BatchedMesh and Object Outlining Solutions

When working with BatchedMesh for optimizing draw calls with thousands of elements and millions of faces, implementing selection outlining can indeed be challenging since traditional approaches like OutlinePass work with individual objects rather than batched geometry.

Potential Solutions

1. Custom Shader Approach

Creating a custom shader for your BatchedMesh is likely the most efficient solution. This approach would allow you to:

• Keep the performance benefits of batching
• Avoid creating duplicate geometry
• Handle multiple selections efficiently

You could implement this by:

• Adding an ID attribute to your geometry that identifies each individual object within the batch
• Passing selected IDs as uniforms to the shader
• In the fragment shader, apply outline effects only to fragments belonging to selected objects

GLSLCopied
// Example fragment shader concept
uniform int selectedIds[MAX_SELECTIONS];
uniform int selectionCount;
varying float objectId;

void main() {
    bool isSelected = false;
    for(int i = 0; i < selectionCount; i++) {
        if(int(objectId) == selectedIds[i]) {
            isSelected = true;
            break;
        }
    }
    
    if(isSelected) {
        // Apply outline effect
    } else {
        // Regular rendering
    }
}

2. Hybrid Approach

If implementing a fully custom shader is too complex:

• Keep using BatchedMesh for regular rendering
• For selected objects, create temporary individual meshes only for the outline pass
• Limit the maximum number of simultaneously selected objects to maintain performance

This approach balances implementation complexity with performance considerations.

3. Post-Processing Alternative

Consider using a post-processing effect that can identify objects based on an ID buffer:

• Render your scene to an ID buffer where each object has a unique color
• Use a post-processing pass to detect edges between different IDs
• Apply outline effects only to the IDs of selected objects

This approach can be more flexible but might have performance implications for complex scenes.

Recommendation

Given your scale (3k elements with millions of faces), I would recommend the custom shader approach as it will likely provide the best performance while maintaining the selection outline functionality. The implementation complexity is higher initially, but it will scale better as your scene grows or as more objects are selected simultaneously.

If you're concerned about development time, the hybrid approach could be a good compromise, especially if you can limit the number of simultaneous selections.