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TobiasSteiner

Bomb Force - AKA Volume Axis Force In All Directions On RBD Fractured Objects

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Hi guys.

I've been muckin' around in Houdini Indie 15.5 trying to get a very simple effect of pushing apart fractured geometry in all directions from the center (or any point of origin actually) and so far not very happy with the results.

I've used the suggested “Magnet Force” method but to be honest this doesn't yield an acceptable result for a true bomb force effect. The various force nodes in Houdini seem to merely generate singular directional forces which is also not desired. I am aware of the method to set up a network of points or a point cloud of sorts that then all have a point force on them to drive dynamics however this is a convoluted and inefficient method at best.

The desired result is very simple. If you are familiar with Maya which also uses Bullet dynamics, you may be familiar with the Volume Axis Force attribute which allows us to perform what I'm describing in one fell swoop. This is what I'd like to achieve in Houdini.

Any insight into successfully achieving a true and accurate dynamic behavior of a bomb force would be greatly appreciated. Thank you kindly for your time and thoughts!

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marty    574

You probably want to post examples of what is acceptable for you so we know the standard you are after. Thanks!

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paulh    10

It might seem convoluted at first, but it's actually quite simple and very powerful to set up your own per-point force object. Sure, the volume axis force in Maya is a one click solution, but it's also very limited. In Houdini you can alter your forces in infinite ways to get exactly what you need. Once you get used to having that kind of control, Maya forces seems laughably simplistic.

Here's a simple file set up in < 5 minutes. There are numerous, and probably more efficient ways to do the same thing.

bomb_force.hip

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Thank you guys for the help.  They all look pretty decent.

21 hours ago, LaidlawFX said:

Here is a pre-built explode force like you want if you don't want to pay the iron price with Alexander example.

http://www.orbolt.com/asset/LaidlawFX::ExplodeForce::1.0

This seemed like the most promising option but the attributes don't appear to animate in 15.5.  I've tried keyframing Scale Force, Transforms (xyz position in the "enable transform" option), and uniform scale (if position in the center of the target object) but the value changes have no effect when keyframed.  Bug or limitation of the asset?

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LaidlawFX    139
1 minute ago, TobiasSteiner said:

 I've tried keyframing Scale Force, Transforms (xyz position in the "enable transform" option), and uniform scale (if position in the center of the target object) but the value changes have no effect when keyframed.  Bug or limitation of the asset?

In Dops make sure when you animate to flip any drop down menu parameters that says "Use Default" of the parmop_* series of parameters to "Set Always" or it will use the "default" which is usually controlled by the "Default operation" parmop_* that is set to "Set Initial" i.e. it only picks the first frame evaluation so the simulation does not have to process an update on each frame. Check out the gravity force for a quick example, and animating that.

The rest of those parameters are controls in the sop geometry node that is branched from the field force(the iron price), so they do not use those style controls and will update each frame if you animate them.

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8 minutes ago, LaidlawFX said:

In Dops make sure when you animate to flip any drop down menu parameters that says "Use Default" of the parmop_* series of parameters to "Set Always" or it will use the "default" which is usually controlled by the "Default operation" parmop_* that is set to "Set Initial" i.e. it only picks the first frame evaluation so the simulation does not have to process an update on each frame. Check out the gravity force for a quick example, and animating that.

The rest of those parameters are controls in the sop geometry node that is branched from the field force(the iron price), so they do not use those style controls and will update each frame if you animate them.

Perfect!  That made it all work just fine.  Thank you so much!

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1 hour ago, LaidlawFX said:

In Dops make sure when you animate to flip any drop down menu parameters that says "Use Default" of the parmop_* series of parameters to "Set Always" or it will use the "default" which is usually controlled by the "Default operation" parmop_* that is set to "Set Initial" i.e. it only picks the first frame evaluation so the simulation does not have to process an update on each frame. Check out the gravity force for a quick example, and animating that.

The rest of those parameters are controls in the sop geometry node that is branched from the field force(the iron price), so they do not use those style controls and will update each frame if you animate them.

Quick question.  If I use the "Enable Transform" option, am I suppose to be able to use the force field as shown in the viewer as a kind of falloff field?  When I move this field far away from the fractured object it still effects it as though I did not move it at all.  Even after toggling the "Set Always" option.

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LaidlawFX    139

It just allows you to re-position where the emitter, there is no inherent fall off per say.

Rollover for Enable Transform - "This allows you to post transform your geometry if the desire shapes shouldn't be at the origin."

There is no inherent falloff value with this setup. I could do one I suppose, but it would have to be next week, and even then it would have mix usefulness.

With most realistically force you animate them on for a quick pulse, and the variety of the impulse i.e. the unevenness of the velocity over the impact area will more realistically stagger the movement of pieces as some will move slower than others. Another relevant example is the "beat" of your explosion, just trigger it on then for a few frames. Or like in boxing with a knockout punch to the face, the impact is delivered very short and quickly, but you see the knockdown as the reciprocal movement as momentum is absorbed, not a direct continuation of the punch force. Hopefully that makes sense.

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12 hours ago, LaidlawFX said:

It just allows you to re-position where the emitter, there is no inherent fall off per say.

Rollover for Enable Transform - "This allows you to post transform your geometry if the desire shapes shouldn't be at the origin."

There is no inherent falloff value with this setup. I could do one I suppose, but it would have to be next week, and even then it would have mix usefulness.

With most realistically force you animate them on for a quick pulse, and the variety of the impulse i.e. the unevenness of the velocity over the impact area will more realistically stagger the movement of pieces as some will move slower than others. Another relevant example is the "beat" of your explosion, just trigger it on then for a few frames. Or like in boxing with a knockout punch to the face, the impact is delivered very short and quickly, but you see the knockdown as the reciprocal movement as momentum is absorbed, not a direct continuation of the punch force. Hopefully that makes sense.

Understood.  Thank you for the explanation.  The effect I am trying to create with a falloff method is to introduce a slow cracking effect along selected regions just before the explosion force.  I figured I could animate the region of influence paired with the scale force rising in order to achieve this.

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LaidlawFX    139
10 minutes ago, TobiasSteiner said:

Understood.  Thank you for the explanation.  The effect I am trying to create with a falloff method is to introduce a slow cracking effect along selected regions just before the explosion force.  I figured I could animate the region of influence paired with the scale force rising in order to achieve this.

I usually would create my own sop emitter for this. I draw a curve along the crack, animate the carve, uv it, resample it, and then use the uvs in a point vop to control the force scale it towards the head of the cracks, use a little noise to randomize the amount of force, and sometime add some more points along the curve to ensure both side have directional emitters, skinning or sweeping and controlling the pscale based on the uvs similar to the velocity scale. 

Another random thing that helped was not even using forces, but just animating a large vornoi fracture base plate underneath your main piece. Hitting it with sticks method, it's very direct, but gets the job done the cracks in your sub layer define where the cracks form on the top.

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23 hours ago, LaidlawFX said:

I usually would create my own sop emitter for this. I draw a curve along the crack, animate the carve, uv it, resample it, and then use the uvs in a point vop to control the force scale it towards the head of the cracks, use a little noise to randomize the amount of force, and sometime add some more points along the curve to ensure both side have directional emitters, skinning or sweeping and controlling the pscale based on the uvs similar to the velocity scale. 

Another random thing that helped was not even using forces, but just animating a large vornoi fracture base plate underneath your main piece. Hitting it with sticks method, it's very direct, but gets the job done the cracks in your sub layer define where the cracks form on the top.

Great ideas.  I'll give this a crack....pun intended. Thanks!

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