[Bf-committers] particle + mirror modifier bug

[Oliver Mangold]

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.