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# 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 ![image](https://github.com/user-attachments/assets/84894c6d-42e4-451b-b3e2-811266486ede) where after the fix with ```rust my_gizmos.arc_2d(Isometry2d::IDENTITY, FRAC_PI_2, 75.0, ORANGE_RED); ``` we get ![image](https://github.com/user-attachments/assets/16b0aba0-f7b5-4600-ac49-a22be0315c40) 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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README.md |