Stable Random per Point with rand(@ptnum)

What it does

Uses rand(@ptnum) to produce a random value that is unique per point but constant over time, because the seed (the point number) does not change frame to frame. Assigning it to a vector like @Cd seeds R, G and B independently.

When to use it

Use it for any per-point variation that should stay put: random colour, random scale, random offsets, random group membership. It is the seed you reach for before adding time or class-based variation.

The VEX code

// rand() of the point number is stable per point and per frame.
@Cd = rand(@ptnum);                       // vector: random RGB
f@size = fit01(rand(@ptnum), 0.5, 1.5);   // float: random scale

Inputs and assumptions

• Runs per point in an Attribute Wrangle over points.
• @ptnum is the seed; it is stable, so the result does not flicker.
• Assigning rand() to a vector randomises each channel; to a float, a single value.

Common mistakes

• Seeding off a value that changes each frame (like @Time) when you wanted a stable look - use @ptnum.
• Expecting different results for two points that share a point number after a merge or copy - offset the seed (e.g. rand(@ptnum + @primnum * 7919)).
• Wanting per-piece (not per-point) variation - seed off a @class or @name instead of @ptnum.
• Assuming rand() is uniform enough for cryptographic use - it is a fast hash for look variation only.

Frequently asked questions

Why use rand(@ptnum) instead of rand(@Time)?

@ptnum is stable per point, so the randomness stays put frame to frame. Seeding off @Time makes values change every frame, which flickers.

How do I randomise per piece instead of per point?

Seed off a per-piece attribute such as i@class or s@name (for example rand(i@class)) so every point in a piece gets the same value.

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