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Set Driven Key


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What's up

I come from a background in Maya, and have always appreciated the use of set driven key in Maya. I was curious as to how you could implement a set driven key setup in houdini. For those of you that have not used Maya, set driven key allows you to have one attribute of an object drive an attribute of another object. Here is an example:

Lets say you take the Y translate of a sphere to be at 1 unit in your scene and then lets say you have another sphere with scale 1 1 1 and you key this relationship. Then you move the first sphere up in Y to 4 units in the scene and scale up the second sphere to be 3 3 3, then you key this relationship.

Basically what you will have is when the first sphere moves up in Y from 1 to 4 you will see the second sphere scaling up in size from 1 1 1 to a size of 3 3 3. Set driven key gives you the ability to make relationships between objects, or bones, basically anything and have that relationship set in stone to be used later on for keyframe animation. Hope that made some sense.

In houdini I came across the Paste Copied Relative Refs and this leans towards the set driven key in Maya but not exactly, because it sets a linear relationship between two parameters, it is more like the Connection editor in Maya.

So after all of that is there any takers. If you still are not sure what the heck I am talking about please post that you don't get it and I will try to explain it better.

Mahalo

Benjamin

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Hi Benjamin.

I've used Maya quite extensively in the past. But I am going to tell you that Houdini's "set-driven key" is far more flexible and powerful. :)

There are MANY different ways to handle "set driven key" in Houdini. It's all depending on what you want to do. One of hte most common way to do is to use the ch() function. This will allow you to "steal" value from other channel and reference it to another channel. THere is an array of ch() function built into Houdini. You can type "exhelp -k ch" (or just "exhelp" for all expression related help) to find a host of ch- related function. Look into Expression Language pdf for this. Usually, the ch() can be accompanied by if() function -- which will allow you to set a condition to do certain task if certain conditions are met or not met.

If you want something much more elegant, you can head out to CHOP for this.

DO NOT let expression scares you away!

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channel references are the way to go. you can use a simple fit expression to edit the range. so for you example, it would look something like:

fit(ch("../sphere1/ty"),1,4,111,333)

this may not seem as easy at first (although really there's not much to it), but there's a lot more you can do with it, since you can append all sorts of functions.

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There's no "set driven key" exactly that I've come across, but can do alot more with relative references than just link parameters. The referenced channel can be used in any expression, so you can put a whole expression in the field and manipulate the channel based on changes in the copied reference. Set driven key in Maya makes things simpler granted, but you can accomplish the same results with expressions in Houdini.

The attached file does exactly what you explained in your example using referenced channels and expressions. Take a look at the scale channels on geo2. If you moce geo 1 up, geo two scales by 3 on all axis between the values of 1 and 3.

relative.hipnc

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Thank you all for the info, it makes sense now.

Man trying to understand Houdini is like pulling teeth sometimes, but on the other hand I don't think I will ever use another app. Houdini is the most powerful program I have ever come across, and to think it is only going to get better now that Side Effects is catering to the general public, more GUI!! :D

BTW I have another question. I ran into a little utility for faking GI in Houdini. I downloaded the necessary files I needed but I can't seem to get Houdini to see those files, or I have put the files in the wrong place. Here is the link if anyone is interested in helping me figure this out, the utility looks cool enough to atleast take the time to get it to work:

http://www.lewkiw.com/html/global.html

mahalo

Ben

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your right about houdini being th emost powerful app around the only problem is the over comlpexety (untill you know it all).

i've tryed runnning tht Gl fin ebefore and i couldn't get it to run i think you have to complie it in unix. i only say this as to use other things like shaders you have to complie them. things like this make it a bit of a pain to use.

bren

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Daniel Kramer made a Houdini friendly tcl-tk version of Maya's SetDriven Key called the HouCon Editor. http://www.odforce.net/codex/interface.htm [ Note: the .htm, the old odforce is still alive]

This was a while back and I don't know if it is friendly to 5.5 versions. Someone could update the code I guess (if they found the time, laid off the character work, and the l-systems :) ).

Basically it created a series of chops and used the chf expression to reference those channels so mimic the setdriven key interface. I remember looking at the code for it and he used a GUI builder for the tk so the code isn't very "friendly" at first.

It is possible to do setdriven key effects in Houdini, but the Houdini way is to start from the ground up, it is getting better, but getting the process started is a bit of work.

Check out the chf and fit expressions as a start point. Once you tap into the power of chf to grab channels like an array, it will get easier.

Oh, if there was only more time

-k

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hmmm...well...

I thought the scripts were gone...but I was trying the site and not the link <_< .... anyway...

damn....that looks just like Maya's connection editor..LOL..cool...and it seems to work quite well...

but the crazy stuff in the news sure is interesting...

this is a little test to see what was up with the Maya To Houdini scripts from Daniel Kramer and Pete Shinners...

MayaToHoudiniSkin.mel is busted in Maya 4.5...

oops!...my bad...it does work...at least it does spit out an *.ocapt file...if yo can find it...

maya loads MayaToHoudiniSkel.mel just fine and it kinda works:

from_maya_01.jpg

or should I say......Huuuuuhhhhh?????

from_maya_02.jpg

LOL

anyway...it's a start....

and really I'm not too interested in getting rigs out of Maya and into Houdini but rather building rigs in Houdini ....so look for updates...or more l-system stuff depending on my mood... :P

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