[xorangejuicex]

So I'm trying to make a platformer game but I can't figure out how to make my character jump. Here's my code:

            if (currentKBState.IsKeyDown(Keys.Space) == true)
            {
                Jump(gameTime);
                charPosition.Y -= jumpHeight;
                jumpHeight++;
                if (jumpHeight == 20)
                {
                    jumpHeight = 0;
                }
            }

Jump(gameTime) just takes care of switching the animation.

When I press space, the sprite with jump but it doesn't fall. It just keeps going up. And if I change my code to this

            if (currentKBState.IsKeyDown(Keys.Space) == true)
            {
                Jump(gameTime);
                charPosition.Y -= jumpHeight;
                jumpHeight++;
                if (jumpHeight == 20)
                {
                    charPosition.Y += jumpHeight;
                    jumpHeight--;
                }
            }

it will fall but continues to fall until I let go of space. However it won't go up again if I press spacebar. So if anyone could help me, that'd be great. Also, how would I got about making it so that the spacebar only has to be pressed to jump instead of being held down?

[Kilorn]

The best way to accomplish this would be to create a previousKBState variable to keep track of the button that was pressed in the previous frame. Here's how it's done:

// In the declarations section of your code
KeyboardState currentKBState;
KeyboardState previousKBState;

...

// Then you need to place this at the beginning of your jump logic
previousKBState = currentKBState;
currentKBState = Keyboard.GetState();

...

// Then when you check for the spacebar being pressed
if (currentKBState.IsKeyDown(Keys.Space) && !previousKBState.IsKeyDown(Keys.Space))
{
    // Do some stuff
}

[lesPaul456]

As far as jumping goes, I like to assign the player a velocity and an acceleration:

public void Update(GameTime gameTime)
{
    float deltaTime = (float)gameTime.ElapsedGameTime.TotalSeconds;
    float deltaTimeSquaredDividedByTwo = (deltaTime * deltaTime) / 2;

    Position.X += (Velocity.X * deltaTime) + (Acceleration.X * deltaTimeSquaredDividedByTwo);
    Position.Y += (Velocity.Y * deltaTime) + (Acceleration.Y * deltaTimeSquaredDividedByTwo);

    Velocity.X += Acceleration.X * deltaTime;
    Velocity.Y += Acceleration.Y * deltaTime;
}

When I initialize the player, I would set the acceleration:

player.Acceleration = new Vector2(0, 10);

Then, when the player wants to jump, I would set the velocity:

if (currentKBState.IsKeyUp(Keys.Space) && previousKBState.IsKeyDown(Keys.Space))
{
    Velocity.Y = -20;
}

Just another option.

[Kilorn]

That's not a bad idea, lesPaul456. That gives the programmer the option to control how fast the character actually rises and falls with the jump.

[macosxnerd101]

Also, with lesPaul456's formula, you can extrapolate the maximum height and total time by differentiating the function and finding when the time that solves for when the derivative = 0.

I have a tutorial on Math and 2D Gravity you may find helpful.

[Kilorn]

That's awesome macosxnerd101. I'll definitely be reading that later.

[Kilorn]

Double Post!

I just wanted to mention that I just read through all three of your math tutorials, macosxnerd101 and now my head hurts a bit. Then again, I'm about half asleep right now because work is so boring today, so that might have something to do with it. Awesome little series of tutorials for game programming related math.

[xorangejuicex]

Sorry for the late reply, but thank you guys for your tips and whatnot, it's definitely helped a bunch.

[Kilorn]

Did you manage to get the jumping logic working?