Applying Postprocessing to WebGLRenderTarget

Yes, it's definitely possible to apply postprocessing effects (like vignetting) to a scene rendered inside a cube mesh using a WebGLRenderTarget, while keeping the main scene unaffected.

Approach

The key is to understand the rendering pipeline when using render targets:

1. First, render your inner scene to a WebGLRenderTarget
2. Apply postprocessing effects to this render target
3. Use the processed result as a texture on your cube in the main scene

Implementation Steps

Here's how you can implement this:

JAVASCRIPTCopied
// 1. Create your render target
const renderTarget = new THREE.WebGLRenderTarget(width, height);

// 2. Set up your postprocessing for the render target
const composer = new EffectComposer(renderer, renderTarget);
composer.addPass(new RenderPass(innerScene, innerCamera));

// Add your vignette effect
const vignettePass = new ShaderPass(VignetteShader);
vignettePass.uniforms.darkness.value = 1.5;
vignettePass.uniforms.offset.value = 0.7;
composer.addPass(vignettePass);

// 3. In your render loop
function animate() {
    requestAnimationFrame(animate);
    
    // Render the inner scene with postprocessing
    composer.render();
    
    // Use the processed result as a texture on your cube
    cubeMaterial.map = renderTarget.texture;
    cubeMaterial.needsUpdate = true;
    
    // Render the main scene (which includes the cube)
    renderer.setRenderTarget(null); // Reset to default (canvas)
    renderer.render(mainScene, mainCamera);
}

Performance Considerations

This approach is generally efficient because:

1. You're only applying the postprocessing to the smaller inner scene
2. The render target can be sized appropriately for the cube (doesn't need to be full screen)
3. The main scene render doesn't require additional passes

To optimize performance:

• Use an appropriate size for your render target (not larger than needed)
• Consider using THREE.HalfFloatType for the render target if full precision isn't needed
• If the inner scene doesn't change every frame, you can render it less frequently

This approach gives you the flexibility to apply any postprocessing effect to just the inner scene while maintaining good performance for your overall application.