GSoC 2025: Compositor setup for Renders

[PardhavMaradani]

Hi @BradyAJohnston and @yuxuanzhuang , this discussion is about the compositor setup needed for renders.

Rendering support for annotations was added as part of PR #909 . However, as noted in that PR, the changes there were limited to generating the PIL image with the annotations that could be composited over the final render image - as can be seen in manual compositing step in the video there. We needed the PIL annotation image because the annotations in the viewport using blf and gpu modules cannot be rendered directly as part of the render process.

The compositor is scene specific and is something that users could create, modify and adjust as per their needs. So any changes there have to be non-destructive in nature (i.e., we cannot re-create a composite node tree solely as per our needs) and also provide users the ability to turn on / off any modifications we make. Compositor cannot be setup during the register of the add-on due to the restricted context (i.e., no access to bpy.data, bpy.context.scene, etc) and also because it is scene specific. The ideal place to set it up and verify that it is correctly setup (or fix) is in the pre-render handler.

With these in mind, I made the following changes that will allow automatic compositor setup that is definitely needed for annotations for now, but can be easily extended for other composition needs in the future:

• I added a separate compositor node group called MN Compositor that currently takes in an image (say from a render layer), overlays (simple alpha over) the annotations PIL image and returns this image. We could easily extend this node group with additional composite features we might have in the future
• Auto compositor setup that will add the MN Compositor right before the final composite - this is done as part of the pre-render handler, which is also where the annotations PIL image is generated today
• A user configurable option for the auto compositor setup (defaults to True) - this can be configurable in the Molecular Nodes tab in the Composite editor (n-panel). There is also an operator in the UI that will allow manual setup if needed by the user
• The composite node tree is arranged using the same code (arrange_tree) used for our Geometry Nodes

Here is a short video that shows the above:

auto-compositor-setup.mp4

The video shows the following:

• Default composite setup at the beginning (empty)
• The Auto Setup being True in the Molecular Ndes tab of the n-panel in the Composite editor
• The composite node tree being correctly setup after a render (happens during the pre-render handler) - including linking to a viewer node if present (which is a default from 4.5.0)
• The details of the MN Compositor node group

Please let me know if you have any thoughts on the above. I will create a PR with these changes once the current open ones regarding the density component are merged. Thanks

[PardhavMaradani]

Closing this as changes described above are part of PR #926