Point instances lose material after applying Geometry Node Modifier

Question

• I use geometry nodes (blender 2.93) to distribute cylinders on the cube (Point distribute, Point instance nodes are used)
• I define materials if Materials Properties for cube (red color) and cylinder (blue color). On this stage everything is OK: red cube is covered with blue cylinders.
• After it I create a sphere (green color) and define particle system for the sphere (render as object, object - Cube). I expect, that green sphere is covered with red cubes, which are covered with blue cylinders. But in reality green sphere is covered with red cubes. No cylinders.

The reasonable solution seems to me to apply geometry nodes modifier for cube. After it I see green sphere is covered with red cubes, red cubes are covered with red (not blue) cylinders.

So point instances lose their material after applying modifier.

• Is this situation is expected?
• How to keep material of point instances when using the object (cube) as particle?

I would be grateful for the answer :)

Answer 1

Applying the modifier loses information for two reasons :

• Your cube doesn't have cylinder_mat (or whatever the blue material is called) in its material slots.
• You cylinder object's material slots aren't setup in the right order.

Explanation :

When making the GN "real", it's all converted to one mesh. Every face has a material_index attribute which tells the object and renderer how to display it. If your cylinder object only has 1 material, all its faces will have a material index of 0 (first element). It will then naturally point to the first material slot in the cube object, which is cube_mat (red).

In order to keep the materials, you have to :

1. Add an empty slot in the cylinder material, and add the actual material as the 2nd slot. Make sure to apply the correct material to all faces.

2. Add this material to the 2nd slot of the cube.

Applying the modifier keeps the correct materials :

You should do this for every subsequent object / material you are adding. As long as the material slot(s) in the base object are the same as in the instancer object, the materials will be the same.

However on my build I didn't have trouble instancing the (Cube + Cylinders) object on another object, without applying the GN modifier. Make sure you are using the latest stable version, the behaviour might have changed recently.

Note :

• This setup will destroy UVs. It's only good for materials that don't rely on them.
• If you are to use a modifier or somehow modify the geometry after instancing, all links to materials will be lost. In that case the end of this answer might help : https://blender.stackexchange.com/a/231024/86891