Linear Interpolation

You can find this sample in the tutorial project: Menu → Linear Iterpolation

Explanation

This C# Beginner tutorial covers linear interpolation which is often shortened to 'Lerp'. Sometimes you want to gradually change a value from a start value to a target value. This process is called linear interpolation. Stride exposes several Lerp functions for various types. Among them are Vector2, Vector3 and Vector4.

Code

The example consists of a simple timer that resets after a couple seconds. When the timer starts, a start position and a randomly generated target position are stored. A box will move between these two positions. Every frame a 'Lerp value' is calculated. The lerp value is used to determined what the current position of a moving box should be. Once the timer is done, the current position will become the start position and a new target position is again randomly generated.

using System;
using Stride.Core.Mathematics;
using Stride.Engine;

namespace CSharpBeginner.Code
{
    /// <summary>
    /// Liniear interpolation or in short 'Lerp' can be used to graduatly change a value from a start value to a target value 
    /// This is used during animation of models, ui elements, camera movements and many other scenarios
    /// This example uses Lerp to graduatly move from a start vector3 coordinate to target Vector3 coordinate
    /// The same thing can be done with Vector2 and Vector4
    /// </summary>
    public class LerpDemo : SyncScript
    {
        public float AnimationTimer = 5.0f;

        private float elapsedTime = 0;
        private Random random = new Random();
        private Vector3 startPosition;
        private Vector3 targetPosition;

        public override void Start()
        {
            SetNewLerpTargetAndResetTimer();
        }

        public override void Update()
        {
            // Keep track of elapsed time
            var deltaTime = (float)Game.UpdateTime.Elapsed.TotalSeconds;
            elapsedTime += deltaTime;

            // In order to make use of the lerp method, we need to calculate the 'interpolation value': a value going from 0 to 1.
            var lerpValue = elapsedTime / AnimationTimer;

            // The Vector3 class exposes a 'Lerp' method that requires a start and target position. The third argument is the lerp value.
            Entity.Transform.Position = Vector3.Lerp(startPosition, targetPosition, lerpValue);

            // If the elapsedTime passes the animation timer we reset the timer and set a new target
            if (elapsedTime > AnimationTimer)
            {
                SetNewLerpTargetAndResetTimer();
            }

            DebugText.Print("Elapsed time: " + elapsedTime, new Int2(480, 120));
            DebugText.Print("Lerp value: " + lerpValue, new Int2(480, 140));
            DebugText.Print("Start position: " + startPosition, new Int2(480, 160));
            DebugText.Print("Target position: " + targetPosition, new Int2(480, 180));
        }

        /// <summary>
        /// Resets timer, stores the current position and randomly sets a new target position
        /// </summary>
        private void SetNewLerpTargetAndResetTimer()
        {
            elapsedTime = 0;
            startPosition = Entity.Transform.Position;
            targetPosition = new Vector3(random.Next(-2, 2), random.Next(0, 3), random.Next(-1, 1));
        }
    }
}