glTF export: enhancement & fixes:

* implement KHR_materials_unlit export
* Fix primitive restart values
* Fix jpg uri/mime type
* Fix bug when image has no color channels
* Check animation has actions
* Ignore meshes without primitives
* Fix materials when not selected in export settings
* Improve error message for invalid animation target type
* Animation with errors are ignored, but export continues
* Export of BaseColorFactor

11 files changed, 115 insertions, 33 deletions

diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_extract.py b/io_scene_gltf2/blender/exp/gltf2_blender_extract.py
index 0add794a..42be29b7 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_extract.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_extract.py
@@ -975,7 +975,12 @@ def extract_primitives(glTF, blender_mesh, blender_vertex_groups, modifiers, exp
 
         #
 
-        range_indices = 65536
+        # NOTE: Values used by some graphics APIs as "primitive restart" values are disallowed.
+        # Specifically, the value 65535 (in UINT16) cannot be used as a vertex index.
+        # https://github.com/KhronosGroup/glTF/issues/1142
+        # https://github.com/KhronosGroup/glTF/pull/1476/files
+
+        range_indices = 65535
 
         #
 
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_sampler_keyframes.py b/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_sampler_keyframes.py
index 6ef7fb0b..351a036a 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_sampler_keyframes.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_sampler_keyframes.py
@@ -46,7 +46,7 @@ class Keyframe:
         }.get(self.__target)
 
         if length is None:
-            raise RuntimeError("Unknown target type {}".format(self.__target))
+            raise RuntimeError("Animations with target type '{}' are not supported.".format(self.__target))
 
         return length
 
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_gather_animations.py b/io_scene_gltf2/blender/exp/gltf2_blender_gather_animations.py
index bfbc03ed..7a7cda02 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_gather_animations.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_gather_animations.py
@@ -17,6 +17,7 @@ import typing
 
 from io_scene_gltf2.io.com import gltf2_io
 from io_scene_gltf2.blender.exp import gltf2_blender_gather_animation_channels
+from io_scene_gltf2.io.com.gltf2_io_debug import print_console
 
 
 def gather_animations(blender_object: bpy.types.Object, export_settings) -> typing.List[gltf2_io.Animation]:
@@ -48,13 +49,18 @@ def __gather_animation(blender_action: bpy.types.Action,
     if not __filter_animation(blender_action, blender_object, export_settings):
         return None
 
-    animation = gltf2_io.Animation(
-        channels=__gather_channels(blender_action, blender_object, export_settings),
-        extensions=__gather_extensions(blender_action, blender_object, export_settings),
-        extras=__gather_extras(blender_action, blender_object, export_settings),
-        name=__gather_name(blender_action, blender_object, export_settings),
-        samplers=__gather_samplers(blender_action, blender_object, export_settings)
-    )
+    name = __gather_name(blender_action, blender_object, export_settings)
+    try:
+        animation = gltf2_io.Animation(
+            channels=__gather_channels(blender_action, blender_object, export_settings),
+            extensions=__gather_extensions(blender_action, blender_object, export_settings),
+            extras=__gather_extras(blender_action, blender_object, export_settings),
+            name=name,
+            samplers=__gather_samplers(blender_action, blender_object, export_settings)
+        )
+    except RuntimeError as error:
+        print_console("WARNING", "Animation '{}' could not be exported. Cause: {}".format(name, error))
+        return None
 
     # To allow reuse of samplers in one animation,
     __link_samplers(animation, export_settings)
@@ -159,7 +165,8 @@ def __get_blender_actions(blender_object: bpy.types.Object
     if blender_object.type == "MESH" \
             and blender_object.data is not None \
             and blender_object.data.shape_keys is not None \
-            and blender_object.data.shape_keys.animation_data is not None:
+            and blender_object.data.shape_keys.animation_data is not None \
+            and blender_object.data.shape_keys.animation_data.action is not None:
         blender_actions.append(blender_object.data.shape_keys.animation_data.action)
 
