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

Sebastian H. Schmidt sebastian.h.schmidt at googlemail.com
Thu Aug 20 19:33:01 EDT 2009


hi jake,

there are 2 ways:

1st way: use chops, import color, and than with the slope chop (switch type
to "acceleration")
2nd way:
imagine this small mindgame:
  color is a position
  change in color = change in position --> velocity of "colorchange"
  and if you consider the alteration in this velocity of "colorchange" you
get the accelaration

i hope my old math teacher is not rotating in his grave ;)

cheers
Seb

On Fri, Aug 21, 2009 at 12:01 AM, jacob fisher <jacobfshr at gmail.com> wrote:

> 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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