2 commits

Author	SHA1	Message	Date
Jan Hohenheim	c92ee31779	Allow ordering variable timesteps around fixed timesteps (#14881) ... # Objective - Fixes #14873, see that issue for a whole lot of context ## Solution - Add a blessed system set for this stuff. See [this Discord discussion](https://discord.com/channels/691052431525675048/749335865876021248/1276262931327094908). Note that the gizmo systems, [LWIM](https://github.com/Leafwing-Studios/leafwing-input-manager/pull/522/files#diff-9b59ee4899ad0a5d008889ea89a124a7291316532e42f9f3d6ae842b906fb095R154) and now a new plugin I'm working on are all already ordering against `run_fixed_main_schedule`, so having a dedicated system set should be more robust and hopefully also more discoverable. --- ## ~~Showcase~~ ~~I can add a little video of a smooth camera later if this gets merged :)~~ Apparently a release note is not needed, so I'll leave it out. See the changes in the fixed timestep example for usage showcase and the video in #14873 for a more or less accurate video of the effect (it does not use the same solution though, so it is not quite the same) ## Migration Guide [run_fixed_main_schedule](https://docs.rs/bevy/latest/bevy/time/fn.run_fixed_main_schedule.html) is no longer public. If you used to order against it, use the new dedicated `RunFixedMainLoopSystem` system set instead. You can replace your usage of `run_fixed_main_schedule` one for one by `RunFixedMainLoopSystem::FixedMainLoop`, but it is now more idiomatic to place your systems in either `RunFixedMainLoopSystem::BeforeFixedMainLoop` or `RunFixedMainLoopSystem::AfterFixedMainLoop` Old: ```rust app.add_systems( RunFixedMainLoop, some_system.before(run_fixed_main_schedule) ); ``` New: ```rust app.add_systems( RunFixedMainLoop, some_system.in_set(RunFixedMainLoopSystem::BeforeFixedMainLoop) ); ``` --------- Co-authored-by: Tau Gärtli <git@tau.garden>	2024-08-23 16:19:42 +00:00
Jan Hohenheim	d0e606b87c	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.	2024-07-09 14:23:10 +00:00