     # Remove duplicate actions.
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_gather_image.py b/io_scene_gltf2/blender/exp/gltf2_blender_gather_image.py
index a694cac3..6b969668 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_gather_image.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_gather_image.py
@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import re
 
 import bpy
 import typing
@@ -30,13 +31,17 @@ def gather_image(
         export_settings):
     if not __filter_image(blender_shader_sockets_or_texture_slots, export_settings):
         return None
+
+    uri = __gather_uri(blender_shader_sockets_or_texture_slots, export_settings)
+    mime_type = __gather_mime_type(uri.filepath if uri is not None else "")
+
     image = gltf2_io.Image(
         buffer_view=__gather_buffer_view(blender_shader_sockets_or_texture_slots, export_settings),
         extensions=__gather_extensions(blender_shader_sockets_or_texture_slots, export_settings),
         extras=__gather_extras(blender_shader_sockets_or_texture_slots, export_settings),
-        mime_type=__gather_mime_type(blender_shader_sockets_or_texture_slots, export_settings),
+        mime_type=mime_type,
         name=__gather_name(blender_shader_sockets_or_texture_slots, export_settings),
-        uri=__gather_uri(blender_shader_sockets_or_texture_slots, export_settings)
+        uri=uri
     )
     return image
 
@@ -51,7 +56,7 @@ def __gather_buffer_view(sockets_or_slots, export_settings):
     if export_settings[gltf2_blender_export_keys.FORMAT] != 'GLTF_SEPARATE':
         image = __get_image_data(sockets_or_slots, export_settings)
         return gltf2_io_binary_data.BinaryData(
-            data=image.to_image_data(__gather_mime_type(sockets_or_slots, export_settings)))
+            data=image.to_image_data(__gather_mime_type()))
     return None
 
 
@@ -63,9 +68,13 @@ def __gather_extras(sockets_or_slots, export_settings):
     return None
 
 
-def __gather_mime_type(sockets_or_slots, export_settings):
-    return 'image/png'
-    # return 'image/jpeg'
+def __gather_mime_type(filepath=""):
+    extension_types = {'.png': 'image/png', '.jpg': 'image/jpeg', '.jpeg': 'image/jpeg'}
+    default_extension = extension_types['.png']
+
+    matches = re.findall(r'\.\w+$', filepath)
+    extension = matches[0] if len(matches) > 0 else default_extension
+    return extension_types[extension] if extension.lower() in extension_types.keys() else default_extension
 
 
 def __gather_name(sockets_or_slots, export_settings):
@@ -98,6 +107,8 @@ def __get_image_data(sockets_or_slots, export_settings):
     # in a helper class. During generation of the glTF in the exporter these will then be combined to actual binary
     # ressources.
     def split_pixels_by_channels(image: bpy.types.Image, export_settings) -> typing.List[typing.List[float]]:
+        assert image.channels > 0, "Image '{}' has no color channels and cannot be exported.".format(image.name)
+
         channelcache = export_settings['gltf_channelcache']
         if image.name in channelcache:
             return channelcache[image.name]
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py b/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py
index 427f07ce..e1c17480 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials.py
@@ -16,7 +16,8 @@ import bpy
 
 from io_scene_gltf2.blender.exp.gltf2_blender_gather_cache import cached
 from io_scene_gltf2.io.com import gltf2_io
-from io_scene_gltf2.blender.exp import gltf2_blender_gather_texture_info
+from io_scene_gltf2.io.com.gltf2_io_extensions import Extension
+from io_scene_gltf2.blender.exp import gltf2_blender_gather_texture_info, gltf2_blender_export_keys
 from io_scene_gltf2.blender.exp import gltf2_blender_gather_material_normal_texture_info_class
 from io_scene_gltf2.blender.exp import gltf2_blender_gather_material_occlusion_texture_info_class
 
@@ -69,11 +70,7 @@ def gather_material(blender_material, export_settings):
 
 
 def __filter_material(blender_material, export_settings):
-    # if not blender_material.use_nodes:
-    #     return False
-    # if not blender_material.node_tree:
-    #     return False
-    return True
+    return export_settings[gltf2_blender_export_keys.MATERIALS]
 
 
 def __gather_alpha_cutoff(blender_material, export_settings):
@@ -113,6 +110,8 @@ def __gather_emissive_texture(blender_material, export_settings):
 def __gather_extensions(blender_material, export_settings):
     extensions = {}
 
+    if gltf2_blender_get.get_socket_or_texture_slot(blender_material, "Background") is not None:
+        extensions["KHR_materials_unlit"] = Extension("KHR_materials_unlit", {}, False)
 
