Fix intensity for line light

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

             //Disc,
         }
 
-        const float k_LineWidth = 0.01f; // Provide a small size of 1cm for line light
+        const float k_MinAreaWidth = 0.01f; // Provide a small size of 1cm for line light
 
         // Used for UI only; the processing code must use LightTypeExtent and LightType
         LightShape m_LightShape;
                     m_AdditionalLightData.lightTypeExtent.enumValueIndex = (int)LightTypeExtent.Rectangle;
                     EditorGUILayout.PropertyField(m_AdditionalLightData.shapeWidth, s_Styles.shapeWidthRect);
                     EditorGUILayout.PropertyField(m_AdditionalLightData.shapeHeight, s_Styles.shapeHeightRect);
-                    m_AdditionalLightData.shapeWidth.floatValue = Mathf.Max(m_AdditionalLightData.shapeWidth.floatValue, k_LineWidth);
-                    m_AdditionalLightData.shapeHeight.floatValue = Mathf.Max(m_AdditionalLightData.shapeHeight.floatValue, k_LineWidth);
+                    m_AdditionalLightData.shapeWidth.floatValue = Mathf.Max(m_AdditionalLightData.shapeWidth.floatValue, k_MinAreaWidth);
+                    m_AdditionalLightData.shapeHeight.floatValue = Mathf.Max(m_AdditionalLightData.shapeHeight.floatValue, k_MinAreaWidth);
                     settings.areaSizeX.floatValue = m_AdditionalLightData.shapeWidth.floatValue;
                     settings.areaSizeY.floatValue = m_AdditionalLightData.shapeHeight.floatValue;
                     settings.shadowsType.enumValueIndex = (int)LightShadows.None;
                     settings.lightType.enumValueIndex = (int)LightType.Point;
                     m_AdditionalLightData.lightTypeExtent.enumValueIndex = (int)LightTypeExtent.Line;
                     EditorGUILayout.PropertyField(m_AdditionalLightData.shapeWidth, s_Styles.shapeWidthLine);
-                    m_AdditionalLightData.shapeWidth.floatValue = Mathf.Max(m_AdditionalLightData.shapeWidth.floatValue, k_LineWidth);
-                    m_AdditionalLightData.shapeHeight.floatValue = Mathf.Max(m_AdditionalLightData.shapeHeight.floatValue, k_LineWidth);
+                    m_AdditionalLightData.shapeWidth.floatValue = Mathf.Max(m_AdditionalLightData.shapeWidth.floatValue, k_MinAreaWidth);
+                    m_AdditionalLightData.shapeHeight.floatValue = Mathf.Max(m_AdditionalLightData.shapeHeight.floatValue, k_MinAreaWidth);
-                    settings.areaSizeY.floatValue = k_LineWidth;
+                    settings.areaSizeY.floatValue = k_MinAreaWidth;
                     settings.shadowsType.enumValueIndex = (int)LightShadows.None;
                     break;
 
                     break;
 
                 case LightShape.Line:
-                    settings.intensity.floatValue = LightUtils.calculateLineLightArea(m_AdditionalLightData.areaIntensity.floatValue, k_LineWidth, m_AdditionalLightData.shapeWidth.floatValue);
+                    settings.intensity.floatValue = LightUtils.calculateLineLightArea(m_AdditionalLightData.areaIntensity.floatValue, m_AdditionalLightData.shapeWidth.floatValue);
                     break;
             }
         }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/LightUtils.cs

         }
 
         // convert intensity (lumen) to nits
-        public static float calculateLineLightArea(float intensity, float lineRadius, float lineWidth)
+        public static float calculateLineLightArea(float intensity, float lineWidth)
-            return intensity / (2.0f * Mathf.PI * lineRadius * lineWidth * Mathf.PI);
+            // The area of a cylinder is this:
+            // float lineRadius = 0.01f; // 1cm
+            //return intensity / (2.0f * Mathf.PI * lineRadius * lineWidth * Mathf.PI);
+            // But with our current line light algorithm we get an insane gap in intensity
+            // following formula (fully empirical) give a better match to a rect light of 1cm of width.
+            // It is basically point light intensity / line width.
+            return intensity / (4.0f * Mathf.PI * lineWidth);
         }
     }
 }