# How to select submesh after CSG2 subtraction?

**URL:** <https://forum.babylonjs.com/t/how-to-select-submesh-after-csg2-subtraction/64085>\
**Category:** Questions\
**Created:** [September 11, 2026, 4:35pm UTC](https://forum.babylonjs.com/t/how-to-select-submesh-after-csg2-subtraction/64085 "2026-09-11T16:35:19Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![wxz](https://avatars.discourse-cdn.com/v4/letter/w/e47c2d/32.png) [@wxz](https://forum.babylonjs.com/u/wxz)\
**Post date:** [September 11, 2026, 4:35pm UTC](https://forum.babylonjs.com/t/how-to-select-submesh-after-csg2-subtraction/64085/1 "2026-09-11T16:35:19Z")

</div>

hi,

I’m trying to select the cutting surface after a subtraction operation. AI suggested to user marker material and submesh extraction, but the code it provided doesn’t work: [https://playground.babylonjs.com/#V3UZQA#3](https://playground.babylonjs.com/#V3UZQA#3)

When I click on the red surface, it still logs clicked the original outer shell

---

<div class="post-metadata">

**Author:** ![Deltakosh](https://sea2.discourse-cdn.com/flex024/user_avatar/forum.babylonjs.com/deltakosh/32/26635_2.png) [@Deltakosh](https://forum.babylonjs.com/u/Deltakosh)\
**Post date:** [September 14, 2026, 2:26pm UTC](https://forum.babylonjs.com/t/how-to-select-submesh-after-csg2-subtraction/64085/2 "2026-09-14T14:26:18Z")

</div>

Let me check that

---

<div class="post-metadata">

**Author:** ![Deltakosh](https://sea2.discourse-cdn.com/flex024/user_avatar/forum.babylonjs.com/deltakosh/32/26635_2.png) [@Deltakosh](https://forum.babylonjs.com/u/Deltakosh)\
**Post date:** [September 14, 2026, 3:24pm UTC](https://forum.babylonjs.com/t/how-to-select-submesh-after-csg2-subtraction/64085/3 "2026-09-14T15:24:14Z")

</div>

Your marker-material idea is valid, but there are two issues in the PG.

First, the call is using the old CSG argument order. `CSG2.toMesh` takes `name, scene, options`, so it should be:

```auto
const finalResultMesh = mainCSG .subtract(cutterCSG) .toMesh("finalResultMesh", scene); 

```

You normally should not replace `finalResultMesh.material` afterward. If the base and cutter have different material instances before `CSG2.FromMesh`, CSG2 automatically creates the appropriate `MultiMaterial`.

Internally, `FromMesh` records the source material’s `uniqueId` as provenance. For `A.subtract(B)`, the new inner boundary is derived from the cutter surface, so it receives the cutter material provenance. The output `materialIndex` is only a sequential slot in the generated `MultiMaterial`, however, so don’t assume the cutter is always index `1`.

The reason `pickResult.subMeshId` reports the outer material is a CSG2/submesh picking quirk. `toMesh()` first creates a global submesh covering the complete index buffer, then appends the actual material submeshes. Picking tests the global submesh first; the material submesh hits the same triangle at the same distance, so the global submesh remains selected.

A non-destructive workaround is to resolve the material submesh from the mesh-wide `faceId`:

```auto
const multi = finalResultMesh.material; const cutMaterialIndex = multi.subMaterials.findIndex( (material) => material?.uniqueId === innerCutMaterial.uniqueId );

scene.onPointerDown = (\_, pick) => { if (pick.pickedMesh !== finalResultMesh || pick.faceId < 0) { return; }

const indexOffset = pick.faceId \* 3; const materialSubMesh = finalResultMesh.subMeshes.find( (subMesh) => !subMesh.IsGlobal && indexOffset >= subMesh.indexStart && indexOffset < subMesh.indexStart + subMesh.indexCount );

if (materialSubMesh?.materialIndex === cutMaterialIndex) { console.log("Cut surface selected"); } else { console.log("Original surface selected"); } }; 

```

Another option is to save the generated non-global submesh ranges, call `releaseSubMeshes()`, and recreate only those ranges. Normal `subMeshId` picking will then work without the overlapping global submesh.

One limitation is that this identifies cutter-derived faces through material provenance; it is not a generic “new triangle” flag. CSG2 may split or retriangulate original faces while retaining their original material. Also, the base and cutter must use different material instances, otherwise their faces cannot be distinguished this way.

---

<div class="post-metadata">

**Author:** ![wxz](https://avatars.discourse-cdn.com/v4/letter/w/e47c2d/32.png) [@wxz](https://forum.babylonjs.com/u/wxz)\
**Post date:** [September 15, 2026, 12:35am UTC](https://forum.babylonjs.com/t/how-to-select-submesh-after-csg2-subtraction/64085/4 "2026-09-15T00:35:43Z")

</div>

awesome! thanks a lot.