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Object Group Reorder
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System Information
Windows 7 64bit, 16 GB RAM, AMD 8 Cores 3.11 GHz, AMD Radeon HD 6450

Blender Version
Broken: (2.70a f93bc76)

Short description of error
In a hair particle system which renders a group, the objects are sometimes reordered after saving and reloading.

Exact steps for others to reproduce the error

In the attached file in camera view, the big 'L' should be on the left side of the big 'R'. That's the way the file was saved.
But after reloading the file, they are sometimes swapped.

To correct the order, remove both little letters from the group, and then first add the little 'R' to the group and then the little 'L'. But after saving, the order is wrong again.



Event Timeline

Philip Holzmann (Foaly) raised the priority of this task from to Needs Triage by Developer.
Philip Holzmann (Foaly) updated the task description. (Show Details)
Philip Holzmann (Foaly) set Type to Bug.
Sergey Sharybin (sergey) lowered the priority of this task from Needs Triage by Developer to Normal.Jun 27 2014, 10:54 AM

This is a known limitation of the particle code: It uses objects from the group randomly, but in order to make that repeatable it must expect the group to have some fixed ordering in the first place. This is not well defined, rather than lists groups are conceptually sets, without a defined order (only our use of ListBase structs doesn't enforce the distinction). The dependency graph or other code may reshuffle groups.

A workaround for the problem is to enable the Use Count option, which defines its own ordering instead of relying on the group. This is the only feasible implementation of such group object selection anyway, the random group element picking just doesn't allow repeatable results.

Thank you!
The use count option works. Now I don't have to recreate the groups every time I open my scene.