I'm a little bit of a new programmer, kinda doing relatively basic stuff. Over the past while (although I haven't been working on it very often) I've been, with the help of SDL, been making a 2D game. Right now, I'm working on the code for causing the player to jump when pressing the w key (controls are wsad). I quickly realized that I couldn't keep the speed constant because this would cause the jump to go more like the graph of y=-|x| than y=-x^2, which would look more like an actual jump than the former. Here's the bits of code that make it work.
//The part which happens when the jump key is pressed (w)
case SDLK_w:
if(InAir != true) {
StartOfJump = PlayerYCoord;
InAir = true;
velocity = 0.01;
break;
}
//And skip to a different part.
if(InAir == true) {
if(StartOfJump - PlayerYCoord < JumpHeight) {
PlayerYCoord = PlayerYCoord - velocity;
velocity = velocity * velocity;
velocity is there to make the speed that the Y coord of the player increases, increase exponentially. Right now, it's set to 0.01 at the start of the program.
JumpHeight is the number of pixels the player jumps. This is for the purpose of being able to modify it with ease. As it is, JumpHeight is set to 100 pixels.
StartOfJump is there to ensure that the jump is cut off at the right point, because while at the moment the map for the game is flat I don't expect to keep it that way.
And PlayerYCoord is fairly self-explanatory, but basically it's the Y coordinate of the player on the plane. it is a double.
And yes, I know that right now there is no code for falling. That's not really important when I can't even get the rising part.
Anyway, I'm not really certain why, but for some reason the player's position just increases by one pixel on the pressing of the w key, and it doesn't do anything else. I tried modifying the code to say this
if(InAir == true) {
if(StartOfJump - PlayerYCoord < JumpHeight) {
//PlayerYCoord = PlayerYCoord - 0.1;
PlayerYCoord = PlayerYCoord - velocity;
velocity = velocity * velocity;
cout << fixed << setprecision(4);
cout << "The velocity is" << velocity << " " << endl;
Uuuunfortunately, it starts by printing out 0.0001 then prints 0.000 forever. But that's not right, is it?... The value of velocity is 0.01... so, I changed that last line to say this:
cout << "The velocity is" << velocity * 100 << " " << endl;
Sure enough, it output 0.0100 and then output 0.0000 infinitely like last time. I'm honestly unsure what's causing this issue, but I think it has something to do with the fact that the player's Y coordinate only increases by one pixel then stops.
I'm probably making a really stupid mistake that can be fixed very easily. Anyway, if there's any other bits of code that I need to include (first time on programming forums, I haven't quite got this stuff nailed down yet) I'd be happy to include it. Thanks in advance for the help!
P.S. In case any of my code is system-dependent, I'm on Ubuntu 13.10, 32 bit.
You've implemented gravity incorrectly. You need to start off with an upwards velocity. You then need to subtract the downwards acceleration from gravity each unit of time. This will handle both rising and falling. Your code does... strange things.
velocity = 50
each unit of time while player is in the air:
playerY -= velocity
velocity -= acceleration
I don't quite agree with the above solution.
Your player should always be affected by gravity, right? Even if we are on the ground we are affected by gravity?
Always affect the player by gravity but when we press a jump button or so, add counter force that flings the player upwards (but only once) and then the gravity will outtake this force and push the player down.
The boolean InAirshould only determine if the player can jump again. So if we are in the air we shouldn't be able to execute another jump command. But if we are grounded (InAir = false ) Then we should be able to jump again.
This also makes it more efficient to add double jump or such.
Happy coding!
Related
I want to teach myself some basic physics programming by creating a simple 2d platformer with SDL 2. It seems I'm falling at the first hurdle though, because I can't get movement using both velocity and acceleration per time unit, rather than per frame, to work.
I start by calculating the time per frame in the usual way:
previous_time = current_time;
current_time = SDL_GetTicks();
delta_time = current_time - previous_time;
Then, after the movement flag is set to true by pressing a directional button, this is passed to a function to handle the movement.
