###### Houdini

## The joy of xyzdist() and primuv()

I’m going to try to make a nice easy introduction to my two favorite functions in Houdini VEX (besides `fit01 `

and `chramp `

of course): `xyzdist`

and `primuv`

. These functions are at the core of a lot of really useful and cool tricks in Houdini, including rivets, the attributeInterpolate SOP, the old “droplets falling down a soda can” effect, and some really awesome stuff with volume shaders. I’ll do a little example of each as a way of showing off what you can do with these clever little tools.

First, let’s take a look at the VEX definition (the third overload here is the most frequently used):`float xyzdist(string geometry, vector pt, int &prim, vector &uv, float maxdist)`

At its most basic, `xyzdist`

will return the distance from the sample point `pt`

to the nearest point on the surface `geometry`

. Note that this doesn’t mean the nearest actual *point*, but the interpolated surface in between those points.

Those little “&” symbols mean that this function will *write* to those parameters, rather than just read from them. 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.

So, what can we do with this? Click below to find out…

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