# Objective
- Adds gizmo line joints, suggestion of #9400
## Solution
- Adds `line_joints: GizmoLineJoint` to `GizmoConfig`. Currently the
following values are supported:
- `GizmoLineJoint::None`: does not draw line joints, same behaviour as
previously
- `GizmoLineJoint::Bevel`: draws a single triangle between the lines
- `GizmoLineJoint::Miter` / 'spiky joints': draws two triangles between
the lines extending them until they meet at a (miter) point.
- NOTE: for very small angles between the lines, which happens
frequently in 3d, the miter point will be very far away from the point
at which the lines meet.
- `GizmoLineJoint::Round(resolution)`: Draw a circle arc between the
lines. The circle is a triangle fan of `resolution` triangles.
---
## Changelog
- Added `GizmoLineJoint`, use that in `GizmoConfig` and added necessary
pipelines and draw commands.
- Added a new `line_joints.wgsl` shader containing three vertex shaders
`vertex_bevel`, `vertex_miter` and `vertex_round` as well as a basic
`fragment` shader.
## Migration Guide
Any manually created `GizmoConfig`s must now set the `.line_joints`
field.
## Known issues
- The way we currently create basic closed shapes like rectangles,
circles, triangles or really any closed 2d shape means that one of the
corners will not be drawn with joints, although that would probably be
expected. (see the triangle in the 2d image)
- This could be somewhat mitigated by introducing line caps or fixed by
adding another segment overlapping the first of the strip. (Maybe in a
followup PR?)
- 3d shapes can look 'off' with line joints (especially bevel) because
wherever 3 or more lines meet one of them may stick out beyond the joint
drawn between the other 2.
- Adding additional lines so that there is a joint between every line at
a corner would fix this but would probably be too computationally
expensive.
- Miter joints are 'unreasonably long' for very small angles between the
lines (the angle is the angle between the lines in screen space). This
is technically correct but distracting and does not feel right,
especially in 3d contexts. I think limiting the length of the miter to
the point at which the lines meet might be a good idea.
- The joints may be drawn with a different gizmo in-between them and
their corresponding lines in 2d. Some sort of z-ordering would probably
be good here, but I believe this may be out of scope for this PR.
## Additional information
Some pretty images :)
<img width="1175" alt="Screenshot 2024-03-02 at 04 53 50"
src="https://github.com/bevyengine/bevy/assets/62256001/58df7e63-9376-4430-8871-32adba0cb53b">
- Note that the top vertex does not have a joint drawn.
<img width="1440" alt="Screenshot 2024-03-02 at 05 03 55"
src="https://github.com/bevyengine/bevy/assets/62256001/137a00cf-cbd4-48c2-a46f-4b47492d4fd9">
Now for a weird video:
https://github.com/bevyengine/bevy/assets/62256001/93026f48-f1d6-46fe-9163-5ab548a3fce4
- The black lines shooting out from the cube are miter joints that get
very long because the lines between which they are drawn are (almost)
collinear in screen space.
---------
Co-authored-by: Pablo Reinhardt <126117294+pablo-lua@users.noreply.github.com>