//Pass the movement flag and the milliseconds per frame to the right movement function.
if ( player.get_x() <= 740 ) {
player.x_movement_right(delta_time, 1, moving_right);
}
The integer that's passed doesn't do anything yet. Anyway, the function then determines the acceleration based on if the movement flag is set to true, and what the current velocity is:
void Player::x_movement_right(float dt, int direction, bool moving_right) {
dt /= 1000;
if (moving_right == true && _x_velocity <= 200 ) {
_x_acceleration = 50;
}
else if ( moving_right == false && _x_velocity > 0 ) {
_x_acceleration = -50;
}
else {
_x_acceleration = 0;
}
_x_velocity += _x_acceleration * dt;
_x_position += _x_velocity * dt;
}
The same process occurs if the left movement flag is activated, with inverted values of course. Yet after compiling I hit the directional keys and nothing happens.
What I've already tried:
I removed the dt's at the bottom of the movement function. The player avatar moves with incredible speed, since the acceleration is now per frame, rather than per second.
Same thing when I don't divide dt at the beginning of the movement function, since it's now per millisecond rather than per second.
I tried rounding the velocity times dt at the bottom, since I suspected SDL might have trouble calculation positions with floating point numbers rather than integers. Still no movement.
Based on this I suspect it has something to do with the numbers being too small, but I can't quite wrap my head around what the problem is or how to solve it. So, does anyone know what undoubtedly obvious thing I'm missing? Thanks in advance!
There is no way to know with the information shown, but there are several points that may help you:
Are your _x_velocity and the like floating point types? In what units are you measuring distance? It may be that your increment has not enough resolution to be nonzero.
Have you printed the values of each variable or run the program in a debugger?
What do you mean by "SDL might have trouble calculation positions with floating point numbers"? If you are using SDL's basic 2D renderer, you just need to give it the type it needs in whatever units they ask. The conversions are up to you.
Overall, I'd recommend trying to code the simulation outside SDL or graphics in general. Getting acquainted with C++, debugging and floating-point is also a plus.
So, I ask question about my 2D game, actually it is not game, but it is something like poor game. Well i made moving system and now is time to make jump. I made some kind of gravity with tutorial but I don't like it. It is simple and nice but there is problems in that gravity example: 1. You can hold up arrow and "fly" with jump. I tried to fix that and make some kind of limit to jump but it was bad try. Second problem is that when you jump and you release up arrow (jumping key) then my sprite fall very fast and I can't really control that sprite in air.
This is my first game and I want spend time to do that, but now i have problem that i can't solve with my self so if you can help, please do it. I use C++ with CodeBlocks and SFML multimedia library.
So there is little parts of my code.
First there is velocity and gravity configuration:
//gravity & velocity
const float gravity = 1;
int ground = 600; //height of ground
Vector2f velocity(Vector2f(0,0));
float movingSpeed = 0.5f, jumpingSpeed = 2.0f;
Then there is simple when you press button you jump:
if(Keyboard::isKeyPressed(Keyboard::Up)){
skeleton.move(0,velocity.y = -jumpingSpeed);
source.y = Up;
isWalking = true;
}
And there is system what recognize if you jump and this make you fall:
if(skeleton.getPosition().y + skeleton.getScale().y < ground || velocity.y < 0){
velocity.y += gravity;
}else{
skeleton.setPosition(skeleton.getPosition().x, ground - skeleton.getScale().y);
}
So my sprite name is "skeleton". And yeah this is first time with game physics. I'm not very good game programmer so these code can be poor and yeah... if you invent a solution share it.
so I am working on an AI system with C++ and SFML in which I want the enemies to follow the player. Im going to create it so the enemies are moving towards the point at which the player was 30 frames ago (so it will constantly updating). But my question is simple, what is the math behind getting the enemy to move to a specific point? Like let's say the player is at (230, 400) and the enemy is at (100, 200) how do I get the enemy (using the .move() function with a speed of 3) to move to that point? Any help would be fantastic!
