Hi,
I would like to fix a bug, concerning disconnect/reconnect of hairs on an
object with a mirror modifier. I already reported it
(https://developer.blender.org/T54488), but now I also dug a little into the
relevant code. My findings are, that in the function remap_hair_emitter(),
which does the main work of reconnecting hair to the mesh, there is a weird
case decision
if (target_psmd->dm_final->deformedOnly) {
/* we don't want to mess up target_psmd->dm when converting to
global coordinates below */
dm = target_psmd->dm_final;
}
else {
dm = target_psmd->dm_deformed;
}
where in my tests the second branch is taken, thus dm_deformed is used for the
further steps, in particular to build the BVH which is used to search for the
nearest face to connect the a hair to.
bvhtree_from_mesh_get(&bvhtree, dm, BVHTREE_FROM_FACES, 2);
It seems that in dm_final the mirrored faces are present, while in dm_deformed
they are not. Thus was happens is, that the hair on the mirrored side get
connected to the nearest point of the nearest unmirrored face. The problem goes
away when removing the case decision and always using dm_final. Checking the
history, once it worked that way, and was changed to the current situation in
this commit:
commit 0666de06f3119ff1640f1eb66833cd4feb669209
Date: Thu Jan 15 18:15:52 2015 +0100
Fix for particle system copy: This has to make sure the ORIGSPACE data
layer is available.
Otherwise particle mapping to the new mesh cannot work with subdivided
and constructively-modified meshes.
I have to admit, that I got lost at that point. I'm not familiar enough with
the code yet, to understand what really is the idea behind selecting dm_final
or dm_deformed, and what problem exactly was fixed by that. Would be great if
someone could explain. How can this bug be fixed without breaking the fix of
the commit above?
Best regards,
O.
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