Slick2D中的跳跃难度

Question

I have researched how to implement a rudimentary gravity system in slick2d. Here is the code that I have (This is in the update function):

if (input.isKeyDown(Input.KEY_UP)) {
        spressed = true; //Has the UP key been pressed?
    }
    if (spressed) { 
        if (!sjumping) {//if so, are we already in the air? 
             Sub_vertical_speed = -1.0f * delta;//negative value indicates an upward movement 
             sjumping = true;//yes, we are in the air
        }
        if (sjumping) { //if we're in the air, make gravity happen
             Sub_vertical_speed += 0.04f * delta;//change this value to alter gravity strength 
        } 
        Sub.y += Sub_vertical_speed;
    }
    if (Sub.y == Sub.bottom){//Sub.bottom is the floor of the game
        sjumping = false;//we're not jumping anymore
        spressed = false;//up key reset
    }

Here is where the problem arises. When I press the up key, the sprite jumps and comes down normally, but pressing the up key again does nothing. I originally thought it was cause I didn't reset spressed, so I added the line to set it to false, but you can still only jump once. :/

Answer 1

It looks like your Sub.y needs to be Clamped to your Sub.bottom so it doesn't go above it. Try:

if(Sub.y >= Sub.bottom) {
    Sub.y = Sub.bottom;
    sjumping = false;
    spressed = false;
}

Answer 2

I've done something similar before, and my assumption here is that Sub.y never equals Sub.bottom. Depending on the y position and the vertical speed, the object's y position will never be exactly the value of Sub.bottom. The code below will test for this:

if (Sub_vertical_speed + Sub.y > Sub.bottom){ //new position would be past the bottom
    Sub.y = Sub.bottom;
    sjumping = false; //we're not jumping anymore
    spressed = false; //up key reset
}