How can I apply the **same** random value to multiple instances in geometry nodes?

Question

.blend file:

I have a simple geometry node tree for adding wings to a butterfly body. I would like to randomize the X & Y scales for the wings, however I need the same random value (X) and the same random value (Y) to be applied to each wing (so that they match).

Here is how the wings look without any randomization applied:

I have tried two approaches to randomize the scale of the wings:

1. Add a random value to the Scale vector input of the Instance on Points node (everything in red in the below screenshot):

This results in different random values being applied to each wing:

2. I also tried scaling the instances after realizing them, however this had the same effect:

Scaling the instances after realizing them does not appear to be an option as they will no longer scale from their origins.

Answer 1

Random numbers are seeded not only by the Seed input, but also by the ID. If you don't connect anything to the ID socket, the default ID will be used, which will differ for each instance. You can remedy this by connecting an input to the ID socket, just as you do with the Seed socket.

In the following two images, the Viewer column in the spreadsheet shows the generated random number. The first one is using a forced ID, while the second one uses the default ID. I did this with a simple setup of my own, before I noticed that you had actually included a blend file. I'm using the images I had already made, but I have verified that it works with your blend as well.