Falling Sand 2D + 3DDocumentation · Geometry Nodes falling-sand sandbox for Blender 4.5+

1Files and requirements

Both files work in Blender 4.5 LTS and newer (tested with 4.5 and 5.2). There is nothing to install: the simulation is made of Geometry Nodes only. Each file contains a README text block (Scripting workspace or any Text Editor) with a short version of this page.

2Quick start

1. Open FallingSand_GN.blend or FallingSand3D_GN.blend. The viewport opens in Material Preview, looking through the camera.
2. Press Space with the mouse over the 3D Viewport. The simulation starts at frame 1 and runs while the timeline plays.
3. Go back to frame 1 (Shift+Left) to reset it. Changes to the level or to the canvas settings show up from frame 1.

3Painting with brushes

Each demo has five brushes parked next to the canvas or box: Sand, Water, Fire, Stone and Eraser. A brush is just an object in the Emitters collection with an FS Emitter modifier.

1. Start playback.
2. Click a brush to select it, press G and move it over the canvas or into the box. It paints every frame it overlaps. In 3D, press Z after G to move it up and down only.
3. Right-click (or Esc) puts it back where it was; left-click leaves it there and it keeps emitting.

To change what a brush paints, open its FS Emitter modifier (Properties ▸ Modifiers) and change Element, Mode, Rate or the object's scale. Duplicate a brush (Shift+D) to make more.

4Emitters and levels

The canvas (2D) and the box (3D) read two collections:

Objects without an FS Emitter modifier use the default element. What counts as covered:

• Meshes: in 2D, the faces of the mesh seen from the top (XY). In 3D, closed meshes fill their inside and open meshes such as planes fill a one-voxel layer.
• Curves: become discs (2D) or tubes (3D) of the curve radius, times the modifier's Stroke Width.
• Grease Pencil: strokes work like curves. Select the "Level Drawing" object, switch to Draw mode, draw, then go back to frame 1.

Positions are read relative to the canvas or box object, so you can move the whole setup around. In 3D every level object is tested on its own, so overlapping shapes stamp their exact union.

FS Emitter modifier

5The 2D canvas

The FS Canvas object carries an FS Canvas Geometry Nodes modifier. It outputs a grid of faces in the XY plane, one face per cell, centred on the object, with the cell value in the face attribute fs_cell. The material FS Pixels turns that into colours.

6The 3D box

The FS3 Box object carries an FS3 Box modifier. The box is centred on the object in X and Y and starts at its origin in Z. The voxels live in a point cloud with the point attribute fs_cell; the display turns the visible ones into cubes that carry fs_cell as an instance attribute for the FS3 Voxels material.

7Elements

8Use it in your own scene

The easiest way is to append the whole setup:

1. In your file choose File ▸ Append, open FallingSand_GN.blend (or the 3D file), go into Collection and append Falling Sand (or Falling Sand 3D). The canvas or box, its material, the level, the emitters and the brushes come with it.
2. Delete the demo level and emitter objects you do not need and add your own to the Level and Emitters collections.

To start from scratch instead:

1. Append the node group FS Canvas (or FS3 Box), the node group FS Emitter and the material FS Pixels (or FS3 Voxels).
2. Add any object (a single vertex or an empty mesh is fine), add a Geometry Nodes modifier and pick FS Canvas or FS3 Box.
3. Make two collections, set them as Level and Emitters in the modifier and set the Material.
4. Add objects to the collections. Give them a Geometry Nodes modifier with FS Emitter to choose their element.

9Rendering and baking

• Use Material Preview or Rendered shading to see colours; Solid shading shows plain grey.
• Both EEVEE and Cycles work. The 2D material is emission only and looks the same everywhere. The 3D material is a Principled BSDF with glowing lava, fire and acid.
• The 2D demo uses the Standard view transform so the pixel colours stay exact.
• To render an animation, render from frame 1; the simulation is computed frame by frame. You can also bake it first from the modifier panel (Manage ▸ Bake), like any simulation zone, and then scrub or render any frame.

10Speed and memory

Simulation time only, measured without the viewport on a Ryzen 9 7950X. The time grows with the number of cells and with Steps per Frame. Empty space is cheap; the rules only run where something can change.

Memory

Like every simulation zone, the canvas keeps every played frame in memory so you can scrub back. It stores one value per cell per frame: about 150 KB per frame for the 256 × 144 canvas and about 1–3 MB per frame for the 64 × 64 × 48 box. Going back to frame 1 or changing a setting clears it. For long animations, bake to disk.

11Tips and limits

• Faster motion: raise Steps per Frame. Finer grains: raise the cell count and lower Cell Size.
• Liquids level out from the surface. A large pile of water on a wide, gentle slope flattens slowly; more Steps per Frame or a lower emitter Rate helps.
• 3D inside test: closed meshes are filled using the nearest face, so very concave or non-manifold shapes can fill imperfectly. Split them into simpler parts.
• Nothing shows up? Check that you are at or after frame 1, that the object is inside the canvas or box, and that Mode is Add only where cells are empty.
• Changes do not show: level objects are stamped on the first frame. Go back to frame 1.
• The canvas and box are axis-aligned in their own object space; rotating the object rotates the whole simulation, including gravity.

12How it works

Each cell stores one integer: element + 16 × extra, where extra is a colour variation, a liquid's flow direction, or the remaining life of fire, smoke and steam.

• Movement uses Margolus 2×2 blocks with an alternating offset. Every move is a swap inside a block, so mass is conserved and there are no write conflicts. The rules (fall, slide, sink, spread, rise) are pair tables indexed by the two elements involved.
• 3D runs the same block rules on vertical XZ and YZ slices, alternating every step.
• Reactions sum what each cell's neighbours are (heat, water, acid, lava, dissolvable) and pick a new element from per-element tables.
• Speed: a static lattice with one point per block finds the blocks that can change; only those are solved, on a dense copy. Look-up tables are attributes of a small point cloud.
• A Simulation Zone holds the state between frames and a Repeat Zone runs the steps of a frame.