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When running in the context of a node such as a wrangle SOPthis argument can be an integer representing the input number starting at 0 to read the geometry from. Alternatively, the argument can be a string specifying a geometry file for example, a.

Returns number of elements where an integer or string attribute has a certain value. Copies the value of a geometry attribute into a variable and returns a success flag. Interpolates the value of an attribute at a certain parametric u, v position and copies it into a variable. Returns position derivative on a primitive at a certain parametric u, v position.

Returns one of the set of unique values across all values for an int or string attribute. Returns the set of unique values across all values for an int or string attribute. Interpolates the value of an attribute at certain UV coordinates using a UV attribute.

Returns the albedo percentage of reflected light for a bsdf given the outgoing light direction. Returns an anisotropic volumetric BSDF, which can scatter light forward or backward. Returns the value of a CHOP local transform channel at the specified sample and evaluation time. Returns 1 if the edge specified by the point pair is in the group specified by the string. This function computes the intersection of the specified ray with the geometry in uv space.

Returns the linear vertex number of the next vertex sharing a point with a given vertex. Returns the linear vertex number of the previous vertex sharing a point with a given vertex. Returns 1 if the point specified by the point number is in the group specified by the string.

Returns 1 if the primitive specified by the primitive number is in the group specified by the string. Returns 1 if the vertex specified by the vertex number is in the group specified by the string. Returns the point into which the vertex following the destination vertex of a half-edge in its primitive is wired.

Returns the vertex following the destination vertex of a half-edge in its primitive. Returns the point into which the vertex that precedes the source vertex of a half-edge in its primitive is wired. Returns the vertex that precedes the source vertex of a half-edge in its primitive. Finds and returns a half-edge with a given source point or with given source and destination points. Returns the default name of the alpha plane as it appears in the compositor preferences.

Returns the default name of the bump plane as it appears in the compositor preferences. Returns the default name of the color plane as it appears in the compositor preferences.

Returns the default name of the depth plane as it appears in the compositor preferences. Returns the number of components in the plane named planename in the specified input.Creates two point attributes. Creates two attributes, one primitive attribute for the total length of each curve and one point attribute for the partial length of the curve up to that point. Creates a orient attribute along each curve. A point normal vector attribute pointing along the curve is needed.

Uncomment line 21 if you have a unique up vector point attribute for each curve that you want to use. Let's you carve several curves at one based on a unique primitive carve attribute for each curve. Based on point number and depends on a point U Map attribute between on each curve. Adds a point at the center of each primitive and removes the primitive, useful for a bunch of stuff. Good for creating constraints. Beware though it creates overlapping primitives, i. See the next VEX snippet for a solution that removes overlapping primitives.

I had to squash a primitive together, from on point towards the opposite of the point the one point that wasn't connected to the "main" point. For this I needed the direction and distance to move the point.

Object Merge the cameras camOrigin into SOP's with two extra point added on the add node in the camera. If you use orient to align objects before a Bullet simulation, you need to multiply the orient attribute you get from bullet with your initial orient attribute. In DOP's when creating new points with a Wrangle node in a SOP Solver, it's tricky to assign unique id attribute when several points are created on the same time step.

Assign -1 as the id on all the points that is created and run them trough a second Wrangle node. Calculates how much a deformed geometry is squashed and skewed. For each point it takes the angle between perpendicular N and each point neighbor and compares the value between rest and deformed geometry. Plug in the rest geometry into the first input and the deformed geometry into the other.

houdini vex align

Point order has to match. Curve length Creates two attributes, one primitive attribute for the total length of each curve and one point attribute for the partial length of the curve up to that point Run in a Wrangle SOP Run over - Primitives. Curve Orient - Runs over several primitives Creates a orient attribute along each curve. Curve Multi Carve Let's you carve several curves at one based on a unique primitive carve attribute for each curve. Add a point at the center of each primitive Adds a point at the center of each primitive and removes the primitive, useful for a bunch of stuff.

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Create a poly line between adjacent points Good for creating constraints. Remove overlapping primitives Mainly a complement to the previous VEX snippet.

