[ATOM-15854] Adjusting alpha on grazing angles so it doesn't affect the diffuse response. (#1708)

Signed-off-by: Ken Pruiksma <pruiksma@amazon.com>

Gems/Atom/Feature/Common/Assets/Materials/Types/EnhancedPBR_ForwardPass.azsl

@@ -324,16 +324,21 @@ PbrLightingOutput ForwardPassPS_Common(VSOutput IN, bool isFrontFace, out float
     // Finalize Lighting
     lightingData.FinalizeLighting(surface.transmission.tint);
 
+    PbrLightingOutput lightingOutput = GetPbrLightingOutput(surface, lightingData, alpha);
+
+    // ------- Opacity -------
+
     if (o_opacity_mode == OpacityMode::Blended || o_opacity_mode == OpacityMode::TintedTransparent)
     {
-        float fresnelAlpha = FresnelSchlickWithRoughness(lightingData.NdotV, alpha, surface.roughnessLinear).x; // Increase opacity at grazing angles.
+        // Increase opacity at grazing angles for surfaces with a low m_opacityAffectsSpecularFactor.
+        // For m_opacityAffectsSpecularFactor values close to 0, that indicates a transparent surface
+        // like glass, so it becomes less transparent at grazing angles. For m_opacityAffectsSpecularFactor
+        // values close to 1.0, that indicates the absence of a surface entirely, so this effect should
+        // not apply.
+        float fresnelAlpha = FresnelSchlickWithRoughness(lightingData.NdotV, alpha, surface.roughnessLinear).x;
         alpha = lerp(fresnelAlpha, alpha, MaterialSrg::m_opacityAffectsSpecularFactor);
     }
 
-    PbrLightingOutput lightingOutput = GetPbrLightingOutput(surface, lightingData, alpha);
-
-    // ------- Opacity -------
-
     // Note: lightingOutput rendertargets are not always used as named, particularly m_diffuseColor (target 0) and
     // m_specularColor (target 1). Comments below describe the differences when appropriate.
 
@@ -347,11 +352,13 @@ PbrLightingOutput ForwardPassPS_Common(VSOutput IN, bool isFrontFace, out float
         // It's done this way because surface transparency doesn't really change specular response (eg, glass).
 
         lightingOutput.m_diffuseColor.rgb *= lightingOutput.m_diffuseColor.w; // pre-multiply diffuse
-
+        
         // Add specular. m_opacityAffectsSpecularFactor controls how much the alpha masks out specular contribution.
         float3 specular = lightingOutput.m_specularColor.rgb;
         specular = lerp(specular, specular * lightingOutput.m_diffuseColor.w, MaterialSrg::m_opacityAffectsSpecularFactor);
         lightingOutput.m_diffuseColor.rgb += specular;
+
+        lightingOutput.m_diffuseColor.w = alpha;
     }
     else if (o_opacity_mode == OpacityMode::TintedTransparent)
     {
@@ -374,7 +381,7 @@ PbrLightingOutput ForwardPassPS_Common(VSOutput IN, bool isFrontFace, out float
         specular = lerp(specular, specular * lightingOutput.m_diffuseColor.w, MaterialSrg::m_opacityAffectsSpecularFactor);
         lightingOutput.m_diffuseColor.rgb += specular;
 
-        lightingOutput.m_specularColor.rgb = baseColor * (1.0 - lightingOutput.m_diffuseColor.w); 
+        lightingOutput.m_specularColor.rgb = baseColor * (1.0 - alpha); 
     }
     else
     {