2d Platforms

2d Platforms

• Ant_59

  In order to simulate gravity, you need to more accurately represent motion. You can do this with 2 Vector2Ds: Position and Velocity. It may sound complex, but it's really quite simple. Velocity represents speed in a direction and Position is the objects current position.
  
  Whenever you need to move the object, give it a velocity in the direction you want to move in. For example:
  if(IsKeyDown(Keys.Right)
  Wizard.Velocity.X = 5;
  if(IsKeyDown(Keys.Left)
  Wizard.Velocity.X = -5;
  // And the same for Up and Down using Y instead of X.
  
  When it comes time to update the position of the Wizard, just do:
  Wizard.Position += Wizard.Velocity;
  
  That'll move the Wizards position by it's velocity. So if you have -5 X and +3 Y, he'll move 5 to the left and 3 up.
  
  Now if you want to add gravity, just add to his Y velocity every frame. This will constantly give him movement downwards.
  
  Wizard.Velocity.Y += 2;
  if(Wizard.Velocity.Y > 20)
  Wizard.Velocity.Y = 20; // Don't want him rocketing at the speed of light if he's been falling for a long time.
  
  That should help you get him off of the platform and it'll more accurately represent jumps (jump up fast, then slow down a bit, then begin to fall).
  
  Hope this helps!
  
  

• ray_newbie_SSIS

  The program spans multiple methods, I have the full thing at www.skynes.tripod.com Second on the list, the Wizard but with gravity.

  I actually don't think it's the gravity method. I've commented out the application of gravity and he still didn't jump.

  Moving left and right works, so in the Keyboard.right/left bit I tried adjusting the Y velocity by -40, with no gravity enforced. Technically that should send him flying to the top of the screen, but it didn't. He still didn't move.

  This is what is odd. Moving left and right works grand using mwizard.velocity but when I try to make an up or down motion, it does nothing.

  
  

• AlexBB

  I'm on a Plataform project as well, and I've decided to add some simple elements of physics to the game, being the goal to actually write a small physics engine because I have had trouble trying to use FarSeer's Physics Engine and another one I found out there. I still have A LOT of work to do, but I'll give you a grasp of the concepts of which might be interesting to you:

  Force = mass * acceleration

  mass is a scalar value, every sprite should have one.

  both force and acceleration are vectors, so that is easy to implement.

  Your player may have 80kg and gravity may be 9.8m/s^2 so each call to update in your game you get to calculate gravity for him.

  As far as movement, you can add to his final position a velocity vector like Grozzler put it.

  player input + gravity = player position

  then comes the floor problem, since floors have to stop you from going down, they need to be pushing you up in the same ammount gravity pulls you down, so for that you will need normals. but I guess if you dont want to mess around there, and have specific Y values for your plataforms, you can just do

  if( player.position.Y == value ) {stop gravity}

  Anyway, Id recommend checking codeplex for some XNA components, if you know how to use them into your game. Good Luck!

  
  

• De_Vaddrr

   

  Mass is 80.

  Gravity is 9.

  I divide the total by 120 to get a more reasonable value. So in this case my gravity is 6. I will be playing with my values in the future, so divide by 120 is only to get a smaller number, otherwise I'm forced to have a jump value of a couple of thousand.

  The jump force is -40

  private void ApplyGravity()

  {

  mWizard.Velocity = new Vector2(mWizard.Velocity.X, mWizard.Velocity.Y + grav.GetVelocity(mWizard));

   

  mWizard.Position = new Vector2(mWizard.Position.X + mWizard.Velocity.X, mWizard.Position.Y + mWizard.Velocity.Y);

  }

  //Within jump code

  mWizard.Velocity = new Vector2(0, -40);

   Sorry, I don't know what this forums procedure on posting code extracts is.

   

  
  

• Duncan Stewart

  Everything so far has been a great help and given me a good idea of what to do.

  I'm going to go and re-write how movement works to use velocity and gravity rather than what it was.

  I'll get back to you on what works and what doesn't.

  thanks.

   

  [Edit]

  As odd as it is with playing with physics. Adding gravity, which forced me to change how all the movement worked actually made it all simpler and easier for me to understand!

  I have the next version as www.skynes.tripod.com

  my newest problem is that I won't jump anymore. My movement left and right is fine, works as intended. My jump looks like hes jumping, as in, the sprite changes through the jump pattern. But his feet never leave the ground.

  I set the jump speed to -40, but it never jumps.

  
  

• franziss

  I'd try debbuging the values right when you press the jump button, my guess is that you may have your jump applied one time but at each update call the gravity gets called in and the result is a devastating push down which you dont actually see. But then again, it may be something else. nothing wrong with that code, but if you can post how you apply values to your velocity vector and how you update those in the update method would be helful
  

• mobigital

  Show me your code. What is the mass of the character...The Force (which you apply to the Vertical Velocity) must be greater than the Mass * Gravity in order for the character to move. Just show the code.
  