Random text as a point string attribute. Random Groups. Point Cloud - Base Base code for point cloud opertations. Find opposite point on a quadrilateral 4 points primitive I had to squash a primitive together, from on point towards the opposite of the point the one point that wasn't connected to the "main" point.

Reset orient If you use orient to align objects before a Bullet simulation, you need to multiply the orient attribute you get from bullet with your initial orient attribute. Point unique id In DOP's when creating new points with a Wrangle node in a SOP Solver, it's tricky to assign unique id attribute when several points are created on the same time step.

Cavity map Calculates how much a deformed geometry is squashed and skewed.New to Vex? You might want to read JoyOfVex first.

So you've pushed past hscript, ignored python for now, and seen the awesomeness of vops.

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You'll also probably start to get curious as to why there's at least 4 different ways to define point attributes, all with various idosyncrasies. Time to sweep all that away with Vex, specifically with the Wrangle Sop. Wrangle Sops let you write little bits of Vex code, and come with a few UI niceties to make them quick and efficient to use.

Best way to get into vex and wrangle nodes is to put down a 40x40 grid, append a point wrangle, and follow along:. Hit ctrl-enter, look in the geometry spreadsheet, there you go, float attribute. The tells houdini that you want this to be an attribute, the default type is float.

You want a vector attribute? Prepend 'v' before the ' '. To initialise, use curly braces if its just numbers, or use set if you're doing something with functions or other attributes:. You can set the attribute type using the appropriate prefix. The default is float, so you rarely need the f prefix. The symbol is used to get attributes as well as set them. All the attributes you see in the geometry spreadsheet, you can access them by sticking in front of their name. To access an individual attribute of a vector or colour, use attribute.

Eg to get just the y value of each point and divide in half:. To set it is similar. Eg to set the red channel of each point to the sine of double the x position:.

Vector attributes like Cd or P let you refer to components in several ways, use whatever is convenient. Note that wrangles implicitly know certain common attribute types P, Cd, N, v, orient, id, name, several othersbut if you have your own attributes, Houdini will assume its a float unless told otherwise, easy mistake to make. To ensure Houdini does the right thing, prepend the type. Eg, you've set mycolour as a vector, and try to use it in a wrangle:.

I always forget to do this, shout at my wrangles for a bit, then finally remember to stick 'v' in front of my attribute names. No doubt you'll do the same. If you've used other programming languages, you might be thinking 'Ok, but how does Vex know which point to process? How do I iterate through the geometry? There's too much voodoo here! The answer is that Vex is implicitly working in parallel. You can think of the code we've done so far as working on a single point, so something like.

Is setting the colour of a point based on its position. When you have lots of points, Vex runs that same code on all the points in parallel, substituting the values for Cd and P for each point automatically.

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You don't have to sweat the details for nowjust understand that's what's going on.It's MediaWiki running the Chameleon plugin so that it can use Bootstrap skins.

The particular skin I'm running is Yeti from Bootswatch. I also use the MsUpload plugin to make it easier to attach files. I don't have time unfortunately. In addition I like this to be a quick reference guide, so you can quickly find the example hip and text and get on with your current project, rather than have to sit and watch me ramble for 70 minutes to get to the bit you need.

Maybe I'll get more time in the future. Ends sop, 'unroll'. For those coming from Maya, you know how paintfx is ultimately lots of single curves that are welded together, but they're limited to paintfx, you can't really use them elsewhere? Houdini treats standalone curves as a 'polywire', so edges don't have to belong to a polygon to be valid. Very powerful. Lots of ways, simplest way when starting out is with an attribute randomize sop to create an up vector; the copy sop will now treat N as the primary axis to align the copies, and use up to define the twist around N.

Create an attribute randomize sop, attribute name to up, leave the dimensions as 3, set distribution as 'direction or orientation', enable 'cone angle', and slide it up to taste.

Use an attribute interpolate sop. Scatter on a frozen copy of your geometry use a timeshift with the frame expression disabledon the output attributes tab enable 'sourceprim' and 'sourceprimuv', then wire that to the first input of the attribute interpolate, and the moving geometry to the second.

