Add spot reflector option to modulate light intensity with spot angle.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Lighting/HDLightEditor.Styles.cs

 
             public readonly GUIContent shape = new GUIContent("Type", "Specifies the current type of light. Possible types are Directional, Spot, Point, Rectangle and Line lights.");
             public readonly GUIContent[] shapeNames;
+            public readonly GUIContent spotBackReflector = new GUIContent("Enable light reflector", "Is the light reflected in the back in the spot cone.");
 
             // Additional shadow data
             public readonly GUIContent shadowResolution = new GUIContent("Resolution", "Controls the rendered resolution of the shadow maps. A higher resolution will increase the fidelity of shadows at the cost of GPU performance and memory usage.");

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Lighting/HDLightEditor.cs

             public SerializedProperty affectSpecular;
             public SerializedProperty lightTypeExtent;
             public SerializedProperty spotLightShape;
+            public SerializedProperty spotBackReflector;
             public SerializedProperty shapeWidth;
             public SerializedProperty shapeHeight;
             public SerializedProperty aspectRatio;
                 affectSpecular = o.Find(x => x.affectSpecular),
                 lightTypeExtent = o.Find(x => x.lightTypeExtent),
                 spotLightShape = o.Find(x => x.spotLightShape),
+                spotBackReflector = o.Find(x => x.spotBackReflector),
                 shapeWidth = o.Find(x => x.shapeWidth),
                 shapeHeight = o.Find(x => x.shapeHeight),
                 aspectRatio = o.Find(x => x.aspectRatio),
                     if (spotLightShape == SpotLightShape.Cone)
                     {
                         settings.DrawSpotAngle();
+                        EditorGUILayout.PropertyField(m_AdditionalLightData.spotBackReflector, s_Styles.spotBackReflector);
                         EditorGUILayout.Slider(m_AdditionalLightData.spotInnerPercent, 0f, 100f, s_Styles.spotInnerPercent);
                     }
                     // TODO : replace with angle and ratio
+                        EditorGUILayout.PropertyField(m_AdditionalLightData.spotBackReflector, s_Styles.spotBackReflector);
                         EditorGUILayout.Slider(m_AdditionalLightData.aspectRatio, 0.05f, 20.0f, s_Styles.aspectRatioPyramid);
                     }
                     else if (spotLightShape == SpotLightShape.Box)
                 case LightShape.Spot:
                     // Spot should used conversion which take into account the angle, and thus the intensity vary with angle.
                     // This is not easy to manipulate for lighter, so we simply consider any spot light as just occluded point light. So reuse the same code.
-                    settings.intensity.floatValue = LightUtils.ConvertPointLightIntensity(m_AdditionalLightData.punctualIntensity.floatValue);
-                    // TODO: What to do with box shape ?
-                    // var spotLightShape = (SpotLightShape)m_AdditionalLightData.spotLightShape.enumValueIndex;
+
+                    var spotLightShape = (SpotLightShape)m_AdditionalLightData.spotLightShape.enumValueIndex;
+                    if (spotLightShape == SpotLightShape.Cone)
+                    {
+                        settings.intensity.floatValue = LightUtils.ConvertSpotLightIntensity(m_AdditionalLightData.punctualIntensity.floatValue, settings.spotAngle.floatValue * Mathf.Deg2Rad, m_AdditionalLightData.spotBackReflector.boolValue );
+                    }
+                    else if (spotLightShape == SpotLightShape.Pyramid)
+                    {
+                        if (m_AdditionalLightData.spotBackReflector.boolValue)
+                        {
+                            var aspectRatio = m_AdditionalLightData.aspectRatio.floatValue;
+                            // Since the smallest angles is = to the fov, and we don't care of the angle order, simply make sure the aspect ratio is > 1
+                            if ( aspectRatio < 1f ) aspectRatio = 1f/aspectRatio;
+
+                            var angleA = settings.spotAngle.floatValue * Mathf.Deg2Rad;
+
+                            var halfAngle = angleA * 0.5f;
+                            var length = Mathf.Sin(halfAngle);
+                            length *= aspectRatio;
+                            halfAngle = Mathf.Atan(length);
+
+                            var angleB = halfAngle * 2f;
+
+                            settings.intensity.floatValue = LightUtils.ConvertFrustrumLightIntensity(m_AdditionalLightData.punctualIntensity.floatValue, angleA, angleB );
+                        }
+                        else
+                        {
+                            settings.intensity.floatValue = LightUtils.ConvertPointLightIntensity(m_AdditionalLightData.punctualIntensity.floatValue);
+                        }
+                    }
+                    else // box shape, no conversion implemented for the moment. Should use a fresnel light model.
+                    {
+                        settings.intensity.floatValue = LightUtils.ConvertPointLightIntensity(m_AdditionalLightData.punctualIntensity.floatValue);
+                    }
                     break;
 
                 case LightShape.Rectangle:

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Light/HDAdditionalLightData.cs

         // Only for Spotlight, should be hide for other light
         public SpotLightShape spotLightShape = SpotLightShape.Cone;
 
+        // Only for Spotlight, should be hide for other light
+        public bool spotBackReflector = false;
+
         // Only for Rectangle/Line/box projector lights
         public float shapeWidth = 0.5f;