Android Game Development: Collision Detection Failing

Question

I am currently developing a game for Android, and I would like your expertise on an issue that I have been having.

Background:

1. My game incorporates frame rate independent motion, which takes into account the delta time value before performing necessary Velocity calculations.

2. The game is a traditional 2D platformer.

The Issue:

Here's my issue (simplified). Let's pretend that my character is a square standing on top of a platform (with "gravity" being a constant downward velocity of characterVelocityDown).

I have defined the collision detection as follows (assuming Y axis points downwards):

Given characterFootY is the y-coordinate of the base of my square character, platformSurfaceY is the upper y-coordinate of my platform, and platformBaseY is the lower y-coordinate of my platform:

  if (characterFootY + characterVelocityDown > platformSurfaceY && characterFootY + characterDy < platformBaseY) {

                    //Collision Is True
                    characterFootY = platformSurfaceY;
                    characterVelocityDown = 0;

                } else{ 
                    characterVelocityDown = deltaTime * 6;

This approach works perfectly fine when the game is running at regular speed; however, if the game slows down, the deltaTime (which is the elapsed time between the previous frame and the current frame) becomes large, and characterFootY + characterVelocityDown exceed the boundaries that define collision detection and the character just falls straight through (as if teleporting).

How should I approach this issue to prevent this?

Thanks in advance for your help and I am looking forward to learning from you!

Answer 1

What you need to do is to run your physics loop with constant delta time and iterate it as many time as it need with current tick.

const float PHYSICS_TICK = 1/60.f; // 60 FPS
void Update( float dt )
{
    m_dt += dt;
    while( m_dt > PHYSICS_TICK )
    {
        UpdatePhysics( PHYSICS_TICK );
        m_dt -= PHYSICS_TICK;
    }
}

There are various technics used to handle the tick left ( m_dt )
Caps for miniumum tick and maximum tick are also a must.