From 5b2e54c17b6118e2248c0614f3c0b903f34010b1 Mon Sep 17 00:00:00 2001
From: mandryskowski <nedesf@gmail.com>
Date: Wed, 26 Jul 2023 13:39:10 +0200
Subject: [PATCH] Fresnel should darken the SSR reflections instead of blending
 them with specular light.

---
 .../effects/screen_space_reflection.glsl      | 47 ++++++++++++-------
 1 file changed, 29 insertions(+), 18 deletions(-)

diff --git a/servers/rendering/renderer_rd/shaders/effects/screen_space_reflection.glsl b/servers/rendering/renderer_rd/shaders/effects/screen_space_reflection.glsl
index 1e01d945334c..fecf70bd019c 100644
--- a/servers/rendering/renderer_rd/shaders/effects/screen_space_reflection.glsl
+++ b/servers/rendering/renderer_rd/shaders/effects/screen_space_reflection.glsl
@@ -184,7 +184,7 @@ void main() {
 			vec4 test_normal_roughness = imageLoad(source_normal_roughness, test_pos);
 			vec3 test_normal = test_normal_roughness.xyz * 2.0 - 1.0;
 			test_normal = normalize(test_normal);
-			test_normal.y = -test_normal.y; //because this code reads flipped
+			test_normal.y = -test_normal.y; // Because this code reads flipped.
 
 			if (dot(ray_dir, test_normal) < 0.001) {
 				// if depth was surpassed
@@ -203,6 +203,7 @@ void main() {
 
 	if (found) {
 		float margin_blend = 1.0;
+		vec2 final_pos = pos;
 
 		vec2 margin = vec2((params.screen_size.x + params.screen_size.y) * 0.05); // make a uniform margin
 		if (any(bvec4(lessThan(pos, vec2(0.0, 0.0)), greaterThan(pos, params.screen_size)))) {
@@ -219,16 +220,40 @@ void main() {
 			//margin_blend = 1.0;
 		}
 
-		vec2 final_pos;
+		// Fade In / Fade Out
 		float grad = (steps_taken + 1.0) / float(params.num_steps);
 		float initial_fade = params.curve_fade_in == 0.0 ? 1.0 : pow(clamp(grad, 0.0, 1.0), params.curve_fade_in);
 		float fade = pow(clamp(1.0 - grad, 0.0, 1.0), params.distance_fade) * initial_fade;
+
+		// Ensure that precision errors do not introduce any fade. Even if it is just slightly below 1.0,
+		// strong specular light can leak through the reflection.
+		if (fade > 0.999) {
+			fade = 1.0;
+		}
+
 		// This is an ad-hoc term to fade out the SSR as roughness increases. Values used
 		// are meant to match the visual appearance of a ReflectionProbe.
 		float roughness_fade = smoothstep(0.4, 0.7, 1.0 - normal_roughness.w);
-		final_pos = pos;
 
-		vec4 final_color;
+		// Schlick term.
+		float metallic = texelFetch(source_metallic, ssC << 1, 0).w;
+
+		// F0 is the reflectance of normally incident light (perpendicular to the surface).
+		// Dielectric materials have a widely accepted default value of 0.04. We assume that metals reflect all light, so their F0 is 1.0.
+		float f0 = mix(0.04, 1.0, metallic);
+		float m = clamp(1.0 - dot(normal, -view_dir), 0.0, 1.0);
+		float m2 = m * m;
+		m = m2 * m2 * m; // pow(m,5)
+		float fresnel_term = f0 + (1.0 - f0) * m; // Fresnel Schlick term.
+
+		// The alpha value of final_color controls the blending with specular light in specular_merge.glsl.
+		// Note that the Fresnel term is multiplied with the RGB color instead of being a part of the alpha value.
+		// There is a key difference:
+		// - multiplying a term with RGB darkens the SSR light without introducing/taking away specular light.
+		// - combining a term into the Alpha value introduces specular light at the expense of the SSR light.
+		vec4 final_color = vec4(imageLoad(source_diffuse, ivec2(final_pos - 0.5)).rgb * fresnel_term, fade * margin_blend * roughness_fade);
+
+		imageStore(ssr_image, ssC, final_color);
 
 #ifdef MODE_ROUGH
 
@@ -259,20 +284,6 @@ void main() {
 
 #endif // MODE_ROUGH
 
-		final_color = vec4(imageLoad(source_diffuse, ivec2(final_pos - 0.5)).rgb, fade * margin_blend * roughness_fade);
-
-		// Schlick term.
-		float metallic = texelFetch(source_metallic, ssC << 1, 0).w;
-		// F0 is the reflectance of normally incident light (perpendicular to the surface).
-		// Dielectric materials have a widely accepted default value of 0.04. We assume that metals reflect all light, so their F0 is 1.0.
-		float f0 = mix(0.04, 1.0, metallic);
-		float m = clamp(1.0 - dot(normal, -view_dir), 0.0, 1.0);
-		float m2 = m * m;
-		m = m2 * m2 * m; // pow(m,5)
-		final_color.a *= f0 + (1.0 - f0) * m; // Fresnel Schlick term.
-
-		imageStore(ssr_image, ssC, final_color);
-
 	} else {
 #ifdef MODE_ROUGH
 		imageStore(blur_radius_image, ssC, vec4(0.0));