# FieldLines

> Field::new(charges, lines_per_charge, bounds) traces the field lines of point charges once (Charge::new(x, y, q)).

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

`Field::new(charges, lines_per_charge, bounds)` traces the field lines of point charges once
(`Charge::new(x, y, q)`). The lines leave the positive charges, evenly spaced, and end on a
negative charge or at `bounds`. `FieldLines` draws them on together over `draw` seconds, each
with an arrowhead at its middle. Then, with `test_charge: Some(i)`, a small charge follows
line `i`. `field.at(x, y)` gives the field and `field.lines()` the points. Gallery scene:
`FieldLines`.

Gallery clip of `FieldLines`: https://fframes-kit.pages.dev/media/scenes/FieldLines-24d33c1e.mp4

```rust
use std::sync::OnceLock;

use fframes_kit::prelude::*;

fn figure<'a>(ui: &mut Ui<'a, '_, '_>) -> Svgr<'a> {
    static FIELD: OnceLock<Field> = OnceLock::new();
    let field = FIELD.get_or_init(|| {
        Field::new(
            vec![Charge::new(-2.5, 0.0, 1.0), Charge::new(2.5, 0.0, -1.0)],
            16,
            (-7.3, -4.1, 7.3, 4.1),
        )
    });
    ui.show(FieldLines {
        field: Some(field),
        delay: 0.5,
        test_charge: Some(2),
        ..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`).

### FieldLines

| Field | Type | Default | Description |
| --- | --- | --- | --- |
| `field` | `Option<&Field>` | `None` | The traced field to draw, built once in the video's constructor; `None` draws nothing. |
| `delay` | `f32` | `0.0` | Seconds after the local start before the lines begin to draw. |
| `draw` | `f32` | `1.5` | Seconds the lines take to draw on, all together. |
| `arrows` | `bool` | `true` | Draws an arrowhead at the middle of each line. |
| `test_charge` | `Option<usize>` | `None` | Index of the line the small test charge follows after the lines are drawn; `None` for no test charge. |
| `speed` | `f64` | `1.5` | Speed of the test charge along its line in units of `ui.space` per second. |
| `charge_radius` | `f32` | `30.0` | Radius of the charge discs in pixels; the test charge is 0.4 times as big. |
| `positive_color` | `Color` | `theme.colors.highlight` | Colour of positive charges. |
| `negative_color` | `Color` | `theme.colors.accent` | Colour of negative charges. |
| `line_color` | `Color` | `theme.colors.muted` | Colour of the field lines and their arrowheads. |
| `test_color` | `Color` | `theme.colors.title` | Colour of the test charge. |
| `stroke_width` | `f32` | `theme.stroke.thin` | Width of the field lines in pixels. |
| `enter` | `Option<EnterAnim>` | `None` | How the charges draw in; `None` shows them from the start. |
| `exit` | `Option<ExitAnim>` | `None` | How the charges leave at the end of the container; `None` stays. |
| `parts_enter` | `Option<FadeIn>` | `None` | How the lines, signs and the test charge fade in; `None` for none. |
| `parts_exit` | `Option<FadeOut>` | `None` | How the lines and the test charge fade out before the end; `None` for none. |

Feature `physics` · [Source](https://github.com/khanhthanhdev/fframes-kit/blob/main/fframes-kit/src/kits/physics/field_lines.rs#L156)