---UPDATED BELOW WITH CURRENT CODE FOR HANDLING MOVEMENT-----
float angle = atan2(v2iEnemyPos[1].y - rsPlayer.getPosition().y, v2iEnemyPos[1].x - rsPlayer.getPosition().x);
angle =angle * 180 / (atan(1) * 4);
sf::Vector2f newpos((cos(angle))*2, (sin(angle))*2);
std::cout << newpos.x << " " << newpos.y << std::endl;
rsEnemy[1].move(newpos.x, newpos.y);
rwWinOne.draw(rsPlayer);
rwWinOne.display();
rwWinOne.clear();
The direction to move your enemy into is simply the difference between the player position and the enemy position. However, you want the enemies to move with constant speed, so you need to normalize the result. This will give you the direction as a vector of length 1.
Vector2u direction = normalize(player.getPosition() - enemy.getPosition());
You can now multiply this direction by the speed constant of that enemy's type. The result is a vector with it's length depending on the speed factor instead of the distance to the player. Just use the result to move your enemy.
enemy.move(speed * direction);
However, you perform that once a frame and framerates can vary between machines and configurations. Therefore, you should add a the elapsed time since the last move call, which might be your frame time, as factor to the equation. This way, if a frame takes a longer than usual, the enemy will be moved further to comprehend this, and vice versa.
// Initialize a timer
sf::Clock clock;
// Get elapsed time at the beginning of a frame
// I multiply this by target framerate to get a value around one
float delta = clock.restart().asSeconds() * 60;
// Move all objects based on elapsed time
Vector2u direction = normalize(player.getPosition() - enemy.getPosition());
enemy.move(delta * speed * direction);
By the way, basic knowledge of linear algebra is needed very often in game development, so you might pay out to attend an online course.
You should consider the elapsed time (since last frame) - dt - in your calculation.
The enemy has a given speed: enemy_velocity.
I won't use the move() function, but the SetPosition().
You have to calculate the new position of the enemy:
enemy.x += dt * enemy_velocity * cos(angle);
enemy.y += dt * enemy_velocity * sin(angle);
And affect it with SetPosition().
Basically I have a bunch of rectangles floating around at 8 different angles (45 degrees, 90 degrees etc). I have collision detection going on between all of them, but one thing still doesn't work as it should. I don't know if I'm just not thinking or what, but I can't seem to get the resulting angles right. I've also tried searching multiple places, but haven't really gained anything from what I've found.
NOTE: the angle system here starts at 0 at the top and increases clockwise.
NOTE: all rectangles have the same mass
Say one rectangle going straight right (90 degrees) hits another going straight left (270 degrees). They will hit off of each other just fine.
Now say one going left gets hit by one going up. Here I can't simply reverse the angles or anything.
If you have more than one way, consider the following: unless I rearrange the CD so it spreads into the other code, I have the positions of all of the rectangles. The CD checks by seeing if two are overlapping, not by comparing where they are going.
As I've been working on pretty much everything except for the collision detection until now, I only have tonight left to get it working and add a few other small things before I'm done.
If you know of a way to make the angles come out right without hardcoding, great. At this point I'm ready to hardcode it (not too much really) if all I have is which rectangle hits the other (ex 2), or if they both do (ex 1). Either one is really helpful.
I think you mean something like this...
Each Rectangle has this functionality, testing against, say an array of other objects.
Obstacle* obstacle = new Obstacle;
Obstacle** obstacles = obstacle*[];
For(int i = 0; i <3; i++)
{
obstacles[0] = New Obstacle(x,y, etc...);
etc...
