Register shadows OnDemand to the cached shadow system only when rendered for the first time. (#6825)

* Fix a couple of issues

* Changelog

* Change how the registering to the sysem works for ondemand

* Changelog

* Fix wrong warning

Co-authored-by: sebastienlagarde <sebastien@unity3d.com>

Author: FrancescoC-unity

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -80,6 +80,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed an issue where sometimes full screen debug would cause render graph errors.
 - Fixed a nullref exception when creating a new scene while LightExplorer is open.
 - Fixed issue that caused the uber post process to run even if nothing is to be done, leading to different results when disabling every post process manually vs disabling the whole post-processing pipeline.
+- Fixed issue that placed an OnDemand shadow in the atlas before it was ever rendered.
 
 ## [14.0.0] - 2021-11-17
 

com.unity.render-pipelines.high-definition/Editor/Lighting/HDLightUI.cs

@@ -1051,7 +1051,8 @@ static void DrawShadowMapContent(SerializedHDLight serialized, Editor owner)
                     {
                         HDLightEditor editor = owner as HDLightEditor;
                         var additionalLightData = editor.GetAdditionalDataForTargetIndex(0);
-                        if (!HDCachedShadowManager.instance.LightHasBeenPlacedInAtlas(additionalLightData))
+                        // If the light was registered, but not placed it means it doesn't fit.
+                        if (additionalLightData.lightIdxForCachedShadows >= 0 && !HDCachedShadowManager.instance.LightHasBeenPlacedInAtlas(additionalLightData))
                         {
                             string warningMessage = "The shadow for this light doesn't fit the cached shadow atlas and therefore won't be rendered. Please ensure you have enough space in the cached shadow atlas. You can use the light explorer (Window->Rendering->Light Explorer) to see which lights fit and which don't.\nConsult HDRP Shadow documentation for more information about cached shadow management.";
                             // Loop backward in "tile" size to check

com.unity.render-pipelines.high-definition/Runtime/Lighting/Light/HDAdditionalLightData.cs

@@ -1549,7 +1549,9 @@ public ShadowUpdateMode shadowUpdateMode
                 }
                 else if (legacyLight.shadows != LightShadows.None && m_ShadowUpdateMode == ShadowUpdateMode.EveryFrame && value != ShadowUpdateMode.EveryFrame)
                 {
-                    HDShadowManager.cachedShadowManager.RegisterLight(this);
+                    // If we are OnDemand not rendered on placement, we defer the registering of the light until the rendering is requested.
+                    if (!(shadowUpdateMode == ShadowUpdateMode.OnDemand && !onDemandShadowRenderOnPlacement))
+                        HDShadowManager.cachedShadowManager.RegisterLight(this);
                 }
 
                 m_ShadowUpdateMode = value;
@@ -3239,7 +3241,9 @@ internal void RefreshCachedShadow()
 
             if (!ShadowIsUpdatedEveryFrame() && legacyLight.shadows != LightShadows.None)
             {
-                HDShadowManager.cachedShadowManager.RegisterLight(this);
+                // If we are OnDemand not rendered on placement, we defer the registering of the light until the rendering is requested.
+                if (!(shadowUpdateMode == ShadowUpdateMode.OnDemand && !onDemandShadowRenderOnPlacement))
+                    HDShadowManager.cachedShadowManager.RegisterLight(this);
             }
         }
 
@@ -3664,9 +3668,11 @@ internal void CreateHDLightRenderEntity(bool autoDestroy = false)
 
         void OnEnable()
         {
-            if (shadowUpdateMode != ShadowUpdateMode.EveryFrame && legacyLight.shadows != LightShadows.None)
+            if (!ShadowIsUpdatedEveryFrame() && legacyLight.shadows != LightShadows.None)
             {
-                HDShadowManager.cachedShadowManager.RegisterLight(this);
+                // If we are OnDemand not rendered on placement, we defer the registering of the light until the rendering is requested.
+                if (!(shadowUpdateMode == ShadowUpdateMode.OnDemand && !onDemandShadowRenderOnPlacement))
+                    HDShadowManager.cachedShadowManager.RegisterLight(this);
             }
 
             SetEmissiveMeshRendererEnabled(true);

com.unity.render-pipelines.high-definition/Runtime/Lighting/Shadow/HDCachedShadowAtlas.cs

@@ -557,7 +557,6 @@ internal void ScheduleShadowUpdate(HDAdditionalLightData lightData)
             if (!lightData.isActiveAndEnabled) return;
 
             int lightIdx = lightData.lightIdxForCachedShadows;
-            Debug.Assert(lightIdx >= 0);
 
             if (!m_PlacedShadows.ContainsKey(lightIdx))
             {