[Sidefx-houdini-list] DOPs Impact question

Graham Thompson captainhammy at gmail.com
Wed Jan 12 17:32:07 EST 2011

This is all really as I would expect it.  Most if not all of the time
your impacts will occur during sub frames.  Depending on your objects
velocity and the amount of substepping and such the simulation might
not recognize that your objects are impacting the ground until they
have part way through, which is why your impact points appear below
the ground plane.  This also explains why you can have a large number
of impact points as a point is generated at each impact during each
subframe.  Looking at the time field of the data can show what happens
during each step.  By increasing the number of minimum substeps you
can get around this problem by increasing the accuracy of the
collisions so it detects the impact right away, though this will
introduce longer sim times and might not be ideal if your sim is
already looking good.  An alternative would be to use a Ray SOP to ray
the points up to the ground plane where you want them.

As for the data showing up after the impact actually occurred, well
that is related to what I just mentioned.  Your impact occurs during
say frame 21.765342 but since you are using integer frames you can not
really expect to see anything until the next whole number frame in
which case the simulation has advanced and your object probably isn't
impacting anymore.

Finally, your source object isn't actually a DOP object.  It is a
digital asset that is creating a bunch of data, in the case of the
glue object it is creating a bunch of objects which are then simulated
and receive impact data as they hit individually.  As far as the
simulation engine is concerned there is no "sphereObject1" in your
actually simulation you could attach the data to. As for getting the
data from all your piece objects usually people will use a SOP setup
with some copy + stamping using dopnumrecords and dopfield to get at
the data for each record or each object.  Here is a scripting solution
I've posted a couple times on odforce that dramatically simplifies
this and even lets you attach some attributes as well.


On Wed, Jan 12, 2011 at 12:40 PM, Craig Hoffman <craigleehoffman at aol.com> wrote:
>  Hi guys,
> I am doing a DOPs simulation and getting the Impact data out to drive particle systems where impacts happen and am having some issues.
> The DOPs sim looks really accurate- no interpenetrations or anything, but the "Impact" positions coming out of DOPs aren' exactly where I would expect them to be.  Some are a fair bit below the groundPlane collision plane (although others are pretty close).  Is there a way to get more accurate Impact data since the actual simulation seems pretty accurate?  Also, they seem to show up a frame after I would expect them to.  Is "Impact" data generated for the frame after a collision is detected?
> Also, I am using a Glue Object, so the Impact Data I want shows up on "piece1", "piece2", etc. and not on the source object "sphereObject1".  Is there some way to get all the Impact data from the entire simulation rather than having to collect it for each piece at a time using expressions like "dopnumrecords" and "dopfield" (which don't take wildcards).
> Thanks for any help.
> Craig
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