Shader variants reduced by 50% by removing fog EXP and combining DIRLIGHTMAP_COMBINED with LIGHTMAP_ON

ScriptableRenderPipeline/LightweightPipeline/LWRP/LightweightPipeline.cs

             CoreUtils.SetKeyword(cmd, "_MIXED_LIGHTING_SUBTRACTIVE", m_MixedLightingSetup == MixedLightingSetup.Subtractive);
             CoreUtils.SetKeyword(cmd, "_VERTEX_LIGHTS", vertexLightsCount > 0);
             CoreUtils.SetKeyword(cmd, "SOFTPARTICLES_ON", m_Asset.RequireCameraDepthTexture);
+
+            bool linearFogModeEnabled = false;
+            bool exponentialFogModeEnabled = false;
+            if (RenderSettings.fog)
+            {
+                if (RenderSettings.fogMode == FogMode.Linear)
+                    linearFogModeEnabled = true;
+                else
+                    exponentialFogModeEnabled = true;
+            }
+
+            CoreUtils.SetKeyword(cmd, "FOG_LINEAR", linearFogModeEnabled);
+            CoreUtils.SetKeyword(cmd, "FOG_EXP2", exponentialFogModeEnabled);
         }
 
         private bool RenderShadows(ref CullResults cullResults, ref VisibleLight shadowLight, int shadowLightIndex, ref ScriptableRenderContext context)

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightShaderLibrary/Core.hlsl

     return normalize(mul(normalTangent, tangentToWorld));
 }
 
-float ComputeFogFactor(float z)
+half ComputeFogFactor(float z)
 {
     float clipZ_01 = UNITY_Z_0_FAR_FROM_CLIPSPACE(z);
 
     return half(fogFactor);
-#elif defined(FOG_EXP)
-    // factor = exp(-density*z)
-    float unityFogFactor = unity_FogParams.y * clipZ_01;
-    return half(saturate(exp2(-unityFogFactor)));
-    float unityFogFactor = unity_FogParams.x * clipZ_01;
-    return half(saturate(exp2(-unityFogFactor*unityFogFactor)));
+    // -density * z computed at vertex
+    return half(unity_FogParams.x * clipZ_01);
 #else
     return 0.0h;
 #endif
 {
-#if defined(FOG_LINEAR) || defined(FOG_EXP) || defined(FOG_EXP2)
+#if defined (FOG_LINEAR) || defined(FOG_EXP2)
+#if defined(FOG_EXP2)
+    // factor = exp(-(density*z)^2)
+    // fogFactor = density*z compute at vertex
+    fogFactor = saturate(exp2(-fogFactor*fogFactor));
+#endif
     color = lerp(fogColor, color, fogFactor);
 #endif
 }

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightStandard.shader

             #pragma multi_compile _ _ADDITIONAL_LIGHTS
             #pragma multi_compile _ _VERTEX_LIGHTS
             #pragma multi_compile _ _MIXED_LIGHTING_SUBTRACTIVE
+            #pragma multi_compile _ FOG_LINEAR FOG_EXP2
-            #pragma multi_compile _ LIGHTMAP_ON
-            #pragma multi_compile _ DIRLIGHTMAP_COMBINED
-            #pragma multi_compile_fog
+            #pragma multi_compile _ DIRLIGHTMAP_COMBINED LIGHTMAP_ON
+
+            // LW doesn't support dynamic GI. So we save 30% shader variants if we assume
+            // LIGHTMAP_ON when DIRLIGHTMAP_COMBINED is set
+            #ifdef DIRLIGHTMAP_COMBINED
+            #define LIGHTMAP_ON
+            #endif
 
             #pragma vertex LitPassVertex
             #pragma fragment LitPassFragment

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightStandardSimpleLighting.shader

             #pragma multi_compile _ _MAIN_LIGHT_DIRECTIONAL_SHADOW _MAIN_LIGHT_DIRECTIONAL_SHADOW_CASCADE _MAIN_LIGHT_DIRECTIONAL_SHADOW_SOFT _MAIN_LIGHT_DIRECTIONAL_SHADOW_CASCADE_SOFT _MAIN_LIGHT_SPOT_SHADOW _MAIN_LIGHT_SPOT_SHADOW_SOFT
             #pragma multi_compile _ _MAIN_LIGHT_COOKIE
             #pragma multi_compile _ _ADDITIONAL_LIGHTS
-            #pragma multi_compile _ _VERTEX_LIGHTS            
+            #pragma multi_compile _ _VERTEX_LIGHTS
+            #pragma multi_compile _ FOG_LINEAR FOG_EXP2
-            #pragma multi_compile _ LIGHTMAP_ON
-            #pragma multi_compile _ DIRLIGHTMAP_COMBINED
-            #pragma multi_compile_fog
+            #pragma multi_compile _ DIRLIGHTMAP_COMBINED LIGHTMAP_ON
+
+            // LW doesn't support dynamic GI. So we save 30% shader variants if we assume
+            // LIGHTMAP_ON when DIRLIGHTMAP_COMBINED is set
+            #ifdef DIRLIGHTMAP_COMBINED
+            #define LIGHTMAP_ON
+            #endif
 
             #pragma vertex LitPassVertex
             #pragma fragment LitPassFragmentSimple

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightStandardTerrain.shader

             #pragma vertex SplatmapVert
             #pragma fragment SpatmapFragment
 
+            // -------------------------------------
+            // Lightweight Pipeline keywords
+            // We have no good approach exposed to skip shader variants, e.g, ideally we would like to skip _CASCADE for all puctual lights
+            // Lightweight combines light classification and shadows keywords to reduce shader variants.
+            // Lightweight shader library declares defines based on these keywords to avoid having to check them in the shaders
+            // Core.hlsl defines _MAIN_LIGHT_DIRECTIONAL and _MAIN_LIGHT_SPOT (point lights can't be main light)
+            // Shadow.hlsl defines _SHADOWS_ENABLED, _SHADOWS_SOFT, _SHADOWS_CASCADE, _SHADOWS_PERSPECTIVE
-            #pragma multi_compile _ UNITY_SINGLE_PASS_STEREO STEREO_INSTANCING_ON STEREO_MULTIVIEW_ON
-            #pragma multi_compile _ LIGHTMAP_ON
-            #pragma multi_compile _ DIRLIGHTMAP_COMBINED
-            #pragma multi_compile_fog
+            #pragma multi_compile _ FOG_LINEAR FOG_EXP2
+            // -------------------------------------
+            // Unity defined keywords
+            #pragma multi_compile _ UNITY_SINGLE_PASS_STEREO STEREO_INSTANCING_ON STEREO_MULTIVIEW_ON
+            #pragma multi_compile _ DIRLIGHTMAP_COMBINED LIGHTMAP_ON
+
+            // LW doesn't support dynamic GI. So we save 30% shader variants if we assume
+            // LIGHTMAP_ON when DIRLIGHTMAP_COMBINED is set
+            #ifdef DIRLIGHTMAP_COMBINED
+            #define LIGHTMAP_ON
+            #endif
 
             #include "LightweightShaderLibrary/Lighting.hlsl"