Fix fog to draw Fog Cutoff Distance via a plane, not sphere test

Add support for negative Fog Cutoff Distance on fog to draw it in front of the camera Far Plane, no matter the setup.
Fix hot-reloading Fog shader in Editor.

Content/Editor/MaterialTemplates/Features/ForwardShading.hlsl

@@ -151,7 +151,7 @@ void PS_Forward(
 
 #if USE_FOG
 	// Calculate exponential height fog
-	float4 fog = GetExponentialHeightFog(ExponentialHeightFog, materialInput.WorldPosition, ViewPos, 0);
+	float4 fog = GetExponentialHeightFog(ExponentialHeightFog, materialInput.WorldPosition, ViewPos, 0, gBuffer.ViewPos.z);
 
 	// Apply fog to the output color
 #if MATERIAL_BLEND == MATERIAL_BLEND_OPAQUE

Source/Engine/Level/Actors/ExponentialHeightFog.cpp

@@ -41,11 +41,10 @@ void ExponentialHeightFog::Draw(RenderContext& renderContext)
         && _shader->IsLoaded()
         && renderContext.View.IsPerspectiveProjection())
     {
-        // Prepare
         if (_psFog.States[0] == nullptr)
-        {
-            // Create pipeline states
             _psFog.CreatePipelineStates();
+        if (!_psFog.States[0]->IsValid())
+        {
             GPUPipelineState::Description psDesc = GPUPipelineState::Description::DefaultFullscreenTriangle;
             psDesc.DepthWriteEnable = false;
             psDesc.BlendMode.BlendEnable = true;
@@ -59,6 +58,7 @@ void ExponentialHeightFog::Draw(RenderContext& renderContext)
             if (_psFog.Create(psDesc, _shader->GetShader(), "PS_Fog"))
             {
                 LOG(Warning, "Cannot create graphics pipeline state object for '{0}'.", ToString());
+                return;
             }
         }
 
@@ -160,7 +160,7 @@ void ExponentialHeightFog::GetExponentialHeightFogData(const RenderView& view, S
     result.FogAtViewPosition = density * Math::Pow(2.0f, Math::Clamp(-heightFalloff * (viewHeight - height), -125.f, 126.f));
     result.StartDistance = StartDistance;
     result.FogMinOpacity = 1.0f - FogMaxOpacity;
-    result.FogCutoffDistance = FogCutoffDistance;
+    result.FogCutoffDistance = FogCutoffDistance >= 0 ? FogCutoffDistance : view.Far + FogCutoffDistance;
     if (useDirectionalLightInscattering)
     {
         result.InscatteringLightDirection = -DirectionalInscatteringLight->GetDirection();

Source/Engine/Level/Actors/ExponentialHeightFog.h

@@ -55,9 +55,9 @@ public:
     float StartDistance = 0.0f;
 
     /// <summary>
-    /// Scene elements past this distance will not have fog applied. This is useful for excluding skyboxes which already have fog baked in. Setting this value to 0 disables it.
+    /// Scene elements past this distance will not have fog applied. This is useful for excluding skyboxes which already have fog baked in. Setting this value to 0 disables it. Negative value sets the cutoff distance relative to the far plane of the camera.
     /// </summary>
-    API_FIELD(Attributes="EditorOrder(60), DefaultValue(0.0f), Limit(0), EditorDisplay(\"Exponential Height Fog\")")
+    API_FIELD(Attributes="EditorOrder(60), DefaultValue(0.0f), EditorDisplay(\"Exponential Height Fog\")")
     float FogCutoffDistance = 0.0f;
 
 public:

Source/Shaders/ExponentialHeightFog.hlsl

@@ -29,7 +29,7 @@ struct ExponentialHeightFogData
     float StartDistance;
 };
 
-float4 GetExponentialHeightFog(ExponentialHeightFogData exponentialHeightFog, float3 posWS, float3 camWS, float skipDistance)
+float4 GetExponentialHeightFog(ExponentialHeightFogData exponentialHeightFog, float3 posWS, float3 camWS, float skipDistance, float sceneDistance)
 {
     float3 cameraToPos = posWS - camWS;
     float cameraToPosSqr = dot(cameraToPos, cameraToPos);
@@ -78,7 +78,7 @@ float4 GetExponentialHeightFog(ExponentialHeightFogData exponentialHeightFog, fl
 