  

• Marcus Rodrigues

  I couldnt download your sourcecode, I might take a look at it if you can provide a reliable link. :)

  how do you make him jump

  is the gravity taking over the jump impulse

  are you constantly adding up vectors for gravity or jump at each update (giving acceleration - change in velocity over time)

  
  

• AdeptBlue

  Hi

   

  I couldn't grab the code to take a good look but I think it might be worthwhile to not keep creating new Vector2's so I'd probably do something like...

   

  (this is rough code, not sure if it will work and you'll have to fill in bits yourself).  You will want to display your position and velocity on screen so you can see the numbers change.

   

  (that's annoying, in my edit view the code and text font sizes are the other way arround)

   

  * in the wizard class definition have...

   

       Vector2 velocity = Vector2.Zero();

       Vector2 position = WhereIWantToStartPosition;

       BOOL canJump = FALSE;

   

  * Have methods in the wizard's class like...

   

      void UpdateWizard()

      {

            UpdateKeys();

            UpdatePosition();

            UpdateCollision();

            UpdateVelocity();

      }

   

      void UpdateKeys()

      {

           if (pressedLeft)    velocity.X = velocity.X - 40;

           if (pressedRight)  velocity.X = velocity.X + 40;

           if ((pressedJump) && (canJump == TRUE))
           {

                 velocity.Y = 220;

                 canJump = FALSE;

           }

      }

   

      void UpdatePosition()
      {

          position = position + velocity;    // move the player by his velociy

      }

   

      void UpdateVelocity()

      {

             // make our Y velocity adjust for gravity

            velocity.Y = velocity.Y - 40.0f;    

   

            // make sure we don't fall too fast

            if (velocity.Y <  -200.0f)   velocity.Y = -200.0f;         

   

            // allow for a little bit of inertia in the X,

            // 0.0f = no inertia, 1.0f = you never stop sliding, tweak this to get a nice

            // feel for if your character goes on ice make it a different one.

            velocity.X = velocity.X * 0.95f;    

      }

   

      void UpdateCollision()

      {

             // need to put your own stuff here.

   

             // banged head

             if (blockedUp) velocity.Y = 0.0f;      

   

             // run into wall

             if (blockedLeftOrRight) velocity.X = 0.0f;      

   

             // landed on something

             if (blockedDown)              

             {

                    velocity.Y = 0.0f;  

   

                    // we're on some floor so we can jump   

                    canJump = TRUE;   

             }

       }

   

  
  

• Sassie

  Link is working for me now. Haven't had a chance to look at it yet though.

  edit: looks like there's a problem with jumping. Once I get on the platform I can't get down.  I also managed to jump and stay near the top of the window.

  George - I emailed the zip to you.

  
  
  

• Andrew Buyan

  Hey look at these two links. I posted some of my code for my game.
  1: http://xna.multigan.com/pastebin/ page=view&id=1172197702
  2: http://xna.multigan.com/pastebin/ page=view&id=1172197909
  
  The first link is a class for a fighter (in your case a wizard). In your constructor, you want a Position, Velocity, Mass, and Gravity. This is basic physics. You have two methods really. You have your move method which updates the position:
  Final Position = Initial Position + (Velocity*Change in Time)
  
  Update the velocity, which in this case, you only want to worry about gravity and not the horizontal velocity:
  Velocity = Velocity - (Mass*Gravity)
  This is not the way you would learn it in physics class...kinda. It is really Distance/Time.
  
  Then you have the Jump method. You want to send an "impulse" into your velocity to "spring" you upwards.
  Velocity = -450 (I just chose 450 as an example...)
  The reason why I have a negative number is because the way XNA has things oriented and *** or something like that.
  
  In the other class, which is basically your game class (in my case, it is the fighting screen) you have a method which I called checkBorders(). When you are on the ground you want to have 0 Velocity (so you don't move) and you don't want to go through the ground or the roof, so that's why I put those there.
  
  If there are any questions just ask in this thread, I'll check it over the next couple of days. After that, eh...
  
  
  

• LauraCapatina

  The download link wasn't working for me. Can't wait to how far you've built off my original code! I love looking at how far people can take the initial concept and go with it in their own directions.
  
  [Edit: Link works if I right click and Save As in Firefox. If I click it, it doesn't work so all is well]
  
  [Edit: Nix that. Link still doesn't work. File downloaded has no data. So the link still doesn't work for me.]
  
  
  

• jayaraja

  Durrrr... me skynes... me do daft fings... durrrr...

  Tripod doesnt allow remote access... I shoulda remembered that.

  try http://skynes.tripod.com on its own. Link should be there and since it's from inside the site, it should allow downloads also.

  
  

• Sondre - MSFT Regional Director

  Got a suggestion for where I can put the project

  I make him jump by adding a negative value to his Y velocity. Then doing wizard.position += wizard.velocity

  I have commented out the gravity, so the gravity will not affect his jump, yet his jump still doesn't jump.

  
  
• 2d Platforms