Hello, I am working on some physics in my game and more specifically jumping. When I it the jump button I set a bool m_bJumped to true so that in the update function I can alter the Y velocity:

if(event.key.code == sf::Keyboard::Z && m_sfVelocity.y == 0) m_bJumped = true;

Then in the update function:

...

if(m_bJumped == true) {
		
	m_sfVelocity.y = -300 * deltaT.asSeconds();
	m_bJumped = false;
}

...

m_sfVelocity.y += 5 * deltaT.asSeconds();

...

if(checkCollision(std::floor(m_sfEntity.getPosition().x / 32), std::floor((m_sfEntity.getPosition().y + 
                   m_sfSize.y + m_sfVelocity.y) / 32), lvl) == true) 
m_sfVelocity.y = 0;

The check collision function checks to see if the tile below the player is solid and if so to set the Y velocity to 0 so it will stop moving down. The jumping works but is is very random. Some times it jumps 7 tiles and some times it jumps 9. One time it jumped all 20 tiles. How do I make the jumping more stable? Thanks.

EDIT: The deltaT is a sf::Time object that is passed in every frame.

You don't need to multiply -300 by deltaT.asSeconds(). You want the jump velocity to be a constant (-300) and then use delta time to update it (as you are).

You are seeing the effect of slightly different frame times which is making the velocity start at different values.

I changed it to this and the jump is a lot more stable.

So your using delta time...

I would do it like this

int AmountTimeGoingUpOnJump= 250;//Miliseconds THIS WILL VARY ON YOUR DECISION
bool isJumping = false;//If this is true, increase his velocity until he meats max treshold, else decrease it until he meats max treshold or meats a solid surface
int unitPosX, unitPosY;
SomeTimeHoldingClass StartJumpTime;//Time when the jump started
SomeTimeHoldingClass GetCurrentTime();//returns current time
 
//Now the user presses "Z" call startJump.
StartJump(){StartJumpTime = GetCurrentTime();}
LoopJump(){if(CurrentTime - StartJumpTime > AmountTimeGoingUpOnJump){isJumping = false;/*No velocity increase, also somwhere check if nothing bellow him to decrease velocity*/} 
else {isJumping = true, /*make his velocity increase*/}}

if(m_bJumped == true) {
        
    m_sfVelocity.y = -300 * deltaT.asSeconds();
    m_bJumped = false;
}

Also as it stands m_bJumped=false sould be set when the "unit" meats ground so you prevent air jumps

m_sfVelocity.y += 5 * deltaT.asSeconds();

Here you sould check for max velocity if you want.

I mean the more you jump the faster you go?

You go half way speeding(Going up only X amount of time) then you still going up but slowing down, until the velocity goes from + to - and then you start to decline

This would work and thank you for the suggestion, but I wanted the entity to act like a projectile. Thanks for the help.