# Doppler

> A source moving right from start at source_speed m/s sends out a front every period seconds, and the fronts spread at wave_speed m/s.

Page: https://fframes-kit.pages.dev/docs/physics/doppler/
Cargo feature: `physics` (`fframes-kit = { version = "..", features = ["physics"] }`)

A source moving right from `start` at `source_speed` m/s sends out a front every `period`
seconds, and the fronts spread at `wave_speed` m/s. They bunch up ahead of the source and
spread out behind it. At or above the wave speed they pile into a cone. A front fades as it
grows and is gone at `max_radius` meters. Listeners `ahead` and `behind` (meters from
`start`) show the frequency they hear, `f c / (c ∓ v)`, once the first front has reached
them. The source's own frequency sits above it, on a chip of the background. `heard` and
`fronts` give the numbers. Gallery scene: `Doppler`.

Gallery clip of `Doppler`: https://fframes-kit.pages.dev/media/scenes/Doppler-064a0c78.mp4

```rust
use fframes_kit::prelude::*;

fn figure<'a>(ui: &mut Ui<'a, '_, '_>) -> Svgr<'a> {
    ui.show(Doppler {
        start: (-3.5, 0.0),
        scale: 0.7,
        source_speed: 1.0,
        wave_speed: 3.0,
        period: 0.4,
        ahead: Some(8.0),
        behind: Some(-4.0),
        ..ui.defaults()
    })
}
```

## API

Components are plain structs. Set the fields you need and fill the rest with `..ui.defaults()`: the defaults below come from the current theme (`theme.` values are fields of `Theme`).

### Doppler

| Field | Type | Default | Description |
| --- | --- | --- | --- |
| `start` | `(f64, f64)` | `(-4.0, 0.0)` | Where the source starts, in units of `ui.space`; meters below are measured from it. |
| `scale` | `f64` | `1.0` | Units of `ui.space` per meter. |
| `source_speed` | `f64` | `1.0` | Speed of the source to the right in m/s; at or above `wave_speed` the fronts pile into a cone. |
| `wave_speed` | `f64` | `3.0` | Speed the wavefronts spread at in m/s. |
| `period` | `f64` | `0.4` | Seconds between two wavefronts; the source's own frequency is its inverse. |
| `max_radius` | `f64` | `6.0` | Radius in meters where a front is gone; it fades out on the way. |
| `ahead` | `Option<f64>` | `Some(10.0)` | Listener in meters from `start`, to the right of the source; `None` for none. |
| `behind` | `Option<f64>` | `Some(-2.5)` | Listener in meters from `start`, to the left of the source; `None` for none. |
| `labels` | `bool` | `true` | Writes the source's frequency above it and the frequency each listener hears once the first front reaches it. |
| `delay` | `f32` | `0.0` | Seconds after the container's start before the source starts moving. |
| `time_scale` | `f32` | `1.0` | Speed of the motion over local time; 2 runs twice as fast, 1 is real time. |
| `source_color` | `Color` | `theme.colors.accent` | Colour of the source and its frequency label. |
| `wave_color` | `Color` | `theme.colors.text` | Colour of the wavefronts. |
| `listener_color` | `Color` | `theme.colors.highlight` | Colour of the listener marks. |
| `stroke_width` | `f32` | `theme.stroke.thin` | Width of the wavefronts in pixels. |
| `enter` | `Option<EnterAnim>` | `None` | How the listener marks draw in; `None` shows them from the start. |
| `exit` | `Option<ExitAnim>` | `None` | How the listener marks leave at the end of the container; `None` stays. |
| `parts_enter` | `Option<FadeIn>` | `None` | How the source and the fronts fade in; `None` for none. |
| `parts_exit` | `Option<FadeOut>` | `None` | How the source and the fronts fade out before the end; `None` for none. |

Methods:

- `fn source(&self, t: f64) -> f64`: Meters from `start`, the source after `t` seconds.
- `fn heard(&self, x: f64, t: f64) -> Option<f64>`: The frequency in Hz a listener at `x` meters from `start` hears while the source is where it is after `t` seconds: `f c / (c - v)` ahead of it, `f c / (c + v)` behind. `None` ahead of a source at or above the wave speed, where no front has come.
- `fn fronts(&self, t: f64) -> Vec<(f64, f64)>`: The emission time and the radius in meters of every front alive after `t` seconds, oldest first.

Feature `physics` · [Source](https://github.com/khanhthanhdev/fframes-kit/blob/main/fframes-kit/src/kits/physics/doppler.rs#L28)