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[Leonard <question683065@xxxxxxxxxxxxxxxxxxxxx>]

Question #683065 on Yade changed:
https://answers.launchpad.net/yade/+question/683065

Description changed to:
Hi,
In yade, O.interactions.geom.normal gives the unit vector oriented along the interaction, from particle #1, towards particle #2
(https://yade-dev.gitlab.io/trunk/yade.wrapper.html?highlight=o%20interaction#yade.wrapper.ChCylGeom6D.normal).
My question is that the directions seems not always from Id1 towards Id2, for example:
O.bodies.append(sphere((0,0,0),radius=1,color=[1,1,1]))
O.bodies.append(sphere((0,0,1.5),radius=0.5,color=[0,0,1]))
Gl1_Sphere.quality=3
O.engines=[
    ForceResetter(),
    InsertionSortCollider([Bo1_Sphere_Aabb()]),
    InteractionLoop(
        [Ig2_Sphere_Sphere_ScGeom()],
        [Ip2_FrictMat_FrictMat_FrictPhys()],
        [Law2_ScGeom_FrictPhys_CundallStrack()]
    ),
    NewtonIntegrator(damping=0.4),

]
##run for one step to generate interactions
O.step()
After this, I use O.interactions[0,1].geom.normal and obtain the result as:
In [2]: O.interactions[0,1].geom.normal
Out[2]: Vector3(0,0,1).
This is reasonable, however, when I change the Id of the two spheres like:
O.bodies.append(sphere((0,0,1.5),radius=0.5,color=[0,0,1]))
O.bodies.append(sphere((0,0,0),radius=1,color=[1,1,1]))
Gl1_Sphere.quality=3
O.engines=[
    ForceResetter(),
    InsertionSortCollider([Bo1_Sphere_Aabb()]),
    InteractionLoop(
        [Ig2_Sphere_Sphere_ScGeom()],
        [Ip2_FrictMat_FrictMat_FrictPhys()],
        [Law2_ScGeom_FrictPhys_CundallStrack()]
    ),
    NewtonIntegrator(damping=0.4),

]
##run for one step to generate interactions
O.step()
I use O.interactions[0,1].geom.normal  again to check the normal vector and obtain:
In [2]: O.interactions[0,1].geom.normal
Out[2]: Vector3(0,0,1)
which is the same as before, that means the vector normal isn't from Id1 towards Id2. I think it should be Vector3(0,0,-1).
Could you please point out where is the problem?
My yade version is 2018.02b on Ubuntu 18.04
Thanks in advance!

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