Gizmo line joints

[lynn-lumen]

Objective

• Adds gizmo line joints, suggestion of Tracking issue: Amazing Gizmos #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.

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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 GizmoConfigs 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 :)

• Note that the top vertex does not have a joint drawn.

Now for a weird video:

Screen.Recording.2024-03-02.at.05.14.23.mov

• 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.

[pablo-lua]

This was what I was expecting from this feature. Though I'm not sure about the rendering part, I have a couple of maybe silly questions.

[pablo-lua]

The render phase was a bit tricky, but everything right. There is a couple of things in other places that could improve IMO

[pablo-lua]

All good, LGTM

[pablo-lua]

The CI error looks like a external error, not related to this PR. I'll have a better look now just to make sure though.

[pablo-lua]

#12254 found the source of the error. I think you have to use bevy_utils for this now.