Building a Plattformer - How to determine if a player is allowed to jump?

Question

I'm building a simple Plattformer Jump n' Run Style game. I do not use tiles - instead i've geometrical shapes for my level entities (and the player is one too). I finished my collision detection code and everything works fine so far.

Next, I wanted to implement jumping. Just checking if the player hits the appropriate key and add some upwards velocity. Works fine. But it works even if the player is in-air, which is not what I want. ;-)

So, I have to check if the player stands on something. My first idea was to check if there was a collision in the last frame and mark the player as "able to jump", but this would even trigger if the player hits a wall in-air. As my math skills aren't that good, i ask for help - even hints would do how to implement this.

Thanks!

Answer 1

Two options spring to mind:

• First thought is to tag the geometry with an ID and then check to see if the collision is with geometry tagged as floor. This offers the most control of jumpable surfaces but at a cost in level creation time.
• Second is to check the normal of the collision, if it's pointing up then allow jump. Or within some margin of up, depends on if you have slanting floors. This is flexible and doesn't require any tagging, but if you have slanted floors and walls you might get some jumping where you don't want it.

Answer 2

You surely should implement some kind of surface type. Think of it, how will you manage if you can climb up a ladder if you can't know if your character just collided a wall or a ladder? You could simply use OOP to manage a type hierarchy using heritage, but I would suggest you to use "categories" implemented using an enumerated type:

Here is the idea: A "Collisions" enumeration has a flag for each category. For example:

namespace Collisions
{
    enum Type
    {
        None   = 0,
        Floor  = 1 << 0,
        Ladder = 1 << 1,
        Enemy  = 1 << 2,
        ... // And whatever else you need.

        // Then, you can construct named groups of flags.
        Player = Floor | Ladder | Enemy
    };
}

With this method, you will be able to test if the player juste collided anything that you should manage, so your engine can call a "Collided" Method of the entity :

void Player::Collided( Collisions::Type group )
{
   if ( group & Collisions::Ladder )
   {
      // Manage Ladder Collision
   }
   if ( group & Collisions::Floor )
   {
      // Manage Floor Collision
   }
   if ( group & Collisions::Enemy )
   {
      // Manage Enemy Collision
   }
}

The method use bitwise flags and the bitwise "Or" operator to assure each group have a different value, based on category's binary value. This method works fine and is easily scalable so you can create customs collision groups. Each Entity (Player, Enemy, etc.) in your game has some bits called a "filter", which are used to determine what it can collide with. Your collision code should check to see if bits match and react accordingly, with some code that might look like:

void PhysicEngine::OnCollision(...)
{
    mPhysics.AddContact( body1, body1.GetFilter(), body2, body2.GetFilter() );
}

Answer 3

If you consider your character's foot as alway under the character by a certain distance, and if you're not moving away from the surface then your character is on the ground.

In rough pseudo-code:

bool isOnGround(Character& chr)
{
   // down vector is opposite from your characters current up vector.
   // if you want to support wall jumps, animate the vecToFoot as appropriate.
   vec vecToFoot = -chr.up * chr.footDistanceFromCentre;


// if feet are moving away from any surface, can't be on the ground
   if (dot(chr.velocity, down) < 0.0f)
     return false;

// generate line from character centre to the foot position
   vec linePos0 = chr.position;
   vec linePos1 = chr.position + vecToFoot;

// test line against world. If it returns a hit surface, we're on the ground
   if (testLineAgainstWorld(line0, line1, &surface)
     return true;

return false;
}