stop-motion effect

[etudenc]

Hi all!

Still pretty new to Houdini and as an exercise I was trying to make a bunch of copied boxes change their position in world Y on every third frame using a stamp on the Copy SOP.

What I want to see happen is the boxes get translated randomly in world Y every three frames. For the frames that don't get evaluated I want the boxes to stay in their current (randomized) positions and this is what I can't seem to make happen.

Using an IF expression and modulus seemed like a good path. As an example, in the Value Field I created a variable called "move" and for the Value entered

if($F % 3 == 0, rand($PT * $F), $TY)

I then set the TZ component on the box's Xform SOP to

stamp("../copy1", "move", 0)

Problem is, the boxes return to their default positions on the frames where IF is false. I tried "chf("./ty", $F - 1)" in place of $TY in hopes that I could grab the value for $TY but no go.

I hope all the above is clear, and I hate to ask what I fear is a very basic question, but I am stumped!

Any suggestions would be appreciated!

[pclaes]

Something like this? :

in y-channel of your boxes

$FF-($FF%3)

$FF will push the boxes up

-($FF%3) is your offset.

If you wanted you could add a stamp:

+ stamp(..) is a per box offset in y

hmm, rereading what you want... Are you actually trying to accumulate those random y values?

kinda like:

box 1 at frame 1 is at Y value 2,

box 1 at frame 2 is at Y value 2,

box 1 at frame 3 is at Y value (2 + 8 = 10),

box 1 at frame 4 is at Y value 10,

box 1 at frame 5 is at Y value 10,

box 1 at frame 6 is at Y value (10 + 3 = 13),

box 1 at frame 7 is at Y value 13,

...

with "8" and "3" being random numbers?

If that is the case you are accumulating and effectively require a simulation or a feedback loop. You can either do that with a sopsolver, a particle system or a foreach sop. (sopsolver would be my preferred way personally).

Edited June 10, 2011 by pclaes

[etudenc]

hmm, rereading what you want... Are you actually trying to accumulate those random y values?

Hi pclaes,

Thanks so much for the response! I want the boxes to randomly change position in Y on every other frame, so with $F % 2 = 0:

box1 at frame 1 stays put

box1 at frame 2 jumps to a random position in Y

box1 at frame 3 stays in the position it was randomly moved to at frame 2

box1 at frame 4 jumps to a random position in Y

box1 at frame 5 stays in the position it was randomly moved to at frame 4

etc.

Does that make better sense? The accumulation ideas will be good for future study, thanks! And gotta say, the training DVD you created for 3DBuzz is fantastic, I hope you have more in store!

[pclaes]

ah, that makes it easy. Just put the rand($FF-($FF%2)) into your y channel.

This will change the seed for the rand expression every 2 frames.

Video tutorials... might do them again at some point, but tricky to make time for them at the moment. I'm sharing scene files of projects/tests that I'm working on and writing some comments in them, which is a bit more straightforward and equally fast if not faster for intermediate/advanced users to learn from, but might get lost in time. Perhaps when I'm inbetween jobs.

[etudenc]

ah, that makes it easy. Just put the rand($FF-($FF%2)) into your y channel.

This will change the seed for the rand expression every 2 frames.

That's closer (at least they all move every other frame), but they all move together now. I tried adding * $PT at various places in the expression but that seems to cancel out the "every other frame" part.

[pclaes]

Are your point numbers changing? Are you using particles?

If so, you might want to use $ID instead of $PT.

what about: rand(($FF-($FF%2))+$PT)

[etudenc]

Are your point numbers changing? Are you using particles?

If so, you might want to use $ID instead of $PT.

what about: rand(($FF-($FF%2))+$PT)

Got it! Just had to get the parentheses in the right place so it would evaluate in the right order.

rand(($FF - ($FF % 2)) * $PT )

pclaes, any reason you chose to use $FF instead of $F? I plugged both in and they both work fine.

Thanks again for the help!

Edited June 11, 2011 by etudenc

[pclaes]

just habit really... $FF is floating point, it helps to evaluate subframes. I more often deal with floating point than with integers. If I need it as integers I can still cast it int(...) .