Jump into the game

I am doing a 2nd side scroller and for life I can't jump to work. This is how I do the movement left and right:

for(var i = 0; i < time; i++)
     newVelocityX = (oldVelocityX + accelerationX) * frictionX;


then update my player's position.

positionX = oldPositionX + newVelocityX;

This works great and the "time" variable only has the number of ms since I last ran the function. Friction works great and I'm happy that everything is good in the X direction. This is what I have in the Y direction:

for(var i = 0; i < time; i++) {
    accelerationY += gravityAccelerationY;
    newVelocityY = oldVelocityY + accelerationY;


The object falls due to gravity just fine. If I set the acceleration to negative Y when the user presses the up arrow, I can even make the player jump, but on a fast computer they jump very high and on an old computer they jump very low. I'm not sure how to fix this, I thought I was already explaining this by putting it in a loop like I did.


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1 answer

You will need to do a few things in order to change the code to work properly. There are many bugs / performance in the code you posted.

Here's some code to create the basics of the game.

Example code for the jump:

if (jump) {
    velocityY = -jumpHeightSquared; // assuming positive Y is downward, and you are jumping upward
velocityY += gravityAccelerationY * time;
positionY += velocityY * time;
if (positionY > 0) {
    positionY = 0; // assuming the ground is at height 0
    velocityY = 0;


Example code to move sideways:

velocityX += accelerationX * time;
velocityX *= Math.pow(frictionX, time);
positionX += velocityX * time;


Some comments on the code:

Velocity and position variables should keep their values ​​between frames (I assume you figured this out).

gravityAccelerationY and frictionX are constant values ​​if gravity or friction has not changed.

Where I replaced your for loops with * time

, using a single multiplication would be faster than a loop. The only difference would be low frame rates or high acceleration rates, where acceleration seemingly "sped up" from what it should be. You shouldn't have a problem with this.



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