDefault("exposure_compensation", "0.0");
 	settings->setDefault("enable_auto_exposure", "false");
+	settings->setDefault("debanding", "true");
 	settings->setDefault("antialiasing", "none");
 	settings->setDefault("enable_bloom", "false");
 	settings->setDefault("enable_bloom_debug", "false");
@@ -499,6 +500,7 @@ void set_default_settings()
 	settings->setDefault("active_block_range", "2");
 	settings->setDefault("viewing_range", "50");
 	settings->setDefault("leaves_style", "simple");
+	settings->setDefault("debanding", "false");
 	settings->setDefault("curl_verify_cert", "false");
 
 	// Apply settings according to screen size