     # TODO specular glossiness extension
 
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials_pbr_metallic_roughness.py b/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials_pbr_metallic_roughness.py
index 2a6315bf..67e7fa12 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials_pbr_metallic_roughness.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_gather_materials_pbr_metallic_roughness.py
@@ -13,11 +13,13 @@
 # limitations under the License.
 
 import bpy
+from mathutils import Color
 
 from io_scene_gltf2.io.com import gltf2_io
-from io_scene_gltf2.blender.exp import gltf2_blender_gather_texture_info
+from io_scene_gltf2.blender.exp import gltf2_blender_gather_texture_info, gltf2_blender_search_node_tree
 from io_scene_gltf2.blender.exp import gltf2_blender_get
 from io_scene_gltf2.blender.exp.gltf2_blender_gather_cache import cached
+from io_scene_gltf2.io.com.gltf2_io_debug import print_console
 
 
 @cached
@@ -48,9 +50,41 @@ def __gather_base_color_factor(blender_material, export_settings):
         base_color_socket = gltf2_blender_get.get_socket_or_texture_slot(blender_material, "BaseColor")
     if base_color_socket is None:
         base_color_socket = gltf2_blender_get.get_socket_or_texture_slot_old(blender_material, "BaseColorFactor")
-    if isinstance(base_color_socket, bpy.types.NodeSocket) and not base_color_socket.is_linked:
+    if base_color_socket is None:
+        base_color_socket = gltf2_blender_get.get_socket_or_texture_slot(blender_material, "Background")
+    if not isinstance(base_color_socket, bpy.types.NodeSocket):
+        return None
+    if not base_color_socket.is_linked:
         return list(base_color_socket.default_value)
-    return None
+
+    texture_node = __get_tex_from_socket(base_color_socket)
+    if texture_node is None:
+        return None
+
+    def is_valid_multiply_node(node):
+        return isinstance(node, bpy.types.ShaderNodeMixRGB) and \
+               node.blend_type == "MULTIPLY" and \
+               len(node.inputs) == 3
+
+    multiply_node = next((link.from_node for link in texture_node.path if is_valid_multiply_node(link.from_node)), None)
+    if multiply_node is None:
+        return None
+
+    def is_factor_socket(socket):
+        return isinstance(socket, bpy.types.NodeSocketColor) and \
+               (not socket.is_linked or socket.links[0] not in texture_node.path)
+
+    factor_socket = next((socket for socket in multiply_node.inputs if is_factor_socket(socket)), None)
+    if factor_socket is None:
+        return None
+
+    if factor_socket.is_linked:
+        print_console("WARNING", "BaseColorFactor only supports sockets without links (in Node '{}')."
+                      .format(multiply_node.name))
+        return None
+
+    return list(factor_socket.default_value)
+
 
 def __gather_base_color_texture(blender_material, export_settings):
     base_color_socket = gltf2_blender_get.get_socket_or_texture_slot(blender_material, "Base Color")
@@ -58,9 +92,20 @@ def __gather_base_color_texture(blender_material, export_settings):
         base_color_socket = gltf2_blender_get.get_socket_or_texture_slot(blender_material, "BaseColor")
     if base_color_socket is None:
         base_color_socket = gltf2_blender_get.get_socket_or_texture_slot_old(blender_material, "BaseColor")
+    if base_color_socket is None:
+        base_color_socket = gltf2_blender_get.get_socket_or_texture_slot(blender_material, "Background")
     return gltf2_blender_gather_texture_info.gather_texture_info((base_color_socket,), export_settings)
 
 
+def __get_tex_from_socket(blender_shader_socket: bpy.types.NodeSocket):
+    result = gltf2_blender_search_node_tree.from_socket(
+        blender_shader_socket,
+        gltf2_blender_search_node_tree.FilterByType(bpy.types.ShaderNodeTexImage))
+    if not result:
+        return None
+    return result[0]
+
+
 def __gather_extensions(blender_material, export_settings):
     return None
 
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_gather_mesh.py b/io_scene_gltf2/blender/exp/gltf2_blender_gather_mesh.py
index f32eb733..d6058c60 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_gather_mesh.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_gather_mesh.py
@@ -18,6 +18,7 @@ from io_scene_gltf2.blender.exp.gltf2_blender_gather_cache import cached
 from io_scene_gltf2.io.com import gltf2_io
 from io_scene_gltf2.blender.exp import gltf2_blender_gather_primitives
 from io_scene_gltf2.blender.exp import gltf2_blender_generate_extras
+from io_scene_gltf2.io.com.gltf2_io_debug import print_console
 
