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Maniphest T52889

Driver is updating with delay
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Description

System Information
Win7 64bit

Blender Version
Broken: 2.79

Short description of error
"Transform channel" type driver updates with dealy of 1 frame. As you can see value for Transform channel is still 0 when real X location is not. "Single property" is working fine though.

Exact steps for others to reproduce the error
Open file, press right arrow button on keyboard to update driver

Driver_delay.blend89 KBDownload

Event Timeline

Serge Lyatin (SergeL) created this task.Sep 24 2017, 10:23 PM
Serge Lyatin (SergeL) updated the task description. (Show Details)
Joshua Leung (aligorith) changed the task status from Unknown Status to Archived.Sep 25 2017, 2:46 AM
Joshua Leung (aligorith) claimed this task.
Joshua Leung (aligorith) added a subscriber: Joshua Leung (aligorith).

Not a bug.

This is a known limitation of the existing evaluation pipeline, and is the expected behaviour given how the system works. See diagram:

  • The old depsgraph only works on a per-datablock level. To be precise, it can only schedule updates for "all Object-attached stuff" and "all ObData attached stuff".
  • The "Transform Channel" dvar is set to use the "World Space" outputs from the Cube object. This means that the driver depends on the results of same Object-datablock that is currently being calculated when evaluating the driver. In other words, we have a cyclic dependency.
  • The "Single Property" dvar doesn't have this problem, as Blender knows that it only needs to read off the property values, instead of needing any final output of the datablock evaluation step.