Implement conclusion of discussion at: KhronosGroup/glTF#2214

- Set `WATTS_TO_LUMENS` to 683 convention.
- Add compatibility modes for unitless pipelines and raw Blender units.
- Add explanation text. (Currently placeholder, may remove.)
- Renamed "Geometry" export subpanel to "Data".

addons/io_scene_gltf2/__init__.py

@@ -58,7 +58,6 @@ def reload_package_recursive(current_dir, module_dict):
                        BoolProperty,
                        EnumProperty,
                        IntProperty,
-                       FloatProperty,
                        CollectionProperty)
 from bpy.types import Operator
 from bpy_extras.io_utils import ImportHelper, ExportHelper
@@ -279,6 +278,17 @@ def __init__(self):
         default=False,
     )
 
+    export_lighting_mode: EnumProperty(
+        name='Lighting Mode',
+        items=(
+            ('SPEC', 'Standard', 'Export standard glTF lighting units'),
+            ('COMPAT', 'Compatibility', 'Export unitless PBR lighting'),
+            ('RAW', 'Raw', 'Export Blender lighting strengths directly'),
+        ),
+        description='Optional backwards compatibility for non-standard render engines. Applies to lights',# TODO: and emissive materials',
+        default='SPEC'
+    )
+
     export_colors: BoolProperty(
         name='Vertex Colors',
         description='Export vertex colors with meshes',
@@ -488,18 +498,6 @@ def __init__(self):
         default=False
     )
 
-    export_lights_factor: FloatProperty(
-        name='Lights Strength',
-        description='Different platforms scale brightness differently. Setting 1/600 here works well for many engines',
-        default=1.0
-    )
-
-    export_raw_lights: BoolProperty(
-        name='Raw Intensity',
-        description='Export raw light intensity, without converting to glTF units',
-        default=False
-    )
-
     will_save_settings: BoolProperty(
         name='Remember Export Settings',
         description='Store glTF export settings in the Blender project',
@@ -667,8 +665,7 @@ def execute(self, context):
             export_settings['gltf_morph_tangent'] = False
 
         export_settings['gltf_lights'] = self.export_lights
-        export_settings['gltf_lights_factor'] = self.export_lights_factor
-        export_settings['gltf_raw_lights'] = self.export_raw_lights
+        export_settings['gltf_lighting_mode'] = self.export_lighting_mode
 
         export_settings['gltf_binary'] = bytearray()
         export_settings['gltf_binaryfilename'] = (
@@ -774,9 +771,6 @@ def draw(self, context):
         col.prop(operator, 'export_extras')
         col.prop(operator, 'export_cameras')
         col.prop(operator, 'export_lights')
-        if operator.export_lights:
-            col.prop(operator, 'export_lights_factor')
-            col.prop(operator, 'export_raw_lights')
 
 
 class GLTF_PT_export_transform(bpy.types.Panel):
@@ -807,7 +801,7 @@ def draw(self, context):
 class GLTF_PT_export_geometry(bpy.types.Panel):
     bl_space_type = 'FILE_BROWSER'
     bl_region_type = 'TOOL_PROPS'
-    bl_label = "Geometry"
+    bl_label = "Data"
     bl_parent_id = "FILE_PT_operator"
     bl_options = {'DEFAULT_CLOSED'}
 
@@ -905,6 +899,59 @@ def draw(self, context):
 
         layout.prop(operator, 'export_original_specular')
 
+class GLTF_PT_export_geometry_lighting(bpy.types.Panel):
+    bl_space_type = 'FILE_BROWSER'
+    bl_region_type = 'TOOL_PROPS'
+    bl_label = "Lighting"
+    bl_parent_id = "GLTF_PT_export_geometry"
+    bl_options = {'DEFAULT_CLOSED'}
+
+    @classmethod
+    def poll(cls, context):
+        sfile = context.space_data
+        operator = sfile.active_operator
+        return operator.bl_idname == "EXPORT_SCENE_OT_gltf"
+
+    def draw(self, context):
+        layout = self.layout
+        layout.use_property_split = True
+        layout.use_property_decorate = False  # No animation.
+
+        sfile = context.space_data
+        operator = sfile.active_operator
+
+        layout.prop(operator, 'export_lighting_mode')
+
+class GLTF_PT_export_geometry_lighting_info(bpy.types.Panel):
+    bl_space_type = 'FILE_BROWSER'
+    bl_region_type = 'TOOL_PROPS'
+    bl_label = "Info"
+    bl_parent_id = "GLTF_PT_export_geometry_lighting"
+    bl_options = {'DEFAULT_CLOSED'}
+
+    @classmethod
+    def poll(cls, context):
+        sfile = context.space_data
+        operator = sfile.active_operator
+        return operator.bl_idname == "EXPORT_SCENE_OT_gltf"
+
+    def draw(self, context):
+        layout = self.layout
+        layout.use_property_split = True
+        layout.use_property_decorate = False  # No animation.
+
+        sfile = context.space_data
+        operator = sfile.active_operator
+
+        for line in (
+            "The GLTF specification requires lighting to be defined in candela, lux, and nits.",
+            "However, some viewers lorem ipsum.",
+            "Dolor sit amet, consectetur adipisci tempor incidunt ut labore et dolore magna aliqua.",
+            "Veniam, quis nostrud exercitation ullamcorpor s commodo consequat. Duis autem vel.",
+            "Eum irrure esse molestiae consequat, vel illum dolore eu fugi et iusto odio dignissim."
+        ):
+            layout.label(text=line)
+
 
