TrailRenderer that maintains a width in screen space (URP)

PixelPerfectTrail.cs

public class PixelPerfectTrail : MonoBehaviour
{
    [SerializeField] private TrailRenderer trail = null;

    [Tooltip("How small the trail width is allowed to get in pixels.")]
    public float MinimumPixelSize = 1.5f;

    private float startSize = 1f;

    private void Awake()
    {
        startSize = trail.widthMultiplier;
    }

    private void OnEnable()
    {
        RenderPipelineManager.beginCameraRendering += ScaleWithDistance;
    }

    private void OnDisable()
    {
        RenderPipelineManager.beginCameraRendering -= ScaleWithDistance;
    }

    private void ScaleWithDistance(ScriptableRenderContext context, Camera cam)
    {
        float distance = Vector3.Distance(cam.transform.position, transform.position);
        float currentPixelSize = DistanceAndDiameterToPixelSize(distance, startSize, cam);

        if (currentPixelSize < MinimumPixelSize)
        {
            float scaledWidth = PixelSizeAndDistanceToDiameter(MinimumPixelSize, distance, cam);
            trail.widthMultiplier = scaledWidth;
        }
        else
        {
            trail.widthMultiplier = startSize;
        }
    }

    // Get the diameter of an object, given its screen size in pixels and distance. By Elecman:
    // https://forum.unity.com/threads/this-script-gives-you-objects-screen-size-in-pixels.48966/#post-2107126
    private float PixelSizeAndDistanceToDiameter(float pixelSize, float distance, Camera cam)
    {
        float diameter = (pixelSize * distance * cam.fieldOfView) / (Mathf.Rad2Deg * Screen.height);
        return diameter;
    }


    // Get the screen size of an object in pixels, given its distance and diameter. By Elecman:
    // https://forum.unity.com/threads/this-script-gives-you-objects-screen-size-in-pixels.48966/#post-2107126
    private float DistanceAndDiameterToPixelSize(float distance, float diameter, Camera cam)
    {
        float pixelSize = (diameter * Mathf.Rad2Deg * Screen.height) / (distance * cam.fieldOfView);
        return pixelSize;
    }
}