# Houdini

## Getting a camera position in Vex, using optransform and cracktransform

In an earlier post I used a very unwieldy approach to get the position of a null in vex. Since then I have used a method where an object_merge SOP is set to transform “into this object” and then a point function can easily read the first point in the null to arrive at a center position. The problem with this is that you are reading the icon geometry for the object and in the case of a null this is ok, but the camera icon has each point offset from it’s origin.

Enter optransform and cracktransform. The optransform function will let you set a path to the camera (or whatever else you may want to use) and then populates a matrix with the target’s transforms. You can then extract out the position as a vector using the cracktransform function… this function if pretty powerful and at first confusing but essentially it has a switch which let’s you specify the portion of the matrix you need.

In the example above I used these functions to get the camera position and then used that position to compute a direction in “camera space’ to allow a ray SOP to project scattered points onto an object as a camera projection. Sounds complex, but it’s not. Here’s the wrangle which uses optransform and cracktransform to get the camera position:

vector @cdir; vector @raydir; matrix camMatrix = optransform(chs("camera")); //get a matrix with the camera's transforms. @cdir = cracktransform(0, 0, 0, {0,0,0}, camMatrix); //extract out the camera position as a vector @raydir = normalize(@P-@cdir); //get a vector to project pointo from camera

## Attribute Transfer

It had to be done.

## Vex setpointgroup snippet

Since I was making circuit looking stuff I tried a different approach above, and in doing so had to explore setting random points into a group via Vex. Enter the setpointgroup function, very handy indeed. This snippet creates a group of random points based on a threshold, and is useful enough to take note of:

if ( rand(@ptnum+chi("seed")) < ch('threshold') ) { setpointgroup(0, chs("Group"), @ptnum, 1, "set"); }

## Circuit POP

This was fun and quick to do by restricting the direction particles could move to 45 degree increments and keeping them flat on the ground plane. Eventually I’ll be posting .hip files with these posts, once I get this blog’s back end a bit more up to date.

## Dot Product to Cull Points and Establish a Gradient

Here’s another super-simple trick you can perform with the dot product of two vectors. If you create two nulls and get the vector between them, you can compare that with the vector from any point to the first null using a dot product. Any value less than zero is past the plane of the first control normal to the second. That doesn’t read well, but the result is straightforward, you are using the controls to act like a normal and cull points.

Furthermore, the distance between those two nulls acts as a handy scale factor to color points in a gradient between the two. Here for instance we’ve “cut” a sphere in half…

The structure is very simple as you can see in the network view. The wrangle is also very simple. Thanks to Tokeru for inspiration and the basic structure of this wrangle.

vector p1, p2, v1, v2; p1 = point(1,'P',0); //control null 1 p2 = point(2,'P',0); //control null 2 v1 = normalize(p2-p1); //vector between control nulls v2 = @P-p1; //vector from current eval point to control 1 f@angle = dot(v2, v1); //The dot product of the two, indicates 'perpendicularity' if (@angle <= 0){ removepoint(0,@ptnum);} //Cull out any points past p1 float dist = distance(p1,p2); @Cd = dot(v2,v1)/dist; //The distance between the two can also act as a gradient

This works, but there’s a far simpler approach to take using the matrix transforms mentioned a few posts earlier. Transform the points to the origin of a control object’s local space (you saved that wrangle as an asset, right?) and then you can use very simple logic – in this case let’s delete any point greater than zero in z.

if ( @P.z > 0) { removepoint(0,@ptnum); }

Translate the points back to their original world position (in that other wrangle you saved as an asset, right?) and bam, you have a super easy tool to cull points according to a control object. Or make a gradient, or whatever else you want.

## Dot Product and Edges in Vex

Getting the dot product of two vectors is infinitely useful, and easy in Vex. In this example I get a vector pointing from scattered particles to the camera and then get their dot product with the surface normal at that point. This gives you a very good idea of whether the surface is facing or perpendicular to the camera at any given point.

In this case, I simply cull any point which isn’t close to a value of zero, indicating the surface is perpendicular to the camera. If instead I culled particles that were less than zero I would be removing all back-facing (occluded) particles, which is perhaps more useful but less visually interesting. Since I am left with only particles near the edges of the geometry it’s easy to connect them with lines and get an “edge detection” kind of look.

Here’s the Vex wrangle I used. Note an earlier polyframe node gave me point normals on the scattered points to work with.

float threshold = chf("Threshold"); //User input threshold vector camPos = point(1, "P", 0); //The position of the imported camera vector toCam = normalize(camPos - @P); //A vector from each point to the camera) //@N = toCam; //Use to visualize toCam as normals f@angle = dot(toCam, @N); //The dot product of the normal and toCam vectors. if (@angle > threshold || @angle < -1*threshold){ removepoint(0,@ptnum); }

I’ve used this edge culling in production a few times, to create anime style speedlines and particle effects like rain strikes and halos. Culling particles behind geometry is of course also handy, though raycast solutions are much cleaner for that kind of thing. Next, we’ll use a simple dot product calculation to cull points and create a gradient effect.

## HUD elements

I’ve made a small (but growing) library of assets to help generate and animate HUD-style graphics. Here are a few early results…