# TouchDesigner | SOPs and Stamps

Usually when it comes to working with SOPs I tend to focus on thinking about instances instead of copies – but there are some really excellent use cases for the Copy SOP and one of the most powerful pieces of the copy stop is stamping your copies.

If your shaking your head wondering what Stamps are, you certainly aren’t alone – they’re one of the lesser used features of the Copy SOP (in my opinion) but one that has a wide range of use cases. Recently on the forum there was an excellent question about using the data from CHOPs to set the characteristics of SOPs – Multiple Circle SOPs from CHOP samples. One solution to that particular question is using the ability to Stamp copies, and with that in mind we can pull apart how stamping works.

Let’s start with a simple example. Let’s begin by connecting a Circle SOP to a Copy SOP.

When we start there’s nothing particularly exciting that happens in this example. In order to make some changes we need to first change our number of copies:

This still isn’t particularly interesting just yet, all of the copies of our original circle are all in the same place – all stacked on top of one another. Let’s translate our copies by some small number, and turn on our wireframe view of our geometry so we can better see what’s happening:

The first copy is moved 2 units to the right, the second copy is moved 4 units to the right, the next 6, etc. This puts all of our copies neatly next to one another.

What if, however, we wanted to change the number of divisions in each copy… we might start with a triangle, and add one division for each copy. There’s no good way to do that out the gate – and it might be easy to think that you need to use a Script SOP to make this work.

Luckily, however, we can make this work with stamping. Let’s start by heading over to the Stamp Page of the Copy SOPs parameters:

We’re going to start by first turning on our Stamp inputs parameter. Next let’s figure out what we want to stamp each Copy with… if we want to change the number of divisions in a circle based on what Copy it is, we need a way to attach the copy index to that piece of geometry. Lucky of us there’s a handy python member that let’s us do just that. First let’s name our parameter `index`, and next we can use a simple python expression to attach our `copyIndex` to our geometry:

`me.copyIndex` in plain English means – “which copy of the geometry I am.” We can see that our copy index goes up to 9, and like all things programming, we should remember that our indices start at 0 – so our copy index are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. What’s interesting about using Stamps is that we first Stamp our geometry, but then go back upstream in our network to use that stamp.

So to use our stamp we now need to go to our Circle SOP. The syntax of using Stamps looks like this:

`fetchStamp(name, default)`

We just used the name `index` when we stamped our copies, so we might first start with:

`fetchStamp('index', 0)`

Which means, find the Stamp called `index` and if you can’t find it, use the value 0. BUT, before we use this expression we need to remember that a circle needs a minimum of 3 divisions – less than 3 and we’ll just end up with a line. Let’s make sure we add 3 to our stamp value since our number of copies starts at 0:

`fetchStamp('index', 0) + 3`

What does that look like:

Okay… that’s pretty cool. But that’s really just the start of our journey here. Where this gets really interesting is if we start to use this new super power to look at other operators – after all we now have a way to index our copies.

Let’s now look at something a little more interesting.

We might instead think about using a Noise TOP to help describe how we’d like to displace the vertices of our copies. The idea in this example is that we’ll start by creating a grid – we’ll use a point SOP to displace our verticies, a tips SOP to fan our one side, and a copy SOP to create a spiral:

Our Point SOP is going to displace the vertices of our SOP by using the point Index to locate the x sample position, and the copyIndex to locate the y sample index:

The resulting Spiral has a noisy feeling that we can adjust with a combination of our noise TOP and copy SOP:

This isn’t a technique that’s well suited for changing in real-time, but it is a great way to think about creating fixed geometry that you might use for any number of environmental elements in your pieces.