Add support for transparent queue priority

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Material/Unlit/BaseUnlitUI.cs

             public static GUIContent transparentDepthPrepassEnableText = new GUIContent("Enable transparent depth prepass", "It allow to to fill depth buffer to improve sorting");
             public static GUIContent transparentDepthPostpassEnableText = new GUIContent("Enable transparent depth postpass", "It allow to fill depth buffer for postprocess effect like DOF");
             public static GUIContent transparentBackfaceEnableText = new GUIContent("Enable back then front rendering", "It allow to better sort transparent mesh by first rendering back faces then front faces in two separate drawcall");
+
+            public static GUIContent transparentQueuePriorityText = new GUIContent("Transparent Queue Priority", "Allow to define priority (from -100 to +100) to solve sorting issue with transparent");
             public static GUIContent enableTransparentFogText = new GUIContent("Enable fog", "Enable fog on transparent material");
             public static GUIContent enableBlendModePreserveSpecularLightingText = new GUIContent("Blend preserve specular lighting", "Blend mode will only affect diffuse lighting, allowing correct specular lighting (reflection) on transparent object");
 
         protected const string kTransparentDepthPostpassEnable = "_TransparentDepthPostpassEnable";
         protected MaterialProperty transparentBackfaceEnable = null;
         protected const string kTransparentBackfaceEnable = "_TransparentBackfaceEnable";
+        protected MaterialProperty transparentQueuePriority = null;
+        protected const string kTransparentQueuePriority = "_TransparentQueuePriority";
         protected MaterialProperty doubleSidedEnable = null;
         protected const string kDoubleSidedEnable = "_DoubleSidedEnable";
         protected MaterialProperty blendMode = null;
             transparentDepthPostpassEnable = FindProperty(kTransparentDepthPostpassEnable, props, false);
             transparentBackfaceEnable = FindProperty(kTransparentBackfaceEnable, props, false);
 
+            transparentQueuePriority = FindProperty(kTransparentQueuePriority, props, false);
+
             doubleSidedEnable = FindProperty(kDoubleSidedEnable, props, false);
             blendMode = FindProperty(kBlendMode, props, false);
 
             if (transparentBackfaceEnable != null && ((SurfaceType)surfaceType.floatValue == SurfaceType.Transparent))
                 m_MaterialEditor.ShaderProperty(transparentBackfaceEnable, StylesBaseUnlit.transparentBackfaceEnableText);
 
+            if (transparentQueuePriority != null && ((SurfaceType)surfaceType.floatValue == SurfaceType.Transparent))
+            {
+                EditorGUI.BeginChangeCheck();
+                m_MaterialEditor.ShaderProperty(transparentQueuePriority, StylesBaseUnlit.transparentQueuePriorityText);
+                if (EditorGUI.EndChangeCheck())
+                {
+                    transparentQueuePriority.floatValue = Mathf.Clamp((int)transparentQueuePriority.floatValue, -(int)HDRenderQueuePriority.TransparentPriorityQueueRange, (int)HDRenderQueuePriority.TransparentPriorityQueueRange);
+                }
+            }
+
             // This function must finish with double sided option (see LitUI.cs)
             if (doubleSidedEnable != null)
             {
                 material.SetOverrideTag("RenderType", "Transparent");
                 material.SetInt("_ZWrite", 0);
                 var isPrepass = material.HasProperty(kPreRefractionPass) && material.GetFloat(kPreRefractionPass) > 0.0f;
-                material.renderQueue = (int)(isPrepass ? HDRenderQueue.PreRefraction : HDRenderQueue.Transparent);
+                material.renderQueue = (int)(isPrepass ? HDRenderQueue.PreRefraction : HDRenderQueue.Transparent) + (int)material.GetFloat(kTransparentQueuePriority);
 
                 if (material.HasProperty(kBlendMode))
                 {

ScriptableRenderPipeline/HDRenderPipeline/HDRP/HDRenderPipeline.cs

             "Forward Transparent"
         };
 
-        static readonly RenderQueueRange k_RenderQueue_PreRefraction = new RenderQueueRange { min = (int)HDRenderQueue.PreRefraction, max = (int)HDRenderQueue.Transparent - 1 };
-        static readonly RenderQueueRange k_RenderQueue_Transparent = new RenderQueueRange { min = (int)HDRenderQueue.Transparent, max = (int)HDRenderQueue.Overlay - 1 };
-        static readonly RenderQueueRange k_RenderQueue_AllTransparent = new RenderQueueRange { min = (int)HDRenderQueue.PreRefraction, max = (int)HDRenderQueue.Overlay - 1 };
+        static readonly RenderQueueRange k_RenderQueue_PreRefraction = new RenderQueueRange { min = (int)HDRenderQueue.PreRefractionMin, max = (int)HDRenderQueue.PreRefractionMax };
+        static readonly RenderQueueRange k_RenderQueue_Transparent = new RenderQueueRange { min = (int)HDRenderQueue.TransparentMin, max = (int)HDRenderQueue.TransparentMax };
+        static readonly RenderQueueRange k_RenderQueue_AllTransparent = new RenderQueueRange { min = (int)HDRenderQueue.PreRefractionMin, max = (int)HDRenderQueue.TransparentMax };
 
         readonly HDRenderPipelineAsset m_Asset;
 

