Animating thousands of fish with MultiMeshInstance3D

This tutorial explores a technique used in the game ABZU for rendering and animating thousands of fish using vertex animation and static mesh instancing.

In Godot, this can be accomplished with a custom Shader and a MultiMeshInstance3D. Using the following technique you can render thousands of animated objects, even on low end hardware.

We will start by animating one fish. Then, we will see how to extend that animation to thousands of fish.

Animating one Fish

We will start with a single fish. Load your fish model into a MeshInstance3D and add a new ShaderMaterial.

Here is the fish we will be using for the example images, you can use any fish model you like.



The fish model in this tutorial is made by QuaterniusDev and is shared with a creative commons license. CC0 1.0 Universal (CC0 1.0) Public Domain Dedication

Typically, you would use bones and a Skeleton3D to animate objects. However, bones are animated on the CPU and so you end having to calculate thousands of operations every frame and it becomes impossible to have thousands of objects. Using vertex animation in a vertex shader, you avoid using bones and can instead calculate the full animation in a few lines of code and completely on the GPU.

The animation will be made of four key motions:

  1. A side to side motion

  2. A pivot motion around the center of the fish

  3. A panning wave motion

  4. A panning twist motion

All the code for the animation will be in the vertex shader with uniforms controlling the amount of motion. We use uniforms to control the strength of the motion so that you can tweak the animation in editor and see the results in real time, without the shader having to recompile.

All the motions will be made using cosine waves applied to VERTEX in model space. We want the vertices to be in model space so that the motion is always relative to the orientation of the fish. For example, side-to-side will always move the fish back and forth in its left to right direction, instead of on the x axis in the world orientation.

In order to control the speed of the animation, we will start by defining our own time variable using TIME.

//time_scale is a uniform float
float time = TIME * time_scale;

The first motion we will implement is the side to side motion. It can be made by offsetting VERTEX.x by cos of TIME. Each time the mesh is rendered, all the vertices will move to the side by the amount of cos(time).

//side_to_side is a uniform float
VERTEX.x += cos(time) * side_to_side;

The resulting animation should look something like this:


Next, we add the pivot. Because the fish is centered at (0, 0), all we have to do is multiply VERTEX by a rotation matrix for it to rotate around the center of the fish.

We construct a rotation matrix like so:

//angle is scaled by 0.1 so that the fish only pivots and doesn't rotate all the way around
//pivot is a uniform float
float pivot_angle = cos(time) * 0.1 * pivot;
mat2 rotation_matrix = mat2(vec2(cos(pivot_angle), -sin(pivot_angle)), vec2(sin(pivot_angle), cos(pivot_angle)));

And then we apply it in the x and z axes by multiplying it by VERTEX.xz.

VERTEX.xz = rotation_matrix * VERTEX.xz;

With only the pivot applied you should see something like this:


The next two motions need to pan down the spine of the fish. For that, we need a new variable, body. body is a float that is 0 at the tail of the fish and 1 at its head.

float body = (VERTEX.z + 1.0) / 2.0; //for a fish centered at (0, 0) with a length of 2

The next motion is a cosine wave that moves down the length of the fish. To make it move along the spine of the fish, we offset the input to cos by the position along the spine, which is the variable we defined above, body.

//wave is a uniform float
VERTEX.x += cos(time + body) * wave;

This looks very similar to the side to side motion we defined above, but in this one, by using body to offset cos each vertex along the spine has a different position in the wave making it look like a wave is moving along the fish.


The last motion is the twist, which is a panning roll along the spine. Similarly to the pivot, we first construct a rotation matrix.

//twist is a uniform float
float twist_angle = cos(time + body) * 0.3 * twist;
mat2 twist_matrix = mat2(vec2(cos(twist_angle), -sin(twist_angle)), vec2(sin(twist_angle), cos(twist_angle)));

We apply the rotation in the xy axes so that the fish appears to roll around its spine. For this to work, the fish's spine needs to be centered on the z axis.

VERTEX.xy = twist_matrix * VERTEX.xy;

Here is the fish with twist applied:


If we apply all these motions one after another, we get a fluid jelly-like motion.


Normal fish swim mostly with the back half of their body. Accordingly, we need to limit the panning motions to the back half of the fish. To do this, we create a new variable, mask.

mask is a float that goes from 0 at the front of the fish to 1 at the end using smoothstep to control the point at which the transition from 0 to 1 happens.

//mask_black and mask_white are uniforms
float mask = smoothstep(mask_black, mask_white