demonstration of precise race finish line timing

RacingPreciseFinishLine.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

// @kurtdekker - precise timing on two cars passing the finish line at the same instant
//
// from forum post:
//
// https://discussions.unity.com/t/how-do-you-measure-milliseconds-at-runtime/1562308
//
// To use: make a scene, drop it on a GameObject, press PLAY
//
// Optional: create your own cars, drag 'em in.
//
// Now wiggle the cars around, rotate the whole contraption if you like, whatever
//
// Output: tells you the precise time between T1 and T2
// that each car's centroid crosses the finish line plane.

public class RacingPreciseFinishLine : MonoBehaviour
{
	[Header("Make 'em or we make 'em.")]
	public GameObject Car1_T1;
	public GameObject Car1_T2;
	public GameObject Car2_T1;
	public GameObject Car2_T2;

	[Header("Input times:", order = 1)]
	[Header("-> Previous Frame Time:", order = 2)]
	public float TimeT1 = 1.000f;
	[Header("-> Current Frame Time:")]
	public float TimeT2 = 1.020f;

	[Header("By fiat, the normal of finish line is transform.right")]
	[Header("This normal is in the direction of travel.")]
	public Transform FinishLinePlane;

	void Start()
	{
		if (!Car1_T1)
		{
			// create them
			Car1_T1 = GameObject.CreatePrimitive(PrimitiveType.Cube);
			Car1_T2 = GameObject.CreatePrimitive(PrimitiveType.Cube);
			Car2_T1 = GameObject.CreatePrimitive(PrimitiveType.Cube);
			Car2_T2 = GameObject.CreatePrimitive(PrimitiveType.Cube);

			// position them
			float x = -3.0f;
			float y1 = +2.0f;
			float y2 = -2.0f;

			Car1_T1.transform.position = new Vector3(x, y1, 0);
			Car1_T2.transform.position = new Vector3(x + 5, y1, 0);
			Car2_T1.transform.position = new Vector3(x, y2, 0);
			Car2_T2.transform.position = new Vector3(x + 8, y2, 0);

			// name them
			Car1_T1.name = "Car1_T1";
			Car1_T2.name = "Car1_T2";
			Car2_T1.name = "Car2_T1";
			Car2_T2.name = "Car2_T2";
		}

		if (!FinishLinePlane)
		{
			FinishLinePlane = GameObject.CreatePrimitive(PrimitiveType.Cube).transform;
			FinishLinePlane.transform.position = new Vector3(1, 0, 0);
			FinishLinePlane.localScale = new Vector3(0.01f, 8, 4);
			FinishLinePlane.name = "Finish Line Plane";
		}
	}

	void Update()
	{
		Vector3 finishPosition = FinishLinePlane.position;

		// arbitrary: use any direction you like
		Vector3 finishNormal = FinishLinePlane.right;

		// relative position to finish line position
		Vector3 Car1_T1_RelativePosition = finishPosition - Car1_T1.transform.position;
		Vector3 Car1_T2_RelativePosition = finishPosition - Car1_T2.transform.position;
		Vector3 Car2_T1_RelativePosition = finishPosition - Car2_T1.transform.position;
		Vector3 Car2_T2_RelativePosition = finishPosition - Car2_T2.transform.position;

		// linear track space, before to after the finish line
		float car1Pos1 = Vector3.Dot(Car1_T1_RelativePosition, finishNormal);
		float car1Pos2 = Vector3.Dot(Car1_T2_RelativePosition, finishNormal);
		float car2Pos1 = Vector3.Dot(Car2_T1_RelativePosition, finishNormal);
		float car2Pos2 = Vector3.Dot(Car2_T2_RelativePosition, finishNormal);

		// how far in linear space did each car travel this final frame?
		float car1Delta = car1Pos2 - car1Pos1;
		float car2Delta = car2Pos2 - car2Pos1;

		// careful! we're lerping "backwards" from now (T2) to previous (T1)!!
		// that makes these sort of "back fractions..."
		float fraction1 = car1Pos2 / car1Delta;
		float fraction2 = car2Pos2 / car2Delta;

		float car1Time = Mathf.Lerp(TimeT2, TimeT1, fraction1);
		float car2Time = Mathf.Lerp(TimeT2, TimeT1, fraction2);

		// help yourself to whatever precision you want to claim, but
		// single-precision float is about 6-9 decimal places max, and
		string output = System.String.Format(
			"Car1: {0:0.00000}    Car2: {1:0.00000}    ", car1Time, car2Time);

		Debug.Log(output);
	}
}