Grease Pencil: Add LineArt modifier

This adds the LineArt grease pencil modifier.

It takes objects or collections as input and generates various grease
pencil lines from these objects with the help of the active scene
camera. For example it can generate contour lines, intersection lines
and crease lines to name a few.

This is really useful as artists can then use 3D meshes to automatically
generate grease pencil lines for characters, enviroments or other
visualization purposes.

These lines can then be baked and edited as regular grease pencil lines.

Reviewed By: Sebastian Parborg, Antonio Vazquez, Matias Mendiola

Differential Revision: http://developer.blender.org/D8758

release/scripts/startup/bl_ui/properties_material.py

@@ -274,6 +274,38 @@ class MATERIAL_PT_viewport(MaterialButtonsPanel, Panel):
         col.prop(mat, "roughness")
 
 
+class MATERIAL_PT_lineart(MaterialButtonsPanel, Panel):
+    bl_label = "Line Art"
+    bl_options = {'DEFAULT_CLOSED'}
+
+    @classmethod
+    def poll(cls, context):
+        mat = context.material
+        return mat and not mat.grease_pencil
+
+    def draw(self, context):
+        layout = self.layout
+
+        mat = context.material
+        lineart = mat.lineart
+
+        layout.prop(lineart, "use_transparency")
+
+        if lineart.use_transparency:
+
+            layout.label(text="Transparency Masks:")
+
+            row = layout.row(align=True)
+            row.prop(lineart, "transparency_mask_0", text="0", toggle=True)
+            row.prop(lineart, "transparency_mask_1", text="1", toggle=True)
+            row.prop(lineart, "transparency_mask_2", text="2", toggle=True)
+            row.prop(lineart, "transparency_mask_3", text="3", toggle=True)
+            row.prop(lineart, "transparency_mask_4", text="4", toggle=True)
+            row.prop(lineart, "transparency_mask_5", text="5", toggle=True)
+            row.prop(lineart, "transparency_mask_6", text="6", toggle=True)
+            row.prop(lineart, "transparency_mask_7", text="7", toggle=True)
+
+
 classes = (
     MATERIAL_MT_context_menu,
     MATERIAL_UL_matslots,
@@ -282,6 +314,7 @@ classes = (
     EEVEE_MATERIAL_PT_surface,
     EEVEE_MATERIAL_PT_volume,
     EEVEE_MATERIAL_PT_settings,
+    MATERIAL_PT_lineart,
     MATERIAL_PT_viewport,
     EEVEE_MATERIAL_PT_viewport_settings,
     MATERIAL_PT_custom_props,