CODEWITHSUNDEEP
androidcollision-detection
SeveN
SeveN

Reputation: 119

Android Game Development: Collision Detection Failing

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!

Upvotes: 5

Views: 419

Answers (2)

Tom
Tom

Reputation: 7824

I guess the issue here is that slowdowns are inevitable. You can try and optimize the code but you'll always have users with slow devices or busy sections of your game where it takes a little longer than usual to process it all. Instead of assuming a consistent delta, assume the opposite. Code under the realization that someone could try installing it on an abacus.

So basically, as SeveN says, make your game loop handle slowdowns. The only real way to do this (in my admittedly limited experience) would be to place a cap on how large delta can be. This will result in your clock not running on time exactly, but when you think about it, that's how most games handle slowdown. You don't fire up StarCraft on your pentium 66 and have it run at 5 FPS but full speed, it slow down and processes it as normal, albeit at a slideshow.

If you did such a thing, during periods of slowdown in your game, it'd visibly slow down... but the calculations should still all be spot on.

edit: just realised you're SeveN. Well done.

Upvotes: 1

Wojciech
Wojciech

Reputation: 340

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.

Upvotes: 1

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