Fix #104061: RenderEngine Python example does not work in background mode

And add example of how to use bl_use_gpu_context.

doc/python_api/examples/bpy.types.RenderEngine.1.py

@@ -5,8 +5,6 @@ Simple Render Engine
 
 import bpy
 import array
-import gpu
-from gpu_extras.presets import draw_texture_2d
 
 
 class CustomRenderEngine(bpy.types.RenderEngine):
@@ -95,6 +93,10 @@ class CustomRenderEngine(bpy.types.RenderEngine):
     # Blender will draw overlays for selection and editing on top of the
     # rendered image automatically.
     def view_draw(self, context, depsgraph):
+        # Lazily import GPU module, so that the render engine works in
+        # background mode where the GPU module can't be imported by default.
+        import gpu
+
         region = context.region
         scene = depsgraph.scene
 
@@ -116,6 +118,8 @@ class CustomRenderEngine(bpy.types.RenderEngine):
 
 class CustomDrawData:
     def __init__(self, dimensions):
+        import gpu
+
         # Generate dummy float image buffer
         self.dimensions = dimensions
         width, height = dimensions
@@ -134,6 +138,7 @@ class CustomDrawData:
         del self.texture
 
     def draw(self):
+        from gpu_extras.presets import draw_texture_2d
         draw_texture_2d(self.texture, (0, 0), self.texture.width, self.texture.height)
 
 

doc/python_api/examples/bpy.types.RenderEngine.2.py

@@ -0,0 +1,36 @@
+"""
+GPU Render Engine
++++++++++++++++++
+"""
+
+import bpy
+
+
+class CustomGPURenderEngine(bpy.types.RenderEngine):
+    bl_idname = "CUSTOM_GPU"
+    bl_label = "Custom GPU"
+
+    # Request a GPU context to be created and activated for the render method.
+    # This may be used either to perform the rendering itself, or to allocate
+    # and fill a texture for more efficient drawing.
+    bl_use_gpu_context = True
+
+    def render(self, depsgraph):
+        # Lazily import GPU module, since GPU context is only created on demand
+        # for rendering and does not exist on register.
+        import gpu
+
+        # Perform rendering task.
+        pass
+
+
+def register():
+    bpy.utils.register_class(CustomGPURenderEngine)
+
+
+def unregister():
+    bpy.utils.unregister_class(CustomGPURenderEngine)
+
+
+if __name__ == "__main__":
+    register()