How to use an instance position as texture coordinate in shader?

Question

How could be used instance position generated by geometry nodes in material node tree as Texture Coordinate > Object?

I tried to use position vector, but with no luck ...

What I'm trying here is to take attribute of this two instances position (stored under name attribute) and I expect this vector position will be used like an origin of regular Object in shader so I can control up/down position of mask.

Probably could be done simpler by Domain Size node, but I don' know how either :) Also using Value node directly to Output socket as value to be used in shader via Attribute node > Fac > Math as Ad with Texture Coordinate node as Object.

Anyway ... I'm looking for this result - two planes position control min-max of mask.

In this sample node-tree I used real objects for Texture Coordinates.
For Geometry Nodes version I used Attribute node, but result is zero info.

There is a plenty of threads how to bring texture coordinates into geometry nodes, but not an opposite way.

Q2: In GeoNodes I use node group with Add and Subtract node to math negative value, that seems to me complicated. What is the proper way?

Answer 1

In your setup, you are trying to capture the Position vectors (via Store Named Attribute) in the Instance domain, but at that step there aren't any instances created yet so the capture fails. Simply switch to the Point domain and the information will be captured correctly:

However, even now that we're managing to bring that information into our shader, there's one big problem: to the best of my knowledge, those values are written on the very geometry the Geometry Nodes modifier is manipulating—they don't magically become available to any and all objects. When you ask a shader to use values captured by a GN modifier, shader tries to read those values from the geometry it is currently assigned to. We captured two Position vectors on our points, but those points aren't in the Cylinder's mesh, they're in the two Grids we instanced in the GN object, so the shader fails to find anything to read (see the bottom of the post for a visual demonstration).

This insight makes it obvious as to how we can affect the colors of the Cylinder with our GN setup: we need to bring the Cylinder into our GN tree so its geometry becomes available to capture values on:

_{GN SETUP. We sample the position vectors from the points, but capture them as attributes on the Cylinder's geometry. I also took the liberty of swapping out the two Grids with the two Toruses available in the file.}

Now to use that captured information in the shader. Simply comparing the two vectors (or rather their separated Z components) to each other like in your setup doesn't yield the desired result, because what we wanna compare them to is the shader's coordinate system itself. We get that from the Position socket of the Geometry node. If the Position—Z of the geometry the shader is assigned to is lesser than the captured Bottom value OR greater than the captured Top value, use the Green shader, otherwise use the Pink:

_{SHADER SETUP. The OR logic gate node is from this answer}

_{WHOLE SETUP}

BONUS: Here's a demonstration of the shader failing to read values captured by the GN setup if it's not assigned to the same object. Below, you can see the Cylinder as re-created in the GN setup on the left, and on the right the original Cylinder with no GN setup, both using the same shader. The one with the GN modifier can correctly compare the captured position with its own coordinates, but the one on the right cannot read those values which means it gets a blanket value of $0$, so everything greater than $0$ gets colored green, and everything smaller gets colored pink: