d0e606b87c
5 Commits
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Jan Hohenheim
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d0e606b87c
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Add an example for doing movement in fixed timesteps (#14223)
_copy-pasted from my doc comment in the code_ # Objective This example shows how to properly handle player input, advance a physics simulation in a fixed timestep, and display the results. The classic source for how and why this is done is Glenn Fiedler's article [Fix Your Timestep!](https://gafferongames.com/post/fix_your_timestep/). ## Motivation The naive way of moving a player is to just update their position like so: ```rust transform.translation += velocity; ``` The issue here is that the player's movement speed will be tied to the frame rate. Faster machines will move the player faster, and slower machines will move the player slower. In fact, you can observe this today when running some old games that did it this way on modern hardware! The player will move at a breakneck pace. The more sophisticated way is to update the player's position based on the time that has passed: ```rust transform.translation += velocity * time.delta_seconds(); ``` This way, velocity represents a speed in units per second, and the player will move at the same speed regardless of the frame rate. However, this can still be problematic if the frame rate is very low or very high. If the frame rate is very low, the player will move in large jumps. This may lead to a player moving in such large jumps that they pass through walls or other obstacles. In general, you cannot expect a physics simulation to behave nicely with *any* delta time. Ideally, we want to have some stability in what kinds of delta times we feed into our physics simulation. The solution is using a fixed timestep. This means that we advance the physics simulation by a fixed amount at a time. If the real time that passed between two frames is less than the fixed timestep, we simply don't advance the physics simulation at all. If it is more, we advance the physics simulation multiple times until we catch up. You can read more about how Bevy implements this in the documentation for [`bevy::time::Fixed`](https://docs.rs/bevy/latest/bevy/time/struct.Fixed.html). This leaves us with a last problem, however. If our physics simulation may advance zero or multiple times per frame, there may be frames in which the player's position did not need to be updated at all, and some where it is updated by a large amount that resulted from running the physics simulation multiple times. This is physically correct, but visually jarring. Imagine a player moving in a straight line, but depending on the frame rate, they may sometimes advance by a large amount and sometimes not at all. Visually, we want the player to move smoothly. This is why we need to separate the player's position in the physics simulation from the player's position in the visual representation. The visual representation can then be interpolated smoothly based on the last and current actual player position in the physics simulation. This is a tradeoff: every visual frame is now slightly lagging behind the actual physical frame, but in return, the player's movement will appear smooth. There are other ways to compute the visual representation of the player, such as extrapolation. See the [documentation of the lightyear crate](https://cbournhonesque.github.io/lightyear/book/concepts/advanced_replication/visual_interpolation.html) for a nice overview of the different methods and their tradeoffs. ## Implementation - The player's velocity is stored in a `Velocity` component. This is the speed in units per second. - The player's current position in the physics simulation is stored in a `PhysicalTranslation` component. - The player's previous position in the physics simulation is stored in a `PreviousPhysicalTranslation` component. - The player's visual representation is stored in Bevy's regular `Transform` component. - Every frame, we go through the following steps: - Advance the physics simulation by one fixed timestep in the `advance_physics` system. This is run in the `FixedUpdate` schedule, which runs before the `Update` schedule. - Update the player's visual representation in the `update_displayed_transform` system. This interpolates between the player's previous and current position in the physics simulation. - Update the player's velocity based on the player's input in the `handle_input` system. ## Relevant Issues Related to #1259. I'm also fairly sure I've seen an issue somewhere made by @alice-i-cecile about showing how to move a character correctly in a fixed timestep, but I cannot find it. |
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Chris Biscardi
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20eb13f458
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Use smooth_nudge in 2d_top_down_camera example (#13907)
# Objective [`StableInterpolate`](https://dev-docs.bevyengine.org/bevy/math/trait.StableInterpolate.html) was introduced after this example was written (or more accurately, the two PRs were merged on the same day and developed in parallel). The example used `Vec3::lerp` where I believe it is now preferred to use `smooth_nudge`. ## Solution Its not entirely clear to me whether `StableInterpolate` should be preferred in this scenario, although it seems likely. So I figured a PR to make the change would either result in it being merged or denied with a reason. ## Testing ``` cargo run --example 2d_top_down_camera ``` Co-authored-by: François Mockers <mockersf@gmail.com> |
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Jan Hohenheim
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6273227e09
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Fix lints introduced in Rust beta 1.80 (#13899)
Resolves #13895 Mostly just involves being more explicit about which parts of the docs belong to a list and which begin a new paragraph. - found a few docs that were malformed because of exactly this, so I fixed that by introducing a paragraph - added indentation to nearly all multiline lists - fixed a few minor typos - added `#[allow(dead_code)]` to types that are needed to test annotations but are never constructed ([here](https://github.com/bevyengine/bevy/pull/13899/files#diff-b02b63604e569c8577c491e7a2030d456886d8f6716eeccd46b11df8aac75dafR1514) and [here](https://github.com/bevyengine/bevy/pull/13899/files#diff-b02b63604e569c8577c491e7a2030d456886d8f6716eeccd46b11df8aac75dafR1523)) - verified that `cargo +beta run -p ci -- lints` passes - verified that `cargo +beta run -p ci -- test` passes |
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Jan Hohenheim
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48f70789f5
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Add first person view model example (#13828)
# Objective A very common way to organize a first-person view is to split it into two kinds of models: - The *view model* is the model that represents the player's body. - The *world model* is everything else. The reason for this distinction is that these two models should be rendered with different FOVs. The view model is typically designed and animated with a very specific FOV in mind, so it is generally *fixed* and cannot be changed by a player. The world model, on the other hand, should be able to change its FOV to accommodate the player's preferences for the following reasons: - *Accessibility*: How prone is the player to motion sickness? A wider FOV can help. - *Tactical preference*: Does the player want to see more of the battlefield? Or have a more zoomed-in view for precision aiming? - *Physical considerations*: How well does the in-game FOV match the player's real-world FOV? Are they sitting in front of a monitor or playing on a TV in the living room? How big is the screen? ## Solution I've added an example implementing the described setup as follows. The `Player` is an entity holding two cameras, one for each model. The view model camera has a fixed FOV of 70 degrees, while the world model camera has a variable FOV that can be changed by the player. I use different `RenderLayers` to select what to render. - The world model camera has no explicit `RenderLayers` component, so it uses the layer 0. All static objects in the scene are also on layer 0 for the same reason. - The view model camera has a `RenderLayers` component with layer 1, so it only renders objects explicitly assigned to layer 1. The arm of the player is one such object. The order of the view model camera is additionally bumped to 1 to ensure it renders on top of the world model. - The light source in the scene must illuminate both the view model and the world model, so it is assigned to both layers 0 and 1. To better see the effect, the player can move the camera by dragging their mouse and change the world model's FOV with the arrow keys. The arrow up key maps to "decrease FOV" and the arrow down key maps to "increase FOV". This sounds backwards on paper, but is more intuitive when actually changing the FOV in-game since a decrease in FOV looks like a zoom-in. I intentionally do not allow changing the view model's FOV even though it would be illustrative because that would be an anti-pattern and bloat the code a bit. The example is called `first_person_view_model` and not just `first_person` because I want to highlight that this is not a simple flycam, but actually renders the player. ## Testing Default FOV: <img width="1392" alt="image" src="https://github.com/bevyengine/bevy/assets/9047632/8c2e804f-fac2-48c7-8a22-d85af999dfb2"> Decreased FOV: <img width="1392" alt="image" src="https://github.com/bevyengine/bevy/assets/9047632/1733b3e5-f583-4214-a454-3554e3cbd066"> Increased FOV: <img width="1392" alt="image" src="https://github.com/bevyengine/bevy/assets/9047632/0b0640e6-5743-46f6-a79a-7181ba9678e8"> Note that the white bar on the right represents the player's arm, which is more obvious in-game because you can move the camera around. The box on top is there to make sure that the view model is receiving shadows. I tested only on macOS. --- ## Changelog I don't think new examples go in here, do they? ## Caveat The solution used here was implemented with help by @robtfm on [Discord](https://discord.com/channels/691052431525675048/866787577687310356/1241019224491561000): > shadow maps are specific to lights, not to layers > if you want shadows from some meshes that are not visible, you could have light on layer 1+2, meshes on layer 2, camera on layer 1 (for example) > but this might change in future, it's not exactly an intended feature In other words, the example code as-is is not guaranteed to work in the future. I want to bring this up because the use-case presented here is extremely common in first-person games and important for accessibility. It would be good to have a blessed and easy way of how to achieve it. I'm also not happy about how I get the `perspective` variable in `change_fov`. Very open to suggestions :) ## Related issues - Addresses parts of #12658 - Addresses parts of #12588 --------- Co-authored-by: Pascal Hertleif <killercup@gmail.com> |
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Julian
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33dff0d3f7
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2D top-down camera example (#12720)
# Objective This PR addresses the 2D part of #12658. I plan to separate the examples and make one PR per camera example. ## Solution Added a new top-down example composed of: - [x] Player keyboard movements - [x] UI for keyboard instructions - [x] Colors and bloom effect to see the movement of the player - [x] Camera smooth movement towards the player (lerp) ## Testing ```bash cargo run --features="wayland,bevy/dynamic_linking" --example 2d_top_down_camera ``` https://github.com/bevyengine/bevy/assets/10638479/95db0587-e5e0-4f55-be11-97444b795793 |