Making a character jump in XNA/C# (Basic Concept Included)

In the last tutorial, I briefly went over implementing gravity in games. By the end of the tutorial, we are going to program a character jumping when a key is pressed. I included 'Basic Concept Included' in the title because it doesn't use any XNA/C# specific stuff, just some simple math.

Jump?

Well, I'll first start off by telling you want happens in a 'jump'. First, upward force is applied and the character moves up. Somewhere down the road, gravity acts on it and it moves down.
I’m going to define all the variables in one go:

/*GraphicsDeviceManager, SpriteBatch, Texture2D and Vector2 may be found only in XNA. They are just used for drawing objects and defining locations.*/
GraphicsDeviceManager graphics; 
SpriteBatch spriteBatch;
Texture2D charr;
Vector2 charPos;

bool jumping; //Is the character jumping?
float startY, jumpspeed = 0; //startY to tell us //where it lands, jumpspeed to see how fast it jumps

Now, to the Initialize() function, provided by XNA:

charr = Content.Load<Texture2D>("ow"); //Load image
charPos = new Vector2(230, 450);//Char loc, X/Y
startY = charPos.Y;//Starting position
jumping = false;//Init jumping to false
jumpspeed = 0;//Default no speed

And the Update(), where the following is updated every frame:

//Init keyboard
KeyboardState keyState = Keyboard.GetState();

if (jumping)
{
    charPos.Y += jumpspeed;//Making it go up
    jumpspeed += 1;//Some math (explained later)
        if (charPos.Y >= startY)
        //If it's farther than ground
        {
            charPos.Y = startY;//Then set it on
               jumping = false;
        }
    }

else
{
    if (keyState.IsKeyDown(Keys.Space)) 
    {
        jumping = true;
        jumpspeed = -14;//Give it upward thrust
    }
}

We can now draw it with spriteBatch:

spriteBatch.Begin();
spriteBatch.Draw(charr, charPos, Color.White);
spriteBatch.End();

Explanation

We update charPos’s Y axis by jumpspeed. Since jumpspeed is set to –14 when space is pressed, this makes it go up (or down, depends on the game engine/platform you are using). Now, the magic- by adding 1 to jumpspeed every frame after, it will turn positive after a while and move down instead!
The value of jumpspeed will look like this:

-14,-13,-12….0,1,2,3….


If you have any questions, comments, or concerns, remember to leave me a comment. If you enjoyed the post, remember to Subscribe to my RSS feed. Or email me at finaiized(at)gmail.com if you want. I'm welcome to all suggestions you may have, and am willing to help you.

Simple 2D Gravity In Games [Explanation]

Introduction

Sometimes understanding gravity can be hard.
I mean, yeah, it's always around us, it's always been there. But when trying to fake gravity into games- platformers is my example- it can sometimes get confusing. After this article, I hope you'll know how to implement gravity in any language- and since gravity is basically math, I'm not going to recommend a particular programming language you should use to write it.

What's better to start than pull up a definition of gravity? So here's one, located here:

gravitational force: the attraction due to gravitation that the Earth or another astronomical object exerts on an object on or near its surface

We obviously don't have 'Earth' in our game per say, but we know it has to do with attraction. Let's keep this in mind and move on.

(I know the image sucks). Above is a basic representation of gravity. Something is forced up, and gravity acts on it and it moves down gradually. How do we represent this change using math? Well, we’ll need to define gravity of course…

To the basics

I'm going to start be representing super-basic gravity. We'll draw an image and make it move down at an increasingly fast pace, just like in real life.

//Declare gravity and add it to character
float gravity = 0.1f;
character.Y += gravity;
//To make it drop increasingly fast
gravity += 0.1f;

Now that we've done that, we can study the code. I first declare gravity, and assign it as 0.1 (the f is to show it's a float/decimal number, depends on the programming language). Next, I add gravity to the character's Y position (note here that sometimes you might have to subtract y to make it go down) and finally increase gravity so it keeps falling faster. A lot of games I notice actually don't have this feature, and it all comes down to the matter of what you want.

Limiting Gravity

The example above is good and all, but it drops increasingly fast until it's wayyy too fast. I'm going to fix that by simply adding a check:

if (gravity > 0.4f) {
gravity = 0.4f; 
}

There we have it. Super basic gravity.

Using Velocity

I’m going to use velocity in the following examples. Velocity is the rate of change in the position of an object as it moves in a particular direction. We can use velocity coupled with gravity so we can actually travel in an ark- as it can take account of the current direction and add gravity on top of that.

Here’s some code showing that:

Vector2 velocity;
float gravity = 0.1f;

if (keyPressed==Keys.Right) {
velocity = 3.0f;
}
velocity.Y += gravity;
mainChar.X += velocity.X;
mainChar.Y += velocity.Y;

Basically what I am doing here is creating a vector (this is basically a multi-dimensional array, can be replaced by two float/decimals) and, responding to key presses, adding the velocity to the main character’s X and Y. By doing this, I can set multiple velocity increases and have it all affect the character similarly. Now I can easily code things like springs.

Conclusion

Here concludes my gravity tutorial. I’ll be writing a tutorial on jumping next! It's up! If you have any questions, comments, or concerns, remember to leave me a comment. If you enjoyed the post, remember to Subscribe to my RSS feed. Or email me at finaiized(at)gmail.com if you want. I'm welcome to all suggestions you may have, and am willing to help you.