Convert your geo to a volume first, then scatter can use an isooffset sop for this, or a VDB from polygons sop in density mode. You're probably on Apprentice, it limits the kind of things you can export to make sure you can't use it in production. Buy an indie license! Yes they are. Yes it does. Yes it is. But its not as slow for complex things as you'd expect, when put in the right hands it can be compared to most commercial renderers. It's current design goals are to be the jeep of rendering, not an F1 car; its reliable and will get you there, but not in a hurry.

Everyone who wants renders quickly seem to be having great success with Redshift. In a broad sense Houdini is a tool to modify data on geometry. Vex is a direct way to do that, its fast, multithreaded, inherently designed to work with large amounts of geometry in parallel.

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Vex is most often used in attribute wrangle nodes, and can be identified by using P syntax to refer to attributes. Houdini also contains a UI, and a unix flavoured high level overview of your data.

houdini vex align

Hscript is designed to deal at this level; driving UI elements with expressions, moving around your scene with 'cd' and 'ls' if thats what you like. Its the UI expressions that hscript is most associated with, and generally speaking whenever an older tutorial references hscript, there's usually a better vex alternative to use.

It appears there's a long term goal to remove hscript from Houdini, but that won't be for a while. Python is python. If you know what Python is, great, if not, its a handy generic glue language common to most vfx apps, and comes pre installed with linux and osx and is an easy install for windows.

Within Houdini its used to create and delete nodes, define shelves, drive parts of the UI, open files, talk to databases, websites, run the help system Look a little closer and the lines are blurred you can use Python to do geometry processing and define UI parameter expressions, hscript variables can sometimes be understood by vex wrangles in a roundabout waybut thats the general overview.

Vex is the interesting stuff, hscript is the necessary evil, python is for pipeline td's or for when vex doesn't have the function you require.At its most basic, xyzdist will return the distance from the sample point pt to the nearest point on the surface geometry. So if we feed this function an integer and a vector, in addition to the distance to the surface, it will also give us the primitive number prim and the parametric UVs on that primitive uv.

Note that parametric UVs are not the same as regular UVs… this just means the normalized position relative to the individual primitive we found. The easiest example to start with is the rivet. We just want to stick a point onto an object, and have it follow the object around.

The initial setup. The pig itself is being merged in from another network. Connect your point and your template geometry to a Point Wrangle, and start with this code:.

Now what? Given a geometry, a primitive number, and a parametric UV coordinate, primuv will tell you whatever the hell you want to know about the geometry at that point. It will even interpolate attributes for you between points such as point colors, or position. Add this code to your Wrangle:.

You should see your dot stick to the nearest point on surface! You only want to do that computation once. With deforming geometry, you could use Time Shift to pick your reference frame. With just a pig head being transformed at object level, we can use some options on our Object Merge to accomplish something similar. Connecting the second Wrangle and Object Merge. This means we have a static reference point for finding the nearest primitive and point.

Your code should look like this:. If you run the Scatter on static geometry in this case with a Time Shiftwe can use Attribute Interpolate to stick those points onto the moving geometry, via the same two values we can get from xyzdist.

Remember to make sure that the attributes on the Scatter or any other node generating the points and the Attribute Interpolate are the same name.

houdini vex align

Here we are actually going to use some actual old-fashioned UVs, as well as our fancy new functions, to achieve the effect. Using Attribute Interpolate. Note that the output attributes on the Scatter SOP need to be enabled for this to work.

We want the droplets to move down the surface of the can, maybe wiggling back and forth a tiny bit the way that droplets do due to surface tension. We could solve this particle simulation on a cylinder, and use sticky attributes or ray projection to figure out ways to wiggle the particles around and still keep them attached to the surface. Or, we could just solve the problem in an easier space. UVs are nice and simple and flat! We want to make sure these are Point UVs, so we can manipulate points with them later.

Once we have a projection we like, we can move the points into UV space with the following Point Wrangle:. Now you should see the points moved to their UV coordinates, but in real space. We can solve the particle system in this space very easily.

