Thursday, August 11, 2011

Blender 2.5 Compositing from Python

If you are like me then you hate doing the same thing over and over ... which means you like to automate these kinds of processes when it makes sense to do so. I wrote a Blender plugin that imports a HiRISE DTM directly into Blender 2.5x and allows you to easily make fly-throughs of the various Mars locations that a DTM exists for.

My work wants to make automated fly throughs with a trivial piece of compositing to place a foreground and background image into the blender scene. I came across very few working examples or documentation on how to use the compositing part of Blender from python so ... here we go! I'll simply place the final example code here with comments and describe each section a little more in depth below.

# 1) Use compositing for our render, setup some paths
bpy.context.scene.use_nodes = True

fgImageLoc = "/path/to/foreground.tiff"

bgImageLoc = "/path/to/background.tiff"

# 2) Get references to the scene
Scene = bpy.context.scene
Tree = Scene.node_tree
Tree.links.remove( Tree.links[0] )

# 3) The default env will have an input and an output (Src/Dst)
Src = Tree.nodes["Render Layers"]
Dst = Tree.nodes["Composite"]

# 4) Let's create two groups to encapsulate our work
FG_Node =
    "ForegroundImage", type='COMPOSITE')
BG_Node =
    "BackgroundImage", type='COMPOSITE')

# 5) The foreground group has one input and one output"Source", 'RGBA')"Result", 'RGBA')

# 6) The foreground node contains an Image and an AlphaOver node
FG_Image ='IMAGE')
FG_Image.image = fgImageLoc )

# 7) The Image and the Group Input are routed to the AlphaOver
#    and the AlphaOver output is routed to the group's output["Image"], FG_Alpha.inputs[2])["Source"], FG_Alpha.inputs[1])["Result"], FG_Alpha.outputs["Image"])

# 8) Add foreground image compositing to the environment
newFGGroup ="GROUP", group = FG_Node)

# 9) Route the default render output to the input of the FG Group[0], Src.outputs["Image"])

# 10) The background group has one input and one output"Source", 'RGBA')"Result", 'RGBA')

# 11) The background group contains an Image and AlphaOver node
BG_Image ='IMAGE')
BG_Image.image = bgImageLoc )

# 12) Create links to internal nodes["Image"], BG_Alpha.inputs[1])["Source"], BG_Alpha.inputs[2])["Result"], BG_Alpha.outputs["Image"])

# Add background image compositing, similar to 8/9
newBGGroup ="GROUP", group = BG_Node)[0], newFGGroup.outputs[0])[0], Dst.inputs["Image"])

When you run this you will end up with a pipeline that looks like this:

The rendered scene outputs to the foreground image group which then outputs to the background image group which then outputs to the specified file path in Blender. Each group is a composite of primitives in blender. When expanded (via selecting the group and pressing Tab) you will see this:

The group input and Image outputs are routed into the Alpha Over image. The Alpha Over output is routed into to the groups output. This will overlay the Image node onto the scene. A similar setup is produced for the background image.

Here is a slightly more detailed breakdown of the script:

  1. Tell blender that we have a special compositing setup
    • also, store info about where our foreground/background images are kept
  2. Blender's environment is not empty by default.
    • Get a reference to it
    • Clear the link between the Render Layer and the Composite output
  3. Get a reference to the default nodes for later use
    • Src is the source for rendered content from the scene
    • Dst is the node that takes an output and generates a file (or preview)
  4. To simplify our compositing graph, create two groups to encapsulate each function
  5. The group we just created has one input and one output
  6. Create two nodes
    • Image - acts as an output with a static image
    • AlphaOver - overlays two images using the alpha channel defined in the second image
  7. Create links between nodes.
    • It's easier to see these in the image above.
  8. Instantiate the new group in the compositing environment
    • This is where I was a little lost, the group needs to be instantiated and then a new object is returned. The input/output of the returned object are the external input/output ports of the group. If you use the previous object you will make bad connections to the internal structures of the group. Don't do it!
  9. Connect the rendered image to the foreground group input.
  10. through 12. are pretty much the same as 5. through 9.
    • Different connections make the image a background image instead of a foreground image

Thanks to Uncle_Entity and Senshi in #blendercoders@freenode for fixing my initially poor usage.

Here a few links that use the compositing above. Notice that the text hovers above the DTM while the background ... stays in the background:

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