toddcox-visualize/entt-question.md

I have some "composite" objects that I'm not sure how best to map into an ECS. Each composite object has multiple "parts" with various properties. But the parts of a given composite object are intrinsically linked: they transform together and use the same buffers during rendering.

Note that I'm still pretty early in adopting entt for this project, so I'm not particularly tied to any of the architecture here. The only constraint is that, for optimization reasons, I need each composite object to get one buffer on GPU, and each part owns some interval within its parent's buffer. This nested scheme seems the most straightforward way to do that.

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My first attempt has only one entity for each composite object, with a Parts component that holds an std::vector<Part>.

Rendering code looks something like this. In reality I save the draw commands in a separate component and re-use those on each frame.

auto view = registry.view<Parts>();

for (auto [entity, parts]: view.each()) {
  std::vector<Command> commands;

  for (auto part: parts.parts) {
    // assemble draw command
    commands.emplace_back(...);
  }

  // bind buffers for entity
  // issue draw command
}

However it seems appropriate to give each part its own entity. My attempt for this was to switch to std::vector<entt::entity> where each entity has a Part as a component.

Rendering code then looks like:

auto view = registry.view<Parts>();

for (auto [entity, parts]: view.each()) {
  std::vector<Command> commands;

  for (auto part_entity: parts.parts) {
    auto &part = registry.get<Part>(part_entity);
    // assemble draw command
    commands.emplace_back(...);
  }

  // bind buffers for entity
  // issue draw command with shared buffer
}

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In my last attempt, I invert this so each Part holds only parent entity. The Parts component is removed. I can then use a view<Part> directly, but it's trickier to organize the draw calls since they must be grouped by the parent entity itself.

auto view = registry.view<Part>();

for (auto [entity, part]: view.each()) {
  // assemble draw command
  auto &commands = registry.get<Commands>(part.parent);
  commands.emplace_back(...);
}

for (auto [entity, commands]: view
for (auto [entity, commands]: commands) {
   // bind buffers for entity
   auto &commands = registry.get<Commands>(entity);
   // issue draw commands
}

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Is there any more sensible approach to this kind of many-to-one entity composition? Or some way to declare multiple Part components on the same entity? It seems like I almost need some group-by kind of operation, but I don't see such on the registry/view docs.