First implementation of node previews in the shader node editor. Using
the same user interface as compositor node previews, most shader nodes
can now be previewed (except group in/output and material output).
This is currently still an experimental feature, as polishing of the
user experience and performance improvements are planned. These will
be easier to do as incremental changes on this implementation.
See #110353 for details on the work that remains to be done and known
limitations.
Implementation notes:
We take advantage of the `RenderResult` available as `ImBuf` images to
store a `Render` for every viewed nested node tree present in a
`SpaceNode`. The computation is initiated at the moment of drawing nodes
overlays.
One render is started for the current nodetree, having a `ViewLayer`
associated with each previewed node. We separate the previewed nodes in
two categories: the shader ones and the non-shader ones.
- For non-shader nodes, we use AOVs which highly speed up the rendering
process by rendering every non-shader nodes at the same time. They are
rendered in the first `ViewLayer`.
- For shader nodes, we render them each in a different `ViewLayer`, by
rerouting the node to the output of the material in the preview scene.
The preview scene takes the same aspect as the Material preview scene,
and the same preview object is used.
At the moment of drawing the node overlay, we take the `Render` of the
viewed node tree and extract the `ImBuf` of the wanted viewlayer/pass
for each previewed node.
Pull Request: https://projects.blender.org/blender/blender/pulls/110065
b8eb7d18e9