Implementing Multiple Render Targets in Three.js

For your Lattice-Boltzmann fluid simulation requiring 9 distribution function values per grid point, you'll need to use multiple render targets to output data from your shader. While WebGLMultipleRenderTarget is deprecated, you can still achieve this using the current Three.js API.

Solution Approach

You can use multiple WebGLRenderTarget instances or consider using WebGL3DRenderTarget to store your data. Here's how you can implement this:

Option 1: Using Multiple WebGLRenderTargets

1. Create separate render targets for each set of values:

JAVASCRIPTCopied
// Create render targets for your simulation
const renderTargetA = new THREE.WebGLRenderTarget(width, height, {
  format: THREE.RGBFormat,
  type: THREE.FloatType,
  minFilter: THREE.NearestFilter,
  magFilter: THREE.NearestFilter
});

const renderTargetB = new THREE.WebGLRenderTarget(width, height, {
  format: THREE.RGBFormat,
  type: THREE.FloatType,
  minFilter: THREE.NearestFilter,
  magFilter: THREE.NearestFilter
});

const renderTargetC = new THREE.WebGLRenderTarget(width, height, {
  format: THREE.RGBFormat,
  type: THREE.FloatType,
  minFilter: THREE.NearestFilter,
  magFilter: THREE.NearestFilter
});

2. Render to each target sequentially:

JAVASCRIPTCopied
// For each simulation step
function simulationStep() {
  // First render pass - output to renderTargetA
  renderer.setRenderTarget(renderTargetA);
  // Render your scene with the appropriate shader
  renderer.render(sceneA, camera);
  
  // Second render pass - output to renderTargetB
  renderer.setRenderTarget(renderTargetB);
  renderer.render(sceneB, camera);
  
  // Third render pass - output to renderTargetC
  renderer.setRenderTarget(renderTargetC);
  renderer.render(sceneC, camera);
  
  // Reset render target
  renderer.setRenderTarget(null);
}

Option 2: Using WebGL3DRenderTarget

For your specific case, WebGL3DRenderTarget might be more appropriate as it allows you to store multiple layers of data:

JAVASCRIPTCopied
// Create a 3D render target with 3 layers (for your 9 values as RGB in 3 layers)
const renderTarget3D = new THREE.WebGL3DRenderTarget(width, height, 3, {
  format: THREE.RGBFormat,
  type: THREE.FloatType,
  minFilter: THREE.NearestFilter,
  magFilter: THREE.NearestFilter
});

// Initialize the render target
renderer.initRenderTarget(renderTarget3D);

When rendering to this target, you can specify which layer to render to:

JAVASCRIPTCopied
// Render to layer 0
renderer.setRenderTarget(renderTarget3D, 0);
renderer.render(scene, camera);

// Render to layer 1
renderer.setRenderTarget(renderTarget3D, 1);
renderer.render(scene, camera);

// Render to layer 2
renderer.setRenderTarget(renderTarget3D, 2);
renderer.render(scene, camera);

Shader Implementation

In your fragment shader, you'll need to output to the appropriate render target. For the sequential approach, each shader would output to gl_FragColor as normal.

For more advanced setups, you might need to use WebGL 2.0 features and modify your shaders to use multiple render targets with gl_FragData[].

Ping-Pong Technique

For iterative simulations, consider using the ping-pong technique where you alternate between two sets of render targets:

JAVASCRIPTCopied
let currentRenderTargets = [renderTargetA1, renderTargetB1, renderTargetC1];
let nextRenderTargets = [renderTargetA2, renderTargetB2, renderTargetC2];

function simulationStep() {
  // Use currentRenderTargets as input textures
  // Render to nextRenderTargets
  
  // Swap for next iteration
  [currentRenderTargets, nextRenderTargets] = [nextRenderTargets, currentRenderTargets];
}

This approach should allow you to implement your Lattice-Boltzmann simulation with the necessary 9 values per grid point using Three.js render targets.