Heya! 🌸

As discussed on the server, there are some inconsistencies between the polyline component and its immediate drawing equivalent when used for UI.

Reproduction steps

To reproduce the following observations:

• Add a Canvas to your scene
• Set its render mode to Screen Space - Camera
• Add a child Game Object to the Canvas with the polyline component
• Add a child GameObject to the Canvas with a custom polyline script that uses the local matrix and the same inner parameters. Example:

using Shapes;
using UnityEngine;
using ScaleMode = Shapes.ScaleMode;

public class ImmediatePolyline : MonoBehaviour {
    private PolylinePath p;

    private void Awake() {
        Draw.LineThicknessSpace = ThicknessSpace.Meters;
        Draw.LineGeometry = LineGeometry.Flat2D;
        Draw.BlendMode = ShapesBlendMode.Transparent;
        Draw.ScaleMode = ScaleMode.Uniform;
        Draw.PolylineJoins = PolylineJoins.Miter;

        p = new PolylinePath();

        p.AddPoint(new Vector2(-100, -100f));
        p.AddPoint(new Vector2(100, 100f));
    }

    private void OnEnable() => OnPostRenderSubscribe();
    private void OnDisable() => OnPostRenderUnsubscribe();

    private void DrawShapes(Camera cam) {
        Draw.Matrix = transform.localToWorldMatrix;

        Draw.Polyline(p, false,5f, Color.white);
        
        Draw.ResetMatrix();
    }

    private void OnPostRenderSubscribe() {
        Camera.onPostRender += DrawShapes;
    }

    private void OnPostRenderUnsubscribe() {
        Camera.onPostRender -= DrawShapes;
    }

    private void OnDestroy() {
        p.Dispose();
    }
}

Additional notes:

• It works correctly outside UI
• I managed to reproduce it with and without URP

Observations

Differences in thickness

The polyline drawn through immediate drawing:

The polyline drawn with the Polyline component:

Differences in scaling

The immediate-drawn polyline with 0.2 and 3 x-scale:

The component polyline with 0.2 and 3 x-scale:

The immediate-drawn polyline's thickness is unaffected by scale changes (while the component polyline's thickness is only affected by scale inferior to 1), also the end caps are oriented differently as a result of the change in scale, see the difference at 3 x-scale:

Flat2D Immediate Drawing Polyline tries facing the camera

Same setup from the front:

From the side:

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Hope this was clear enough! Have a nice day ✨