Particles will randomly stop on their way down, similar to how droplets sometimes hit a rivulet or another sticky part of the contact surface and slow down. Finally, particles that make it to the bottom of the can are destroyed. Append an Attribute Wrangle and connect the flattened geometry to the second input.Left and Right Primitives. The notions of "left" and "right" which follow depend on context. If an auxiliary input is used, it is always the right primitive and the primary input geometry are all left primitives.

If only one input is used, then for each pair being aligned, there is a left and a right primitive. This means that relative to neighboring primitives, one primitive can be both left and right. Any geometry type. Any geometry type consisting of a single primitive treated as the "right" primitive. If an auxiliary input is used, this location specifies an endpoint for the alignment.

Causes each primitive in the group to be aligned. If off, only the first primitive is aligned and all others are placed relative to it. AlignTube Example for Align geometry node. Adaptive Prune. Agent Clip. Agent Clip Properties. Agent Clip Transition Graph. Agent Collision Layer. Agent Configure Joints.

More stuff

Agent Constraint Network. Agent Definition Cache. Agent Edit. Agent Layer. Agent Look At. Agent Prep. Agent Proxy. Agent Relationship. Agent Terrain Adaptation. Agent Transform Group. Agent Unpack. Agent Vellum Unpack. Loads the geometry from an Alembic scene archive. Alembic Group. Alembic Primitive.

Alembic ROP output driver. Attribute Blur.On the Houdini Discord server s I keep seeing the same kinds of questions over and over again, mostly related to instancing. The most common questions asked seem to be:. Aside from the obvious position of each point, P, there are a few other attributes with obvious effects. First things first. Next, some template point attributes.

If Houdini finds these attributes, it will try to orient each copy so that its Z-axis points towards Nand its Y-axis points towards up. Note that you can sometimes get away with only providing Nbut you might get unpredictable results. Edit: Something worth noting here is the dihedral VEX function. If you only provide an N attribute to the Copy SOP, Houdini has to guess what your up vector might be in order to build a complete orientation matrix.

It does this by calculating the rotation necessary to rotate the world Z-axis onto your N vector, and then applying that same rotation to the Y-axis to figure out the up vector. The dihedral function does exactly this… given two vectors, it computes the rotation matrix necessary to get from one to the other.

A Long-Winded Guide to Houdini Instancing

Basically, the matrix is telling you which way is up, right, and forward. This is how we rotate objects in CG. The third axis can be inferred by computing the cross product of the other two axes.

Take a look at the instances on this deforming mesh:. Note how they sometimes flip around. The same movement, using quaternions for orientation. Note how the copies are much more stable.

A matrix3 can easily be converted to a quaternion these are of type vector4 in Houdini using the VEX quaternion function. Quaternions themselves can easily be blended from one to the next using slerpor spherical linear interpolation. Quaternions are used all over computer graphics, especially in video game engines due to their ability to predictably rotate vectors.

Our point wrangle code for rotating this geometry or any old geometry at the origin looks like this:.

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If we run this, the object should just sit and spin slowly. Easy enough, right?

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We need to make sure each matrix, then, is oriented to that N, along with an up vector for stability. The maketransform function used here instead of ident means our starting matrix is already pointing the way we want it to be before we start rotating.

We define an axis and angle, exactly as before, and spin that matrix around. The last step is just converting the matrix to a quaternion and naming it orientwhich the Copy SOP knows to read. The output result looks like this:.

Now to answer those second two questions… how to vary the objects copied per-point, and how to do it without copy stamping?

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The first point to take home here is that copy stamping is dead. The second point is that the copy stamping functionality can be rebuilt in a slightly different way using For-Each blocks, and Spare Inputs. Spare Inputs are a relatively new trick that allows for some pretty amazing performance tweaks, and occasionally more readable networks.

The first spare input you create, Spare Input 0, can be somewhat counter-intuitively referred to in an Hscript expression as So to grab P. Connect three different shapes into a Switch SOP, then create some points and give them a random integer attribute named index this name is arbitrary with a value between 0 and 2. Our loop will run once per point, discarding all other points during each iteration, then merging everything back together again at the end.

Then output the Copy to Points to the Block End. Now, the fun part.


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