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Old 07-11-2013, 04:00 PM   #1
harefort
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sascha herfort
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nCloth: glueStrength vs. glueStrengthScale

Hey guys!

I'm having trouble wrapping my head around the difference between glueStrength and glueStrengthScale.
The documentation mentions that glueStrengthScale adjusts the distance that the constraint needs to separate before it breaks.
But if it's distance-based, then what's the need for the regular glueStrength?

How do these parameters affect each other and what are the exact conditions for a constraint to break?

I'm using vertex colors a lot to make constraints rip/break over time and I usually have to iterate quite a bit on the painting and work with exponential curves on the glueStrength to get close to what I want.
Is there a setup that allows for precise control of when a constraint breaks - without having to manually place hundrets of them?

Thanks!
-Sascha
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Old 07-11-2013, 08:17 PM   #2
Duncan
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Duncan Brinsmead
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The scale is just a multiplier of the glue strength value. It is simply so you can put the glue strength in a range where it is easier to edit.

The regular glue strength is distance based... the difference in position between the desired constrained position and the position after other constraints (like stretch resistance) have pulled on the point. This difference is a simple measure of strain.

With stress based snapping it is hard to predict the exact moment of failure, and it depends on a lot of factors, such as substeps and stretch resistance. If instead you want exactly predictable breaking you could use an animated map on contraint strength where it suddently goes to zero where you want it to snap. ( you could also map glue strength as an animated map, but it is not really necessary to use glue strength at all if you are manually animated the strength to zero when you want it to snap ).

Last edited by Duncan : 07-11-2013 at 08:24 PM.
 
Old 07-11-2013, 08:44 PM   #3
harefort
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sascha herfort
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Thanks for sheding some light on this, Duncan!

I haven't considered using an animated map so far. I'll give that a shot.
Can I key the constraint vertex color set or what's the best to do this?

Also, can I think of the glue strength in terms of world units then?
Like: breakDistance = glueStrength * glueStrengthScale * glueStrengthMap

I did a test with identically painted glueStrengthMaps. In one I animated glueStrength from 50 down to 0 in the other I did it with glueStrengthScale.
In the case with animated glueStrength, all constraints broke almost exactly when glueStrength turned very low. In the glueStrengthScale case, the broke at different times, depending on their glueStrengthMap color.

I feel like one is behaving differently from the other, which is why I'm asking.
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Old 07-11-2013, 09:02 PM   #4
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One is a multiplier, so if you animate the scale the effective glue strength will change at a different rate than if you animate the glue strength directly.

effectiveGlueStrength = glueStrength * glueStrengthScale

However the initial intent was that one would not animate glue strength but set it to the value where the natural stress on the cloth would tear based on forces.

I don't think you can key the per vertex values. Instead you could try mapping with something like a ramp texture then animating that. One could also use an animated image sequence on a file texture.
 
Old 07-12-2013, 05:24 PM   #5
harefort
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sascha herfort
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Thanks again, Duncan!

I ended up animating glueStrengthScale. It's still a bit tricky, but more predictable than glueStrength and quicker to iterate on that an animated texture.

Cheers!
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Old 07-12-2013, 05:24 PM   #6
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