[usdLux] normalized lights divide by surfaceArea expressed in meters^2

This allows normalized lights to compose correctly into stages with
a different metersPerUnit setting, at a backwards-compatibility cost of
a change in luminance of normalized lights before this, in layers with
metersPerUnit != 1

Author: pmolodo

pxr/usd/usdLux/generatedSchema.usda

@@ -167,7 +167,7 @@ class "LightAPI" (
         displayName = "Normalize Power"
         doc = """Normalizes the emission such that the power of the light
         remains constant while altering the size of the light, by dividing the
-        luminance by the world-space surface area of the light.
+        luminance by the world-space surface area of the light, in meters².
 
         This makes it easier to independently adjust the brightness and size
         of the light, by causing the total illumination provided by a light to
@@ -196,18 +196,34 @@ class "LightAPI" (
         ### Area Lights:
 
         For an area light, the `sizeFactor` is the surface area (in world
-        space) of the shape of the light, including any scaling applied to the
-        light by its transform stack. This includes the boundable light types
+        space, expressed in meters²) of the shape of the light, including any
+        scaling applied to the light by its transform stack. The area should
+        then be multiplied by the scene's `metersPerUnit^2` to convert the
+        surface area into meters². Area lights include the boundable light types
         which have a calculable surface area:
 
         - MeshLightAPI
         - DiskLight
         - RectLight
         - SphereLight
-        - CylinderLight
+        - CylinderLight with treatAsLine == false
 
         <center><b>
-                        sizeFactor<sub>area</sub> = worldSpaceSurfaceArea(light)
+                sizeFactor<sub>area</sub> = worldSpaceSurfaceArea(light) * metersPerUnit²
+        </b></center>
+
+        ### Line Lights:
+
+        Lights that simulate 1-dimensional emissive \"lines\" have a `sizeFactor`
+        equal to the length (in world space, expressed in meters) of the shape
+        of the light, including any scaling applied to the light by its
+        transform stack. The area should then be multiplied by the scene's
+        `metersPerUnit` to convert the length into meters. Line lights include:
+
+        - CylinderLight with length != 0 and treatAsLine == True
+
+        <center><b>
+                sizeFactor<sub>line</sub> = worldSpaceLength(light) * metersPerUnit
         </b></center>
 
         ### DistantLight:

pxr/usd/usdLux/lightAPI.h

@@ -424,7 +424,7 @@ class UsdLuxLightAPI : public UsdAPISchemaBase
     // --------------------------------------------------------------------- //
     /// Normalizes the emission such that the power of the light
     /// remains constant while altering the size of the light, by dividing the
-    /// luminance by the world-space surface area of the light.
+    /// luminance by the world-space surface area of the light, in meters².
     /// 
     /// This makes it easier to independently adjust the brightness and size
     /// of the light, by causing the total illumination provided by a light to
@@ -453,18 +453,34 @@ class UsdLuxLightAPI : public UsdAPISchemaBase
     /// ### Area Lights:
     /// 
     /// For an area light, the `sizeFactor` is the surface area (in world
-    /// space) of the shape of the light, including any scaling applied to the
-    /// light by its transform stack. This includes the boundable light types
+    /// space, expressed in meters²) of the shape of the light, including any
+    /// scaling applied to the light by its transform stack. The area should
+    /// then be multiplied by the scene's `metersPerUnit^2` to convert the
+    /// surface area into meters². Area lights include the boundable light types
     /// which have a calculable surface area:
     /// 
     /// - MeshLightAPI
     /// - DiskLight
     /// - RectLight
     /// - SphereLight
-    /// - CylinderLight
+    /// - CylinderLight with treatAsLine == false
     /// 
     /// <center><b>
-    /// sizeFactor<sub>area</sub> = worldSpaceSurfaceArea(light)
+    /// sizeFactor<sub>area</sub> = worldSpaceSurfaceArea(light) * metersPerUnit²
+    /// </b></center>
+    /// 
+    /// ### Line Lights:
+    /// 
+    /// Lights that simulate 1-dimensional emissive "lines" have a `sizeFactor`
+    /// equal to the length (in world space, expressed in meters) of the shape
+    /// of the light, including any scaling applied to the light by its
+    /// transform stack. The area should then be multiplied by the scene's
+    /// `metersPerUnit` to convert the length into meters. Line lights include:
+    /// 
+    /// - CylinderLight with length != 0 and treatAsLine == True
+    /// 
+    /// <center><b>
+    /// sizeFactor<sub>line</sub> = worldSpaceLength(light) * metersPerUnit
     /// </b></center>
     /// 
     /// ### DistantLight:

pxr/usd/usdLux/schema.usda

@@ -252,7 +252,7 @@ class "LightAPI" (
         displayName = "Normalize Power"
         doc = """Normalizes the emission such that the power of the light
         remains constant while altering the size of the light, by dividing the
-        luminance by the world-space surface area of the light.
+        luminance by the world-space surface area of the light, in meters².
 
         This makes it easier to independently adjust the brightness and size
         of the light, by causing the total illumination provided by a light to
@@ -281,18 +281,34 @@ class "LightAPI" (
         ### Area Lights:
 
         For an area light, the `sizeFactor` is the surface area (in world
-        space) of the shape of the light, including any scaling applied to the
-        light by its transform stack. This includes the boundable light types
+        space, expressed in meters²) of the shape of the light, including any
+        scaling applied to the light by its transform stack. The area should
+        then be multiplied by the scene's `metersPerUnit^2` to convert the
+        surface area into meters². Area lights include the boundable light types
         which have a calculable surface area:
 
         - MeshLightAPI
         - DiskLight
         - RectLight
         - SphereLight
-        - CylinderLight
+        - CylinderLight with treatAsLine == false
 
         <center><b>
-                        sizeFactor<sub>area</sub> = worldSpaceSurfaceArea(light)
+                sizeFactor<sub>area</sub> = worldSpaceSurfaceArea(light) * metersPerUnit²
+        </b></center>
+
+        ### Line Lights:
+
+        Lights that simulate 1-dimensional emissive "lines" have a `sizeFactor`
+        equal to the length (in world space, expressed in meters) of the shape
+        of the light, including any scaling applied to the light by its
+        transform stack. The area should then be multiplied by the scene's
+        `metersPerUnit` to convert the length into meters. Line lights include:
+
+        - CylinderLight with length != 0 and treatAsLine == True
+
+        <center><b>
+                sizeFactor<sub>line</sub> = worldSpaceLength(light) * metersPerUnit
         </b></center>
 
         ### DistantLight: