# Objective
`arc_2d` wasn't actually doing what the docs were saying. The arc wasn't
offset by what was previously `direction_angle` but by `direction_angle
- arc_angle / 2.0`. This meant that the arcs center was laying on the
`Vec2::Y` axis and then it was offset. This was probably done to fit the
behavior of the `Arc2D` primitive. I would argue that this isn't
desirable for the plain `arc_2d` gizmo method since
- a) the docs get longer to explain the weird centering
- b) the mental model the user has to know gets bigger with more
implicit assumptions
given the code
```rust
my_gizmos.arc_2d(Vec2::ZERO, 0.0, FRAC_PI_2, 75.0, ORANGE_RED);
```
we get
where after the fix with
```rust
my_gizmos.arc_2d(Isometry2d::IDENTITY, FRAC_PI_2, 75.0, ORANGE_RED);
```
we get
To get the same result with the previous implementation you would have
to randomly add `arc_angle / 2.0` to the `direction_angle`.
```rust
my_gizmos.arc_2d(Vec2::ZERO, FRAC_PI_4, FRAC_PI_2, 75.0, ORANGE_RED);
```
This makes constructing similar helping functions as they already exist
in 3D like
- `long_arc_2d_between`
- `short_arc_2d_between`
much harder.
## Solution
- Make the arc really start at `Vec2::Y * radius` in counter-clockwise
direction + offset by an angle as the docs state it
- Use `Isometry2d` instead of `position : Vec2` and `direction_angle :
f32` to reduce the chance of messing up rotation/translation
- Adjust the docs for the changes above
- Adjust the gizmo rendering of some primitives
## Testing
- check `2d_gizmos.rs` and `render_primitives.rs` examples
## Migration Guide
- users have to adjust their usages of `arc_2d`:
- before:
```rust
arc_2d(
pos,
angle,
arc_angle,
radius,
color
)
```
- after:
```rust
arc_2d(
// this `+ arc_angle * 0.5` quirk is only if you want to preserve the
previous behavior
// with the new API.
// feel free to try to fix this though since your current calls to this
function most likely
// involve some computations to counter-act that quirk in the first
place
Isometry2d::new(pos, Rot2::radians(angle + arc_angle * 0.5),
arc_angle,
radius,
color
)
```
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