Add explanations for default constant parameters + define as separate const variables (https://github.com/o3de/o3de/pull/6428#discussion_r776073076, https://github.com/o3de/o3de/pull/6428#discussion_r776072881, https://github.com/o3de/o3de/pull/6428#discussion_r776069391)

Signed-off-by: Santi Paprika <santi.gonzalez.cs@gmail.com>

Gems/Atom/Feature/Common/Assets/ShaderLib/Atom/Features/PBR/BackLighting.azsli

@@ -75,8 +75,9 @@ float3 GetBackLighting(Surface surface, LightingData lightingData, float3 lightI
                 // Increase angle of influence (angle(N,L) -> angle(N,L) + acos(transmissionNdLBias)) to smooth transition regions
                 float3 E = surface.albedo * saturate(lightingData.transmissionNdLBias + dot(-surface.normal, dirToLight));
 
-                // Transmission distance modulated by hardcoded constant (could be exposed as a weight of scattering distance for transmission)
-                float s = transmissionDistance * 100.0;
+                // Transmission distance modulated by hardcoded constant C (could be exposed as a weight of scattering distance for transmission)
+                const float C = 100.0f;
+                float s = transmissionDistance * C;
                 
                 // Use scattering color to weight thin object transmission color
                 const float3 invScattering = rcp(transmissionParams.xyz);

Gems/Atom/Feature/Common/Assets/ShaderLib/Atom/Features/PBR/Lights/CapsuleLight.azsli

@@ -135,8 +135,15 @@ void ApplyCapsuleLight(ViewSrg::CapsuleLight light, Surface surface, inout Light
         float3 closestIntersectionPoint = startPoint + closestT * startToEnd;
         float3 posToLight = closestIntersectionPoint - surface.position;
 
-        // Tranmission contribution
-        lightingData.translucentBackLighting += GetBackLighting(surface, lightingData, lightIntensity, normalize(posToLight), -1.0f, 0.0f);
+        // Transmission contribution
+        // We cannot compute the actual transmission distance so we want to:
+        // - If transmission mode is thick object -> use transmission thickness parameter instead
+        // - If transmission mode is thin object -> ignore back lighting
+        // To detect and apply this behavior in the GetBackLighting function, we need to use a negative transmissionDistance 
+        const float transmissionDistance = -1.0f;
+        // If the transmissionDistance is ignored then the attenuation distance (only used on thin objects) does not have any influence
+        const float attenuationDistance = 0.0f;
+        lightingData.translucentBackLighting += GetBackLighting(surface, lightingData, lightIntensity, normalize(posToLight), transmissionDistance, attenuationDistance);
 
         // Calculate the offset from the nearest point on the reflection vector to the nearest point on the capsule light
         float3 posToClosestPointAlongReflection = dot(posToLight, reflectionDir) * reflectionDir;

Gems/Atom/Feature/Common/Assets/ShaderLib/Atom/Features/PBR/Lights/DirectionalLight.azsli

@@ -20,7 +20,9 @@ void ApplyDirectionalLights(Surface surface, inout LightingData lightingData)
     float litRatio = 1.0f;
     float camToSurfDist = distance(ViewSrg::m_worldPosition, surface.position);
 
-    // Distance travelled by the light inside the object. If not redefined, it will take mode-specific null behaviors (see GetBackLighting())
+    // Distance travelled by the light inside the object. If not redefined to a non-negative value, it will take the following behavior:
+    // - If transmission mode is thick object -> use transmission thickness parameter instead
+    // - If transmission mode is thin object -> ignore back lighting
     float transmissionDistance = -1.0f;
 
     if (o_enableShadows && shadowIndex <  SceneSrg::m_directionalLightCount)

Gems/Atom/Feature/Common/Assets/ShaderLib/Atom/Features/PBR/Lights/DiskLight.azsli

@@ -77,8 +77,9 @@ void ApplyDiskLight(ViewSrg::DiskLight light, Surface surface, inout LightingDat
         // shadow
         float litRatio = 1.0;
 
-
-        // Distance travelled by the light inside the object. If not redefined, it will take mode-specific null behaviors (see GetBackLighting())
+        // Distance travelled by the light inside the object. If not redefined to a non-negative value, it will take the following behavior:
+        // - If transmission mode is thick object -> use transmission thickness parameter instead
+        // - If transmission mode is thin object -> ignore back lighting
         float transmissionDistance = -1.0f;
 
         if (o_enableShadows)

