I'm having trouble with collision detection.

[jlathrop16]

I'm trying to use the coding skills I learned in the Poo Eating game to make Pong. Why will my collision detection not work?

Please, Please help me figure out how to make the stupid ball do what I want it to do.

[EdadTace]

i'm not quite sure what u're trying to do from looking at ur code but i've found a few problems i wanna go over real quick

from line 132

Code: Select all

// Moves ball

ballX = ballX + 2;
		

if ( ballX + 15 < paddleX + 10 )
{
	collide = true;
}


if ( collide == true )
{
	ballX = ballX - 2;
}

if ( ballX == ballX - 2 ) // sense - this makes none
{
	ballX = ballX - 2;
}

basicly what u do here makes no sense at all
first u move the ball to the right 2 pixels, and then u check to see if the ball have collided and if true then u move it back 2 pixels ( in other words it stands still )

now the last piece of that code doesn't make any sense when u think about it - basicly what u're checking to see is that if ballX is equal to less than ballX - in other words what u're asking is that if 4 is equal to 2 and that will ofcourse never be true

what I'd do is that I would make a new variable called ballSpeedX or something like that - and then on collision I'd multiply that with -1 which will make the ball go in the opposite direction

something like this

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// Moving ball
ballX = ballX + ballSpeedX;

// Checking for collision
if( ball collided with pad ){ // sorry for being lazy here
    ballSpeedX = ballSpeedX * -1;
}

last on from line 116

Code: Select all

if ( ballX + 15 > 749 )
{
	ballX = 749 - 15; // remember to add -15 when u add it to ballX above or it won't align right
}

hope that helps

[chili]

Edad beat me to it.

There are a number of issues, but I'd say your first and biggest problem is that you need to have a velocity variable for your ball and you need to have that variable negated (flipped opposite) when the ball collides with the paddle. Get that sorted first bro.

[jlathrop16]

How about this? The ball doesn't move.

And another thing I'm worried about is when ballX == paddleX then speed get's reversed. but after that like 1 pixel movement, ballX is no longer equal to paddleX, so wouldn't the speed go back to being positive?

[EdadTace]

it's kinda late here so i'll just real quick point out a few problems i see in ur code and if needed go more in detail later

first of all - i noticed something in ur DrawBall function - and looking at ur other draw function they have the same problem

Code: Select all

void Game::DrawBall ( int x, int y ) 
{
	for ( y = 220; y <= 235; y++ )
	{
		for ( x = 20; x <= 35; x++)
		{
			gfx.PutPixel ( x + ballX, y + ballY, 255, 255, 255);
		}
	}
}

first of all it makes no sense to feed in x and y into the function when u later just force them to be something else
second u don't want to set y and x to anything else than 0 in this case - as that just means that u'll draw the ball in a completely different place than ballX and ballY, which are they variable u're suppose to control it with
here's what it should look like - this way it'll not be offset from ballX and ballY

Code: Select all

void Game::DrawBall() 
{
	for ( int y = 0; y <= 15; y++ )
	{
		for ( int x = 0; x <= 15; x++)
		{
			gfx.PutPixel ( x + ballX, y + ballY, 255, 255, 255);
		}
	}
}

here's another issue - it's not really big or anything but it would be smarter if u first updated the position, then ran the clamp to window code and then draw it to the screen - so just some rearrangement
ours.:

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DrawPaddle ( paddleX, paddleY );

// Clamps paddle to screen

if ( paddleX < 5 )
{
	paddleX = 5;
}

if ( paddleX > 795 )
{
	paddleX = 795;
}

if ( paddleY < 5 )
{
	paddleY = 5;
}

if ( paddleY > 595 )
{
	paddleY = 595;
}


// Moves Paddle

if ( kbd.UpIsPressed() )
{
	paddleY = paddleY - 3;
}

if ( kbd.DownIsPressed() )
{
	paddleY = paddleY + 3;
}

smart way - as I said it's not a big issue, but it's better practice

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// Moves Paddle

if ( kbd.UpIsPressed() )
{
	paddleY = paddleY - 3;
}

if ( kbd.DownIsPressed() )
{
	paddleY = paddleY + 3;
}

// Clamps paddle to screen

if ( paddleX < 5 )
{
	paddleX = 5;
}

if ( paddleX > 795 )
{
	paddleX = 795;
}

if ( paddleY < 5 )
{
	paddleY = 5;
}

if ( paddleY > 595 )
{
	paddleY = 595;
}

// Draw paddle
DrawPaddle ();

lastly for the move ball code

Code: Select all

// Moves ball


int speed;
speed = 3;

if ( ballX == paddleX )
{
	collide = true; // this is not used for anything
	speed = speed * -1;
}

ballX = ballX + speed;
}

first of all ur colide bool is not used for anything, u just set it to true and nothing else

what u wanna do is take that out and u want to use > instead of == in this case or else the ball can easily slip right past
u also wanna make sure to set ballX to the same value as the one u're checking - like when u're clamping

something like this

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/* move ball code here */

// collide with paddle
if( ballX > paddleX )
{
    speed = speed * -1;
    ballX = paddleX;
}

/* draw ball code here */

it's kinda late here so i havn't tested this code but as far as i can tell it should work

EDIT.:

about u being worried about the speed getting inverted again - yes that is true - u'll need to make speed a member variable of the game class so it will be it's value will be saved - in other words u need to put in the game.h and so on

[jlathrop16]

This is all I have. I am just completely lost as to how to make this darn program work. If it's too much work to help me, I understand. BUT, I just wanted to say to you guys and chili that this forum and chili's videos are very much appreciated. It's so great that there's a place like this forum for people like us to get together and work together. So thanks for that.

I'll post my code. Mostly, I just don't really understand how collision works. I don't really know why it's ballX + 15 instead of just ballX. stuff like that.

Thanks so much for any help!

[chili]

jlathrop16: It seems you have a lack of understanding regarding the fundamentals of collision detection. If you have any specific questions I would be glad to answer them.

For example, you asked this:

I don't really know why it's ballX + 15 instead of just ballX.

ballX is the x-coordinate of the top-left corner of the ball. Therefore, it is located at the left side of the ball. However, it is the right side of the ball which collides with the paddle. Accordingly, we must test whether the x-coordinate of the right side of the ball exceeds the x-coordinate of the left side of the paddle. Since the width of the ball is 15 pixels, the x-coordinate of the right side of the ball is ballX + 15.

EdadTace: Thanks for the posts bro. Above and beyond.

[jlathrop16]

Ok, I get that now. But here's another problem. The ball moves towards the edge of the screen and when it get like an inch from the paddle, it jumps like .5 inches forward and stops before it touches the paddle. It just stays there...

I guess I'm just not ready to make Pong. I'm going to go redo maybe tutorials starting with 8.

You mentioned that you would maybe make a video for doing a small game...It should be pong haha. :p