}
Or something similar... this is a little rusty
void collision(obstacles)
{
for(int i = 0; i < obstacles.sizeOf();i++)
{
//bottom y
if((this.ypos + this.height) > (obstacles[i].ypos - obstacles[i].height))
obstacles[i].doYCollision(this);
//top y
if((this.ypos - this.height) < (obstacles[i].ypos + obstacles[i].height))
obstacles[i].doYCollision(this);
//right x
if((this.xpos + this.width) > (obstacles[i].xpos - obstacles[i].width))
obstacles[i].doXCollision(this);
//left x
if((this.xpos - this.width) < (obstacles[i].xpos + obstacles[i].width))
obstacles[i].doXCollision(this);
}
}
again im a little rusty but if you follow it you should be able to relaise what im doing.
then all you need is the resulting function calls.
void doYCollision(Obstacle obstacle)
{
// Whatever y direction they are going do the opposite
obstacle.yDir = obstacle.yDir * -1;
}
void doXCollision(Obstacle obstacle)
{
// Whatever x direction they are going do the opposite
obstacle.xDir = obstacle.xDir * -1;
}
where yDir, xDir is the x and y velocity of the current object.
i should point out again this is very rusty and without having some code from you this is the best ive got. but either way this should start you off into collision detection, the code above shoudl allow for all collisions with all obstacles/objects/pink flamingos/ whatever youve got. Im hoping also that itll do what you want when it comes to multiple collisions at the same time.
You shouldnt need to worry too much about the exact angle (unless you need it for something else), as velocity is a vector mass so has both speed and direction and you can get direction by treating x and y as two different elements. You can do this using the dot product method aswell => http://www.topcoder.com/tc?d1=tutorials&d2=geometry1&module=Static, but if they are just rectangles this should be fine.
Hopes this helps
I am trying to create a 2D platformer (Mario-type) game and I am some having some issues with handling collisions properly. I am writing this game in C++, using SDL for input, image loading, font loading, etcetera. I am also using OpenGL via the FreeGLUT library in conjunction with SDL to display graphics.
My method of collision detection is AABB (Axis-Aligned Bounding Box), which is really all I need to start with. What I need is an easy way to both detect which side the collision occurred on and handle the collisions properly. So, basically, if the player collides with the top of the platform, reposition him to the top; if there is a collision to the sides, reposition the player back to the side of the object; if there is a collision to the bottom, reposition the player under the platform.
I have tried many different ways of doing this, such as trying to find the penetration depth and repositioning the player backwards by the penetration depth. Sadly, nothing I've tried seems to work correctly. Player movement ends up being very glitchy and repositions the player when I don't want it to. Part of the reason is probably because I feel like this is something so simple but I'm over-thinking it.
If anyone thinks they can help, please take a look at the code below and help me try to improve on this if you can. I would like to refrain from using a library to handle this (as I want to learn on my own) or the something like the SAT (Separating Axis Theorem) if at all possible. Thank you in advance for your help!
void world1Level1CollisionDetection()
{
for(int i; i < blocks; i++)
{
if (de2dCheckCollision(ball,block[i],0.0f,0.0f)==true)
{
de2dObj ballPrev;
ballPrev.coords[0] = ball.coords[0];
ballPrev.coords[1] = ball.coords[1];
ballPrev.coords[2] = ball.coords[2];
ballPrev.coords[3] = ball.coords[3];
ballPrev.coords[0] -= ball.xspeed;
ballPrev.coords[1] -= ball.yspeed;
ballPrev.coords[2] -= ball.xspeed;
ballPrev.coords[3] -= ball.yspeed;
int up = 0;
int left = 0;
int right = 0;
int down = 0;
if (ballPrev.coords[0] < block[i].coords[0] && ballPrev.coords[2] < block[i].coords[0] && (((ball.coords[1] < block[i].coords[1]) || (ball.coords[3] < ball.coords[1])) || ((ball.coords[1] < block[i].coords[3]) || ball.coords[3] < block[i].coords[3])))
{
left = 1;
}
if (ballPrev.coords[0] > block[i].coords[2] && ballPrev.coords[2] > block[i].coords[2] && (((ball.coords[1] < block[i].coords[1]) || (ball.coords[3] < ball.coords[1])) || ((ball.coords[1] < block[i].coords[3]) || (ball.coords[3] < block[i].coords[3]))))