 class GLTF_PT_export_geometry_compression(bpy.types.Panel):
     bl_space_type = 'FILE_BROWSER'
@@ -1349,6 +1396,8 @@ def menu_func_import(self, context):
     GLTF_PT_export_geometry_mesh,
     GLTF_PT_export_geometry_material,
     GLTF_PT_export_geometry_original_pbr,
+    GLTF_PT_export_geometry_lighting,
+    GLTF_PT_export_geometry_lighting_info,
     GLTF_PT_export_geometry_compression,
     GLTF_PT_export_animation,
     GLTF_PT_export_animation_export,

addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_lights.py

@@ -81,21 +81,25 @@ def __gather_intensity(blender_lamp, export_settings) -> Optional[float]:
             emission_strength = emission_node.inputs["Strength"].default_value
     else:
         emission_strength = blender_lamp.energy
-    if export_settings['gltf_raw_lights']:
-        emission_luminous = emission_strength
+    if export_settings['gltf_lighting_mode'] == 'RAW':
+        return emission_strength
     else:
-        # Assume at this point this is still in the appropriate watt-related SI unit, which if everything up to here was done with physical basis it hopefully should be.
-        WATTS_TO_LUMENS = 594
-        # At 580nm, according to WolframAlpha, which is probably as authoritative as a source as I can get for something so variable as physical power to perceptual brightness: https://www.wolframalpha.com/input?i=1+watt+in+lumens
+        # Assume at this point the computed strength is still in the appropriate watt-related SI unit, which if everything up to here was done with physical basis it hopefully should be.
+        WATTS_TO_LUMENS = 683
         if blender_lamp.type == 'SUN': # W/m^2 in Blender to lm/m^2 for GLTF/KHR_lights_punctual.
-            emission_luminous = emission_strength * WATTS_TO_LUMENS
+            emission_luminous = emission_strength
         else:
             # Other than directional, only point and spot lamps are supported by GLTF.
             # In Blender, points are omnidirectional W, and spots are specified as if they're points.
             # Point and spot should both be lm/r^2 in GLTF.
-            emission_luminous = emission_strength / (4*math.pi) * WATTS_TO_LUMENS
-    return emission_luminous * export_settings['gltf_lights_factor']
-    # NOTE: The GLTF spec says light intensity is given in lumens, a unit of *perceptual* luminous flux. Perceptual luminance varies with wavelength. So in order to attempt to actually match the spec, we would have to multiply each RGB component by a different (cone cell) responsivity factor as we convert from watts, a unit of *physical* radiation. However, neither the Three.JS nor the Khronos Group reference GLTF model viewers currently technically implement that part of the spec correctly, and in my personal opinion in this PR a unit of perceptual brightness like "lumens" has no place in a model format designed for physically based workflows anyway (and may not have been intentional). I'll raise this issue on the KhronosGroup GLTF issue tracker: https://github.com/KhronosGroup/glTF/issues/2213
+            emission_luminous = emission_strength / (4*math.pi)
+        if export_settings['gltf_lighting_mode'] == 'SPEC':
+            emission_luminous *= WATTS_TO_LUMENS
+        elif export_settings['gltf_lighting_mode'] == 'COMPAT':
+            pass # Just so we have an exhaustive tree to catch bugged values.
+        else:
+            raise ValueError(export_settings['gltf_lighting_mode'])
+        return emission_luminous
 
 
 def __gather_spot(blender_lamp, export_settings) -> Optional[gltf2_io_lights_punctual.LightSpot]:

addons/io_scene_gltf2/blender/imp/gltf2_blender_light.py

@@ -32,7 +32,7 @@ def create(gltf, vnode, light_id):
         import_user_extensions('gather_import_light_before_hook', gltf, vnode, pylight)
 
         if pylight['type'] == "directional":
-            light = BlenderLight.create_directional(gltf, light_id)
+            light = BlenderLight.create_directional(gltf, light_id) # ...Why not pass the pylight?
         elif pylight['type'] == "point":
             light = BlenderLight.create_point(gltf, light_id)
         elif pylight['type'] == "spot":