Approaches To Dynamics In Houdini

Introduction

• Simulation is not "live" and does not have to be cooked from first frame
• You have complete visual feedback in graph form of all channels
• You have complete control over which points are affected and how much they are affected
• You have complete control over start and end paramaters.
• You have much more control over the behaviour of the simulation as you have dozens of CHOPs at your disposal.

Points Covered

• Building a spline from a series of world-animated nulls
• Place nulls in scene
• Create a new object
• Add an object merge SOP, object merge each nulls "point" SOP
  
• Ensure transform object is "This object"

• Concept of transform object
• If you do not have a transform object defined, you will not get the object's world-space transforms, only the SOP-based ones
• Think of it as putting an apple in a car:  Inside the car, the apple is on the passenger seat always.  But if you consider the position of the car too, it might be in Winnipeg instead of London, (in which case it is probably frozen solid).  
  
• You can use a null locked at 0,0,0 instead of "This object"  as the transform object.
• Animating growth of stem
• Use add SOP to make the points you brought in with the object merge into an open polygon
• Use fit and/or polyspline and/or resample SOPs to smooth and add points
• Use carve SOP to animate growth
  
• Copying objects to a template
• Copy your L-system to the points you have selected with the group SOP by:
• Use the add SOP to remove the geometry but keep the points....
• ... thus point positions don't change when you delete unused points and pipe the rest of the points into the COPY SOP template input
• Using the Polywire SOP
• Any shared points will be smoothly joined... use the facet SOP to share points
• Using stamping
• Stamping allows "feedback" back up the SOP chain
• Allows each copy to be unique
• To access stamping up the chain, use the expression:  param("ID",0), which means, "use the ID value for the template, unless there is none, then use 0".
• Here we use stamping to rotate and scale each copy uniquely.
  

Adding a random rotation for each copy

• Fetching animation into CHOPs
• Use the geometry CHOP to fetch point positions.  Save to disc and then read back in to save cooking time.  
  
• Assigning point colours using $BBY
• Back in SOPs, pipe your L-system SOP into a point SOP, add point colors, and use the expression $BBY
• BBY represents the objects bounding box in Y.  At the bottom of the box, the value is zero, at the top, it's one, no matter the size of the object.  Thus the points at the bottom are black (0,0,0), and white at the top, (1,1,1).

If you examine the hip file available on this page, you will also see the use of the chf expression to set point colours. Here is an explanation of the use of this expression.

The chf function is used to lookup a value at a specified index. In other words:

I have created a spare channel in a point SOP called which I have called "lookup", (ctr+r click in SOP arena to add spare channel). This lookup channel goes from zero to 100 in time. However, in this case, we are not really using this channel in reference to time, we are using it in reference to $CR, which is the red point colour. The point colours go from 0 to 1, which is too small to be useful in the channel editor, so I have multiplied it by 100 so that I have more resolution to play with, and it now goes from zero to 100.

So, where the value of $CR is zero, the expressions evaluates to use the value at frame zero. When the value of $CR is 1, then the expression evaluates to use the value at frame 100, (1*100).

In this way we can draw a nice curve in the graph editor to create or modulate any value. For example, you can use a similar expression to get the alpha of a particle to fade to zero near the end of its life. In a point SOP, alpha channel:

chf("./lookup",$LIFE*100)

• Fetching point colours into CHOPs
• Use the geometry CHOP to fetch the point colours too.  Do this for only one frame as these don't animate.  
  
• Adding lag / isolating lag
• Use the aptly-named lag CHOP to add lag.  
• Modulating lag with point colours
• Subtract the original point position to get the lag only.  

Lag only

• Multiply these values by the point color so that there is more lag at the top of the branch and none at the bottom.
• Add this lag back into the original animation and re-export to SOPs using the Channel SOP

• Simple particle systems
• A simple particle system is timed to coincide with the collapse of the wall to simualte broken mortar and dust.