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This commit allows Bevy to use `multi_draw_indirect_count` for drawing meshes. The `multi_draw_indirect_count` feature works just like `multi_draw_indirect`, but it takes the number of indirect parameters from a GPU buffer rather than specifying it on the CPU. Currently, the CPU constructs the list of indirect draw parameters with the instance count for each batch set to zero, uploads the resulting buffer to the GPU, and dispatches a compute shader that bumps the instance count for each mesh that survives culling. Unfortunately, this is inefficient when we support `multi_draw_indirect_count`. Draw commands corresponding to meshes for which all instances were culled will remain present in the list when calling `multi_draw_indirect_count`, causing overhead. Proper use of `multi_draw_indirect_count` requires eliminating these empty draw commands. To address this inefficiency, this PR makes Bevy fully construct the indirect draw commands on the GPU instead of on the CPU. Instead of writing instance counts to the draw command buffer, the mesh preprocessing shader now writes them to a separate *indirect metadata buffer*. A second compute dispatch known as the *build indirect parameters* shader runs after mesh preprocessing and converts the indirect draw metadata into actual indirect draw commands for the GPU. The build indirect parameters shader operates on a batch at a time, rather than an instance at a time, and as such each thread writes only 0 or 1 indirect draw parameters, simplifying the current logic in `mesh_preprocessing`, which currently has to have special cases for the first mesh in each batch. The build indirect parameters shader emits draw commands in a tightly packed manner, enabling maximally efficient use of `multi_draw_indirect_count`. Along the way, this patch switches mesh preprocessing to dispatch one compute invocation per render phase per view, instead of dispatching one compute invocation per view. This is preparation for two-phase occlusion culling, in which we will have two mesh preprocessing stages. In that scenario, the first mesh preprocessing stage must only process opaque and alpha tested objects, so the work items must be separated into those that are opaque or alpha tested and those that aren't. Thus this PR splits out the work items into a separate buffer for each phase. As this patch rewrites so much of the mesh preprocessing infrastructure, it was simpler to just fold the change into this patch instead of deferring it to the forthcoming occlusion culling PR. Finally, this patch changes mesh preprocessing so that it runs separately for indexed and non-indexed meshes. This is because draw commands for indexed and non-indexed meshes have different sizes and layouts. *The existing code is actually broken for non-indexed meshes*, as it attempts to overlay the indirect parameters for non-indexed meshes on top of those for indexed meshes. Consequently, right now the parameters will be read incorrectly when multiple non-indexed meshes are multi-drawn together. *This is a bug fix* and, as with the change to dispatch phases separately noted above, was easiest to include in this patch as opposed to separately. ## Migration Guide * Systems that add custom phase items now need to populate the indirect drawing-related buffers. See the `specialized_mesh_pipeline` example for an example of how this is done. |
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What is Bevy?
Bevy is a refreshingly simple data-driven game engine built in Rust. It is free and open-source forever!
WARNING
Bevy is still in the early stages of development. Important features are missing. Documentation is sparse. A new version of Bevy containing breaking changes to the API is released approximately once every 3 months. We provide migration guides, but we can't guarantee migrations will always be easy. Use only if you are willing to work in this environment.
MSRV: Bevy relies heavily on improvements in the Rust language and compiler. As a result, the Minimum Supported Rust Version (MSRV) is generally close to "the latest stable release" of Rust.
Design Goals
- Capable: Offer a complete 2D and 3D feature set
- Simple: Easy for newbies to pick up, but infinitely flexible for power users
- Data Focused: Data-oriented architecture using the Entity Component System paradigm
- Modular: Use only what you need. Replace what you don't like
- Fast: App logic should run quickly, and when possible, in parallel
- Productive: Changes should compile quickly ... waiting isn't fun
About
- Features: A quick overview of Bevy's features.
- News: A development blog that covers our progress, plans and shiny new features.
Docs
- Quick Start Guide: Bevy's official Quick Start Guide. The best place to start learning Bevy.
- Bevy Rust API Docs: Bevy's Rust API docs, which are automatically generated from the doc comments in this repo.
- Official Examples: Bevy's dedicated, runnable examples, which are great for digging into specific concepts.
- Community-Made Learning Resources: More tutorials, documentation, and examples made by the Bevy community.
Community
Before contributing or participating in discussions with the community, you should familiarize yourself with our Code of Conduct.
- Discord: Bevy's official discord server.
- Reddit: Bevy's official subreddit.
- GitHub Discussions: The best place for questions about Bevy, answered right here!
- Bevy Assets: A collection of awesome Bevy projects, tools, plugins and learning materials.
Contributing
If you'd like to help build Bevy, check out the Contributor's Guide. For simple problems, feel free to open an issue or PR and tackle it yourself!
For more complex architecture decisions and experimental mad science, please open an RFC (Request For Comments) so we can brainstorm together effectively!
Getting Started
We recommend checking out the Quick Start Guide for a brief introduction.
Follow the Setup guide to ensure your development environment is set up correctly. Once set up, you can quickly try out the examples by cloning this repo and running the following commands:
# Switch to the correct version (latest release, default is main development branch)
git checkout latest
# Runs the "breakout" example
cargo run --example breakout
To draw a window with standard functionality enabled, use:
use bevy::prelude::*;
fn main(){
App::new()
.add_plugins(DefaultPlugins)
.run();
}
Fast Compiles
Bevy can be built just fine using default configuration on stable Rust. However for really fast iterative compiles, you should enable the "fast compiles" setup by following the instructions here.
Bevy Cargo Features
This list outlines the different cargo features supported by Bevy. These allow you to customize the Bevy feature set for your use-case.
Thanks
Bevy is the result of the hard work of many people. A huge thanks to all Bevy contributors, the many open source projects that have come before us, the Rust gamedev ecosystem, and the many libraries we build on.
A huge thanks to Bevy's generous sponsors. Bevy will always be free and open source, but it isn't free to make. Please consider sponsoring our work if you like what we're building.
This project is tested with BrowserStack.
License
Bevy is free, open source and permissively licensed! Except where noted (below and/or in individual files), all code in this repository is dual-licensed under either:
- MIT License (LICENSE-MIT or http://opensource.org/licenses/MIT)
- Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
at your option. This means you can select the license you prefer! This dual-licensing approach is the de-facto standard in the Rust ecosystem and there are very good reasons to include both.
Some of the engine's code carries additional copyright notices and license terms due to their external origins.
These are generally BSD-like, but exact details vary by crate:
If the README of a crate contains a 'License' header (or similar), the additional copyright notices and license terms applicable to that crate will be listed.
The above licensing requirement still applies to contributions to those crates, and sections of those crates will carry those license terms.
The license field of each crate will also reflect this.
For example, bevy_mikktspace has code under the Zlib license (as well as a copyright notice when choosing the MIT license).
The assets included in this repository (for our examples) typically fall under different open licenses. These will not be included in your game (unless copied in by you), and they are not distributed in the published bevy crates. See CREDITS.md for the details of the licenses of those files.
Your contributions
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.