Godot

Multimesh buffer's layout#

The Multimesh property of a MultiMeshInstance3D node exposes its raw data through a Buffer property allowing for instance to apply updated data from a compute shader directly.

Its layout is however fairly opaque and hidden in other wiki pages and in the source code: so here is a snippet to avoid (my) hours of headbanging research.

Each instance has a Transform3D property describing its position, rotate and scale, itself composed of an Vector3 Origin describing its position, and a 3x Vector3 Basis describing its rotation and scale. Each Vector3 is being made of three floats values, forming a set of 12 float values in a (in my opinion) non-intuitive order.

If you enable UseColors=true, four additional values (R, G, B and A, as a Color property) are added at the end of this set.

If you enable UseCustomData=true, four additional values (for well, custom usages) are added at the end of this set.

Depending on the configuration, your data set will be of size 12, 16 or 20, and you need to offset you id's by that factor.

 1 int offset 16; //3D base + color enabled, no custom data
 2 int BufferId = offset * id;
 3 
 4 data[BufferId + 0] = value.Basis.X.X;
 5 data[BufferId + 4] = value.Basis.X.Y;
 6 data[BufferId + 8] = value.Basis.X.Z;
 7 
 8 data[BufferId + 1] = value.Basis.Y.X;
 9 data[BufferId + 5] = value.Basis.Y.Y;
10 data[BufferId + 9] = value.Basis.Y.Z;
11 
12 data[BufferId + 2] = value.Basis.Z.X;
13 data[BufferId + 6] = value.Basis.Z.Y;
14 data[BufferId +10] = value.Basis.Z.Z;
15 
16 data[BufferId + 3] = value.Origin.X;
17 data[BufferId + 7] = value.Origin.Y;
18 data[BufferId +11] = value.Origin.Z;
19 
20 data[BufferId +12] = value.Color.R;
21 data[BufferId +13] = value.Color.G;
22 data[BufferId +13] = value.Color.B;
23 data[BufferId +13] = value.Color.A;

MultiMeshInstance (2D)#

In the case of 2D the same applies, but with Origin being only a Vector2 and Basis being only 2x Vector2, forming a 2+2x2[+ 4][+ 4] data set of size going from 6 to 14.