 
 @cached
@@ -38,6 +39,9 @@ def gather_mesh(blender_mesh: bpy.types.Mesh,
         weights=__gather_weights(blender_mesh, vertex_groups, modifiers, export_settings)
     )
 
+    if len(mesh.primitives) == 0:
+        print_console("WARNING", "Mesh '{}' has no primitives and will be omitted.".format(mesh.name))
+        return None
     return mesh
 
 
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_gather_primitives.py b/io_scene_gltf2/blender/exp/gltf2_blender_gather_primitives.py
index 23d51fa6..bf81e808 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_gather_primitives.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_gather_primitives.py
@@ -72,13 +72,19 @@ def __gather_materials(blender_primitive, blender_mesh, modifiers, export_settin
 def __gather_indices(blender_primitive, blender_mesh, modifiers, export_settings):
     indices = blender_primitive['indices']
 
+    # NOTE: Values used by some graphics APIs as "primitive restart" values are disallowed.
+    # Specifically, the values 65535 (in UINT16) and 4294967295 (in UINT32) cannot be used as indices.
+    # https://github.com/KhronosGroup/glTF/issues/1142
+    # https://github.com/KhronosGroup/glTF/pull/1476/files
+    # Also, UINT8 mode is not supported:
+    # https://github.com/KhronosGroup/glTF/issues/1471
     max_index = max(indices)
-    if max_index < (1 << 16):
+    if max_index < 65535:
         component_type = gltf2_io_constants.ComponentType.UnsignedShort
-    elif max_index < (1 << 32):
+    elif max_index < 4294967295:
         component_type = gltf2_io_constants.ComponentType.UnsignedInt
     else:
-        print_console('ERROR', 'Invalid max_index: ' + str(max_index))
+        print_console('ERROR', 'A mesh contains too many vertices (' + str(max_index) + ') and needs to be split before export.')
         return None
 
     element_type = gltf2_io_constants.DataType.Scalar
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_get.py b/io_scene_gltf2/blender/exp/gltf2_blender_get.py
index 7801190b..08e3a307 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_get.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_get.py
@@ -48,6 +48,9 @@ def get_socket_or_texture_slot(blender_material: bpy.types.Material, name: str):
         if name == "Emissive":
             type = bpy.types.ShaderNodeEmission
             name = "Color"
+        elif name == "Background":
+            type = bpy.types.ShaderNodeBackground
+            name = "Color"
         else:
             type = bpy.types.ShaderNodeBsdfPrincipled
         nodes = [n for n in blender_material.node_tree.nodes if isinstance(n, type)]
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py b/io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py
index 65efafef..ea26eb1c 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_gltf2_exporter.py
@@ -215,7 +215,7 @@ class GlTF2Exporter:
         # TODO: we need to know the image url at this point already --> maybe add all options to the constructor of the
         # exporter
         # TODO: allow embedding of images (base64)
-        return image.name + ".png"
+        return image.name + image.get_extension()
 
     @classmethod
     def __get_key_path(cls, d: dict, keypath: List[str], default=[]):
diff --git a/io_scene_gltf2/blender/exp/gltf2_blender_search_node_tree.py b/io_scene_gltf2/blender/exp/gltf2_blender_search_node_tree.py
index 92b63c7d..72eb5252 100755
--- a/io_scene_gltf2/blender/exp/gltf2_blender_search_node_tree.py
+++ b/io_scene_gltf2/blender/exp/gltf2_blender_search_node_tree.py
@@ -82,14 +82,16 @@ def from_socket(start_socket: bpy.types.NodeSocket,
         for link in start_socket.links:
             # follow the link to a shader node
             linked_node = link.from_node
-            # add the link to the current path
-            search_path.append(link)
             # check if the node matches the filter
             if shader_node_filter(linked_node):
-                results.append(NodeTreeSearchResult(linked_node, search_path))
+                results.append(NodeTreeSearchResult(linked_node, search_path + [link]))
             # traverse into inputs of the node
             for input_socket in linked_node.inputs:
-                results += __search_from_socket(input_socket, shader_node_filter, search_path)
+                linked_results = __search_from_socket(input_socket, shader_node_filter, search_path + [link])
+                if linked_results:
+                    # add the link to the current path
+                    search_path.append(link)
+                    results += linked_results
 
         return results