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Overview
Renders the first input, then all frames of the second input, then the third input. This lets you run the "preprocessing" and "postprocessing" nodes once per job. This is useful for render jobs that use frame-interleaved rendering. If you simply connected the nodes (pre → main → post), the "pre" and "post" nodes would render before and after each frame.
For example, the following network:
…will render: pre1, pre2, pre3, mantra1, mantra2, mantra3, post1, post2, post3.
A simple chain:
…would render pre1, mantra1, post1, pre2, mantra2, post2, pre3, mantra3, post3.
Only the second (main) input is required. The "pre" and "post" connections are optional. You can also connect the inputs but then disable them with the Run pre render operation and Run post render operation, allowing you to base pre and post rendering on the value of an expression.
Note
To render multiple inputs at the same time instead of serially, use the 
Merge ROP.
Limitations
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None of the inputs can refer to the same ROP, as this would cause dependency cycles.
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The Pre render and Post render ROPs cannot branch out to meet each other or the main render branch. They must be separate, self-contained branches.
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ROPs that are dependent on the Pre Post ROP (outputs), either directly or indirectly, should not also be dependent on any nodes in the Pre render, Main job or Post render branches. This could cause cyclic dependencies or double processing of a frame.
Essentially, each input should have its own branch which no other outside ROPs reference.
Parameters
Render
Begins the render with the last render control settings.
Render Control
Opens the render control dialog to allow adjustments of the render parameters before rendering.
Run pre render operation
When enabled, the pre render branch is performed before the main render ob.
Pre Frame Range
Allows the frame range of the Pre render ROP to be overriden.
Pre render ROP’s specified range: Use the frame range specified in the Pre render ROP. If set to Render Current Frame, it will be rendered once at the time of the first frame of the main job.
Main job’s first frame: Run the Pre render ROP once at the frame time of the first frame rendered by the main job.
Main job’s last frame: Run the Pre render ROP once at the frame time of the last frame rendered by the main job.
Main job’s frame range
Run the Pre render ROP across the frame range of the main render job.
Run Post render operation
When enabled, the Post render branch is performed after all the frames of the main render job.
Post Frame Range
Allows the frame range of the Post render ROP to be overriden.
Post render ROP’s specified range: Use the frame range specified in the Post render ROP. If set to Render Current Frame, it will be rendered once at the time of the last frame of the main job.
Main job’s first frame: Run the post-render ROP once at the frame time of the first frame rendered by the main job.
Main job’s last frame: Run the Post render ROP once at the frame time of the last frame rendered by the main job.
Main job’s frame range
Run the Post Render ROP across the frame range of the main render job.
Locals
N
Frame being rendered.
NRENDER
Total number of frames being rendered.