Gems/Atom/Feature/Common/Assets/ShaderLib/Atom/Features/PBR/Lights/LightTypesCommon.azsli

@@ -49,6 +49,16 @@ void AddSampleContribution(
     float3 intensityRgb = float3(intensity, intensity, intensity);
 
     diffuseAcc += GetDiffuseLighting(surface, lightingData, intensityRgb, posToLightSampleDir);
-    translucentAcc += GetBackLighting(surface, lightingData, intensityRgb, posToLightSampleDir, -1.0f, 0.0f);
+
+    // Transmission contribution
+    // We cannot compute the actual transmission distance so we want to:
+    // - If transmission mode is thick object -> use transmission thickness parameter instead
+    // - If transmission mode is thin object -> ignore back lighting
+    // To detect and apply this behavior in the GetBackLighting function, we need to use a negative transmissionDistance 
+    const float transmissionDistance = -1.0f;
+    // If the transmissionDistance is ignored then the attenuation distance (only used on thin objects) does not have any influence
+    const float attenuationDistance = 0.0f;
+    translucentAcc += GetBackLighting(surface, lightingData, intensityRgb, posToLightSampleDir, transmissionDistance, attenuationDistance);
+
     specularAcc += GetSpecularLighting(surface, lightingData, intensityRgb, posToLightSampleDir);
 }

Gems/Atom/Feature/Common/Assets/ShaderLib/Atom/Features/PBR/Lights/PointLight.azsli

@@ -85,7 +85,9 @@ void ApplyPointLight(ViewSrg::PointLight light, Surface surface, inout LightingD
         // shadow
         float litRatio = 1.0;
 
-        // Distance travelled by the light inside the object. If not redefined, it will take mode-specific null behaviors (see GetBackLighting())
+        // Distance travelled by the light inside the object. If not redefined to a non-negative value, it will take the following behavior:
+        // - If transmission mode is thick object -> use transmission thickness parameter instead
+        // - If transmission mode is thin object -> ignore back lighting
         float transmissionDistance = -1.0f;
 
         if (o_enableShadows)

Gems/Atom/Feature/Common/Assets/ShaderLib/Atom/Features/PBR/Lights/QuadLight.azsli

@@ -148,13 +148,21 @@ void ApplyQuadLight(ViewSrg::QuadLight light, Surface surface, inout LightingDat
                 GetDiffuseLighting(surface, lightingData, intensity, dirToLightCenter)
             );
             
+            // Transmission contribution
+            // We cannot compute the actual transmission distance so we want to:
+            // - If transmission mode is thick object -> use transmission thickness parameter instead
+            // - If transmission mode is thin object -> ignore back lighting
+            // To detect and apply this behavior in the GetBackLighting function, we need to use a negative transmissionDistance 
+            const float transmissionDistance = -1.0f;
+            // If the transmissionDistance is ignored then the attenuation distance (only used on thin objects) does not have any influence
+            const float attenuationDistance = 0.0f;
             lightingData.translucentBackLighting +=
             (
-                GetBackLighting(surface, lightingData, intensity, p0, -1.0f, 0.0f) +
-                GetBackLighting(surface, lightingData, intensity, p1, -1.0f, 0.0f) +
-                GetBackLighting(surface, lightingData, intensity, p2, -1.0f, 0.0f) +
-                GetBackLighting(surface, lightingData, intensity, p3, -1.0f, 0.0f) +
-                GetBackLighting(surface, lightingData, intensity, dirToLightCenter, -1.0f, 0.0f)
+                GetBackLighting(surface, lightingData, intensity, p0, transmissionDistance, attenuationDistance) +
+                GetBackLighting(surface, lightingData, intensity, p1, transmissionDistance, attenuationDistance) +
+                GetBackLighting(surface, lightingData, intensity, p2, transmissionDistance, attenuationDistance) +
+                GetBackLighting(surface, lightingData, intensity, p3, transmissionDistance, attenuationDistance) +
+                GetBackLighting(surface, lightingData, intensity, dirToLightCenter, transmissionDistance, attenuationDistance)
             );
             
             // Calculate specular by choosing a single representative point on the light's surface based on the reflection ray