[Sidefx-houdini-list] tracking via cops/pic()

Ivan DeWolf ivan at martian-labs.com
Thu Aug 20 19:11:39 EDT 2009


trace is better at this. The pic has a motionless grid with animating  
point attribs (color). Kind of hard to see what it's doing.

A trace SOP will give you an animating polygon. It's centroid is an  
animating channel that you can make use of.



Quoting "jacob fisher" <jacobfshr at gmail.com>:

> alvin,
> are you saying create two grids that have the sequence pic()'d to?
> or are you saying create a null that is attached to the grid and moves
> according to the movement of the red sphere that is duplicated and
> time shifted?
>
> ivan, are you saying use trace because its better at this? or you just
> don't like the use of the pic() expression?
>
> sebastian,
> do you have a suggestion of how to get the acceleration in your step 1?
> that's where i'm really stuck.
>
> -jake
>
>
> On Thu, Aug 20, 2009 at 6:58 PM, Alvin Yap<alvinyapvfx at gmail.com> wrote:
>> Hmmm... let's say you have a 1 red sphere moving on a black background.
>> Use the pic expression to pull in the color data from cops to sops
>> Time shift the geometry by -1F.
>> Merge the timeshifted geo with the current time's geo.
>> Instance geometry onto those red points.
>> Use a bounding box or some xmax-xmin expression to determine how large the
>> bounding box is for this frame and last frame.
>> Hence if the bounding box between the last and current frame is very small,
>> less movement. If the returned bounding box is huge, lots of movement.
>> Use that to drive particles?
>>
>> Too lazy to boot up houdini now- whole day on it ;-)
>>
>> Regards,
>> Alvin
>>
>> On Thu, Aug 20, 2009 at 11:31 PM, jacob fisher <jacobfshr at gmail.com> wrote:
>>
>>> Hopefully this will be a simple question I have and you all won't
>>> think less of me, but here goes.
>>>
>>> I have an image sequence I imported into COPs and I have it copied to
>>> a grid via a pic() command in the point node.
>>>
>>> What I'd like to do is have a reaction based off the movement of an
>>> object in the image sequence.
>>> As a sphere moves from right to left, I would like a particle
>>> simulation to occur.  However, I would like the particle simulation to
>>> be varied depending on how quickly the sphere is moving.  So if it's
>>> moving at 3 pixels/rows/columns/whatever a frame it will release more
>>> particles than if it moves at 1 pixel per frame.
>>>
>>> How would I go about tracking this?
>>>
>>> Thanks for any tips you can think of.
>>> -jake
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