ScriptableRenderPipeline/HDRenderPipeline/HDRP/HDRenderQueue.cs

 
 namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
+    // In HD we don't expose HDRenderQueue instead we create as much value as needed in the enum for our different pass
+    // and use inspector to manipulate the value.
+    // In the case of transparent we want to use RenderQueue to help with sorting. We define a neutral value for the RenderQueue and priority going from -X to +X
+    // going from -X to +X instead of 0 to +X as builtin Unity is better for artists as they can decide late to sort behind or in front of the scene.
+
+    public enum HDRenderQueuePriority
+    {
+        TransparentPriorityQueueRange = 100
+    }
+
     public enum HDRenderQueue
     {
         Background = UnityEngine.Rendering.RenderQueue.Background,
+        // For transparent pass we define a range of 200 value to define the priority
+        // Warning: Be sure no range are overlapping
+        PreRefractionMin = 2750 - HDRenderQueuePriority.TransparentPriorityQueueRange,
+        PreRefractionMax = 2750 + HDRenderQueuePriority.TransparentPriorityQueueRange,
+        TransparentMin = UnityEngine.Rendering.RenderQueue.Transparent - HDRenderQueuePriority.TransparentPriorityQueueRange,
+        TransparentMax = UnityEngine.Rendering.RenderQueue.Transparent + HDRenderQueuePriority.TransparentPriorityQueueRange,
         Overlay = UnityEngine.Rendering.RenderQueue.Overlay
     }
 }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/LayeredLit/LayeredLit.shader

         [ToggleUI] _AlphaCutoffEnable("Alpha Cutoff Enable", Float) = 0.0
 
         _AlphaCutoff("Alpha Cutoff", Range(0.0, 1.0)) = 0.5
+        _TransparentQueuePriority("_TransparentQueuePriority", Float) = 0
 
         // Stencil state
         [HideInInspector] _StencilRef("_StencilRef", Int) = 2 // StencilLightingUsage.RegularLighting

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/LayeredLit/LayeredLitTessellation.shader

         [ToggleUI] _AlphaCutoffEnable("Alpha Cutoff Enable", Float) = 0.0
 
         _AlphaCutoff("Alpha Cutoff", Range(0.0, 1.0)) = 0.5
+        _TransparentQueuePriority("_TransparentQueuePriority", Float) = 0
 
         // Stencil state
         [HideInInspector] _StencilRef("_StencilRef", Int) = 2 // StencilLightingUsage.RegularLighting

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/Lit.shader

         [ToggleUI] _TransparentDepthPrepassEnable("_TransparentDepthPrepassEnable", Float) = 0.0
         [ToggleUI] _TransparentBackfaceEnable("_TransparentBackfaceEnable", Float) = 0.0
         [ToggleUI] _TransparentDepthPostpassEnable("_TransparentDepthPostpassEnable", Float) = 0.0
+        _TransparentQueuePriority("_TransparentQueuePriority", Float) = 0
 
         // Transparency
         [Enum(None, 0, Plane, 1, Sphere, 2)]_RefractionMode("Refraction Mode", Int) = 0

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/LitTessellation.shader

         [ToggleUI] _TransparentDepthPrepassEnable("_TransparentDepthPrepassEnable", Float) = 0.0
         [ToggleUI] _TransparentBackfaceEnable("_TransparentBackfaceEnable", Float) = 0.0
         [ToggleUI] _TransparentDepthPostpassEnable("_TransparentDepthPostpassEnable", Float) = 0.0
+        _TransparentQueuePriority("_TransparentQueuePriority", Float) = 0
 
         // Transparency
         [Enum(None, 0, Plane, 1, Sphere, 2)]_RefractionMode("Refraction Mode", Int) = 0

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Unlit/Unlit.shader

 
         [ToggleUI]  _AlphaCutoffEnable("Alpha Cutoff Enable", Float) = 0.0
         _AlphaCutoff("Alpha Cutoff", Range(0.0, 1.0)) = 0.5
+        _TransparentQueuePriority("_TransparentQueuePriority", Float) = 0
 
         // Blending state
         [HideInInspector] _SurfaceType("__surfacetype", Float) = 0.0