{
right = 1;
}
if(ballPrev.coords[1] < block[i].coords[1] && block[i].coords[1] < ballPrev.coords[3] && ballPrev.coords[3] < block[i].coords[3])
{
up = 1;
}
if(block[i].coords[1] < ballPrev.coords[1] && ballPrev.coords[1] < block[i].coords[3] && block[i].coords[3] < ballPrev.coords[3])
{
down = 1;
}
cout << left << ", " << right << ", " << up << ", " << down << ", " << endl;
if (left == 1)
{
ball.coords[0] = block[i].coords[0] - 18.0f;
ball.coords[2] = block[i].coords[0] - 2.0f;
}
else if (right == 1)
{
ball.coords[0] = block[i].coords[2] + 2.0f;
ball.coords[2] = block[i].coords[2] + 18.0f;
}
else if (down == 1)
{
ball.coords[1] = block[i].coords[3] + 4.0f;
ball.coords[3] = block[i].coords[3] + 20.0f;
}
else if (up == 1)
{
ball.yspeed = 0.0f;
ball.gravity = 0.0f;
ball.coords[1] = block[i].coords[1] - 17.0f;
ball.coords[3] = block[i].coords[1] - 1.0f;
}
}
if (de2dCheckCollision(ball,block[i],0.0f,0.0f)==false)
{
ball.gravity = -0.5f;
}
}
}
To explain what some of this code means:
The blocks variable is basically an integer that is storing the amount of blocks, or platforms. I am checking all of the blocks using a for loop, and the number that the loop is currently on is represented by integer i.
The coordinate system might seem a little weird, so that's worth explaining.
coords[0] represents the x position (left) of the object (where it starts on the x axis).
coords[1] represents the y position (top) of the object (where it starts on the y axis).
coords[2] represents the width of the object plus coords[0] (right).
coords[3] represents the height of the object plus coords[1] (bottom).
de2dCheckCollision performs an AABB collision detection.
Up is negative y and down is positive y, as it is in most games.
Hopefully I have provided enough information for someone to help me successfully. If there is something I left out that might be crucial, let me know and I'll provide the necessary information. Finally, for anyone who can help, providing code would be very helpful and much appreciated.
Thank you again for your help!
Edit 2: I have updated my code with a new algorithm that checks where the ball was previously before collision. Corner cases work on that single platform correctly now, and when I have a wall of objects, I can slide against it correctly now. The only remaining problem is that there is a small jittering effect that happens when I am on the ground, where the ball is constantly going up and down as if it is being pulled by gravity and then the ball falls back into the object again.
Edit: Here is a URL to an image trying to show the kinds of problems I am having:
http://img8.imageshack.us/img8/4603/collisionproblem.png
In case the explanation in the picture doesn't make too much sense, the ball cannot move left past the corner of an object unless I jump over it. However, the ball can move right, but it gets repositioned to the right of the object while moving, which is not needed. This creates a skipping movement essentially, where it appears as the the ball is skipping over half of the object or so when I move right. If this doesn't make sense, please ask me and I'll try to clarify more.
One problem with your code is that you only detect situations like this:
If the circle happens to be fully inside the block, you don't reposition at all. And that's a problem.
You're trying to think about your simulation as if it were continuous, but keep in mind it's discrete. In general, if you only look at the current state of the ball, you really cannot know which side it collided with. Look at these two possibilities:
The first solution that comes to mind is to look at the last position of the ball as well; more precisely, look at the delta vector. See if the delta vector intersects a wall. If it does, reposition in an axis-aligned direction towards the wall intersected by the delta vector.
Edit: When I said "delta vector", I forgot that you're moving a square and not a single point. So, if you just look at the delta vector of the top-left corner, that's not going to be enough because it may not detect that part of the ball entered a block. Instead, you can look at the delta vectors of all 4 corners.