How to render an object on a background via the composite node editor

Question

I am using a node setup to render objects on top of a background image through the Python API.

EDIT: To make it more clear, I have reproduced the situation in Blender GUI:

My Note Setup:

def node_setting_init():
"""node settings for render rgb images

mainly for compositing the background images
"""

bpy.context.scene.use_nodes = True
tree = bpy.context.scene.node_tree
links = tree.links

for node in tree.nodes:
    tree.nodes.remove(node)

image_node = tree.nodes.new('CompositorNodeImage')
scale_node = tree.nodes.new('CompositorNodeScale')
alpha_over_node = tree.nodes.new('CompositorNodeAlphaOver')
render_layer_node = tree.nodes.new('CompositorNodeRLayers')
file_output_node = tree.nodes.new('CompositorNodeOutputFile')

# Scales image to dimensions set in the Render panel, my case 1280x720
scale_node.space = g_scale_space 

# Select output folder, i.e. where to store rendered images
file_output_node.base_path = g_syn_rgb_folder

# Scale background image
links.new(image_node.outputs[0], scale_node.inputs[0])

# Set background image as background image input to alpha node
links.new(scale_node.outputs[0], alpha_over_node.inputs[1]) #1

# Set rendered object as the foreground image to alpha node
links.new(render_layer_node.outputs[0], alpha_over_node.inputs[2]) #2

# Final image is the output image
links.new(alpha_over_node.outputs[0], file_output_node.inputs[0])

My rendering function:

def render(obj_path, viewpoint):
"""render rbg image 

render a object rgb image by a given camera viewpoint and
choose random image as background, only render one image
at a time.

Args:
    obj_path: a string variable indicate the obj file path
    viewpoint: a vp parameter(contains azimuth,elevation,tilt angles and distance)
"""

background_images = os.listdir(g_background_image_path)

# Set camera to the viewpoint
vp = viewpoint
cam_location = camera_location(vp.azimuth, vp.elevation, vp.distance)
cam_rot = camera_rot_XYZEuler(vp.azimuth, vp.elevation, vp.tilt)

cam_obj = bpy.data.objects['Camera']
cam_obj.location[0] = cam_location[0]
cam_obj.location[1] = cam_location[1]
cam_obj.location[2] = cam_location[2]

cam_obj.rotation_euler[0] = cam_rot[0]
cam_obj.rotation_euler[1] = cam_rot[1]
cam_obj.rotation_euler[2] = cam_rot[2]

# Make dir to store output rgb images
if not os.path.exists(g_syn_rgb_folder):
    os.mkdir(g_syn_rgb_folder)

# get random background from background_images folder
image_name = random.choice(background_images)
image_path = os.path.join(g_background_image_path, image_name)
print("\n\nBackground chosen:{}\n\n".format(image_path))

# Load background image and set output file
image_node = bpy.context.scene.node_tree.nodes[0]
image_node.image = bpy.data.images.load(image_path)
file_output_node = bpy.context.scene.node_tree.nodes[4]
file_output_node.file_slots[0].path = 'blender-######.color.png' # blender placeholder #

#start rendering
bpy.ops.render.render(write_still=True)

#"Current Frame, to update animation data from python frame_set() instead"
current_frame = bpy.context.scene.frame_current
#"Set scene frame updating all objects immediately"
bpy.context.scene.frame_set(current_frame + 1)

So I initiliaze:

def init_all():
"""init everything we need for rendering
an image
"""
scene_setting_init(g_gpu_render_enable)
node_setting_init()
cam_obj = bpy.data.objects['Camera']
cam_obj.rotation_mode = g_rotation_mode

#bpy.data.objects['Lamp'].data.energy = 50
#bpy.ops.object.lamp_add(type='SUN')
#xavier:
bpy.types.SpotLight.energy = 50

And call render()

So I am setting node[0], to my background image and I render it. However, my object is rendered on a grey background; my image nowhere to be found. I am pretty sure this worked for version 2.79, but I am running 2.83. I know quite a lot changed between these versions, but I can't find a proper tutorial or answer online.

(I have also tried to set the background image via the camera object, but still no luck.)

This is the result. No background present.

If I put the background image as foreground, and the object as background:

# Set background image as FOREGROUND image input to alpha node
links.new(scale_node.outputs[0], alpha_over_node.inputs[2]) #1

# Set rendered object as the BACKGROUND image to alpha node
links.new(render_layer_node.outputs[0], alpha_over_node.inputs[1]) #2

Now I only see the background, which makes sense I guess.

So something is going wrong with combining the images.

Answer 1

You have to set the background transparent, otherwise you will render the grey part onto the images.

In Blender GUI:

Go to the Render properties tab > Film dropdown and check Transparent. This will tell Blender to only render the cube as opposed to the cube and the gray background.

In Python API:

bpy.context.scene.render.film_transparent = True