     // Disable fog after a certain distance
     FLATTEN
-    if (exponentialHeightFog.FogCutoffDistance > 0 && cameraToPosLen > exponentialHeightFog.FogCutoffDistance)
+    if (exponentialHeightFog.FogCutoffDistance > 0 && sceneDistance > exponentialHeightFog.FogCutoffDistance)
     {
         expFogFactor = 1;
         directionalInscattering = 0;
@@ -87,4 +87,9 @@ float4 GetExponentialHeightFog(ExponentialHeightFogData exponentialHeightFog, fl
     return float4(inscatteringColor * (1.0f - expFogFactor) + directionalInscattering, expFogFactor);
 }
 
+float4 GetExponentialHeightFog(ExponentialHeightFogData exponentialHeightFog, float3 posWS, float3 camWS, float skipDistance)
+{
+    return GetExponentialHeightFog(exponentialHeightFog, posWS, camWS, skipDistance, distance(posWS, camWS));
+}
+
 #endif

Source/Shaders/Fog.shader

@@ -24,41 +24,17 @@ Texture2D Depth : register(t0);
 Texture3D IntegratedLightScattering : register(t1);
 #endif
 
-// Get world space position at given pixel coordinate
-float3 GetWorldPos(float2 uv)
-{
-	float depth = SAMPLE_RT(Depth, uv).r;
-	GBufferData gBufferData = GetGBufferData();
-	float3 viewPos = GetViewPos(gBufferData, uv, depth);
-	return mul(float4(viewPos, 1), gBufferData.InvViewMatrix).xyz;
-}
-
-float4 CalculateCombinedFog(float3 posWS, float sceneDepth, float3 volumeUV)
-{
-	float skipDistance = 0;
-
-#if VOLUMETRIC_FOG
-	skipDistance = max(ExponentialHeightFog.VolumetricFogMaxDistance - 100, 0);
-#endif
-
-	float4 fog = GetExponentialHeightFog(ExponentialHeightFog, posWS, GBuffer.ViewPos, skipDistance);
-
-#if VOLUMETRIC_FOG
-	float4 volumetricFog = IntegratedLightScattering.SampleLevel(SamplerLinearClamp, volumeUV, 0);
-	fog = float4(volumetricFog.rgb + fog.rgb * volumetricFog.a, volumetricFog.a * fog.a);
-#endif
-
-	return fog;
-}
-
 META_PS(true, FEATURE_LEVEL_ES2)
 META_PERMUTATION_1(VOLUMETRIC_FOG=0)
 META_PERMUTATION_1(VOLUMETRIC_FOG=1)
 float4 PS_Fog(Quad_VS2PS input) : SV_Target0
 {
-	// Calculate pixel world space position
-	float3 posWS = GetWorldPos(input.TexCoord);
-	float3 viewVector = posWS - GBuffer.ViewPos;
+    // Get world space position at given pixel coordinate
+	float rawDepth = SAMPLE_RT(Depth, input.TexCoord).r;
+	GBufferData gBufferData = GetGBufferData();
+	float3 viewPos = GetViewPos(gBufferData, input.TexCoord, rawDepth);
+	float3 worldPos = mul(float4(viewPos, 1), gBufferData.InvViewMatrix).xyz;
+	float3 viewVector = worldPos - GBuffer.ViewPos;
 	float sceneDepth = length(viewVector);
 
 	// Calculate volumetric fog coordinates
@@ -67,17 +43,28 @@ float4 PS_Fog(Quad_VS2PS input) : SV_Target0
 
 	// Debug code
 #if VOLUMETRIC_FOG && 0
-	volumeUV = posWS / 1000;
+	volumeUV = worldPos / 1000;
 	if (!all(volumeUV >= 0 && volumeUV <= 1))
 		return 0;
 
 	return float4(IntegratedLightScattering.SampleLevel(SamplerLinearClamp, volumeUV, 0).rgb, 1);
 	//return float4(volumeUV, 1);
-	//return float4(posWS / 100, 1);
+	//return float4(worldPos / 100, 1);
 #endif
 
-	// Calculate fog color
-	float4 fog = CalculateCombinedFog(posWS, sceneDepth, volumeUV);
+    float skipDistance = 0;
+#if VOLUMETRIC_FOG
+	skipDistance = max(ExponentialHeightFog.VolumetricFogMaxDistance - 100, 0);
+#endif
+
+	// Calculate exponential fog color
+	float4 fog = GetExponentialHeightFog(ExponentialHeightFog, worldPos, GBuffer.ViewPos, skipDistance, viewPos.z);
+
+#if VOLUMETRIC_FOG
+    // Sample volumetric fog and mix it in
+	float4 volumetricFog = IntegratedLightScattering.SampleLevel(SamplerLinearClamp, volumeUV, 0);
+	fog = float4(volumetricFog.rgb + fog.rgb * volumetricFog.a, volumetricFog.a * fog.a);
+#endif
 
 	return fog;
 }