# Glue Constraint Relationship parameters questions

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Hi,

In DOP, "Glue Constraint Relationship" node, there are two parameters:

A) "Half-life" : Houdini Docs say it's how fast glue strength decays with time. I understand that this glue creates an impulse to counter the forces/torques applied on the attached pieces, so logically, as long as there are applied forces/torques the glue would keep this impulse, so how/where the decay takes part here? meaning: in each substep dt there would applied forces/torques that are countered by the glue impulse, so how time decay is used here?

B) "Propagation-Rate" : it specifies how much impacts propagate through the constraint network. I don't see any unit to specify this parameter, default value is 1! If I set it to 1, what does it mean? for example, does it mean that 100% of the impacts is propagated to neighboring constraints?

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I know, that this is an older question, but I have to warm it up, cause I have exactly the same questions and I only see marginal differences in changing the values.

So Propagation-Rate of 1? is this some real-world-value like meters
and Half-Life-Decay: does that mean the impact data can travel longer ways with higher values , cause the decay is not that much reduced ?

how are those 2 interact with each other? I really would like to understand what this values do in a glue network...

cheerz de Heavy

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funnily enough i've been playing with thse today myself - i tried tweaking that propagation rate to wild values and barely saw much change.

the half life seemed to give me more 'falloff' from the point of impact..

Maybe its worth setting up a simple scene... so a tall leaning column when hit with a ball hitting it at the top

I imagine it will maintain most of its structure when the half life is say at 0.1 ...but if you up that to 1 then i think it means the impact zone spreads out 10x as much... so the top pieces crumble and the pieces beneath break their glue constraints and it crumbles away more noticeably.

the propagation rate probably means how long it takes to crumble to this maximum distance?

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Half life controls how quickly the 'impact' attribute on the glue bond decays back to zero (assuming the glue didn't break). So a longer half-life means that successive smaller impacts can break the glue

The propagation rate is a value between 0 and 1 that specifies how much of the impulse is spread across the glue bond to the neighbouring object when an impact occurs - 0 means that nothing happens, and 1 means that the impulse is evenly split with the neighbour.

If e.g. there is a glue bond between object A and B with a propagation rate of 0.5, a glue bond between A and C with a propagation rate of 1, and A has an impulse of 5000, then after 1 iteration the impulses are:
A: 2000
B: 1000
C: 2000

The number of propagation iterations specifies how many rounds of this diffusion to perform, and therefore controls how far an impact spreads through the glue network in a single timestep.

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