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Im testing out collision detection for a game, The ball is traveling and i need to know when the ball hits a sloped line.

For this example i have used the whole screen, with a sloped line going from the top left corner to the bottom right corner.

I am working out the gradient using Y = MX + C

private int m;
private int c;
private float slopeY;
private void tryCollision(){
    m = getHeight() / getWidth(); // change in Y / change in X
    c = 0 // where the line crosses the x axis
    slopeY = m * ball.x + c;
    if(ball.y >= slopeY){
        ball.ballStart = false; // ball will only move if it ballStart = true
    }
}

Ball stopped moving where ever i place it on the screen. The problem is that working with gradients, Your meant to work with the y axis starting at the bottom going to the top, but in java the y axis starts at the top left corner going down. I have been told to invert the getHeight() number but i am unsure.

If you need to ask a question for you to help please leave a comment :)

Many Thanks, Charlton Santana

Found the answer :) ... when calculating M the equation would have turned out to 0.5265, as the variable being an int it would have automatically changed to 0 meaning the gradient would be a straight line so by changing the variable type it was able to work properly. I cant believe how simple the problem was. Thank for your help.! Hopefully people looking for collision detection for a gradient will be able to look at it as an example.!

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The orientation of the Y axis shouldn't matter as far as the calculations are concerned. This only becomes a factor when you plot to the screen. I have one concern about the line slopeY = m *= ball.x + c;: Is there a reason for the *= in there? It seems to me that it should just be *. – Code-Guru Aug 21 '12 at 20:19
I am not sure why i have done that, i ran it and the ball stopped moving where ever i place it on the screen. – Charlton Santana Aug 21 '12 at 20:25
The reason it may have to be converted is when i am working out M. – Charlton Santana Aug 21 '12 at 20:27
If the line slopes from the top left to the bottom right of the screen, then you have calculated m correctly with m = getHeight() / getWidth();. You should not later change m with the *= assignment operator, as far as I can tell. I suggest using just y = m * ball.x + c; in the line under discussion. – Code-Guru Aug 21 '12 at 20:58
I went to edit and see if it would work, but when asking the question.. i wasent ment to put the = in the question and in my code it was y = m * ball.x + c; sorry.. – Charlton Santana Aug 21 '12 at 21:09
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closed as too localized by Tetrad♦ Mar 12 at 4:36

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