Help with javascript problem.

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  • I downloaded a couple of Javascript tutorials about 'Baasic physics', and i'm stuck on page 1.

    Note. I have done a bit more research and think i understand most of it, but i can't work out what the symbols marked in red mean?

    // Version 3 - Basic collision (viewbox), perfectly elastic (no energy loss)

    var ball = {

    x: 300,

    y: 200,

    r: 15,

    vx: 0,

    vy: 0


    // Update ball (with Physics! =)

    // 1 - apply velocity to position (vx -> x)

    ball.x += ball.vx;

    ball.y += ball.vy;

    // 2 - apply drag/friction to velocity

    ball.vx *= .99;

    ball.vy *= .99;

    // 3 - apply gravity to velocity

    ball.vy += .25;

    // 4 - check for collisions

    if (ball.y + ball.r > canvas.height) {

    ball.y = canvas.height - ball.r;

    ball.vy = -Math.abs(ball.vy);


    // Draw ball;

    ctx.translate(ball.x, ball.y);

    ctx.fillStyle = "#fff";


    ctx.arc(0, 0, ball.r, 0, Math.PI * 2, true);




    }, 1000 / 77);

    please could somebody have a look at the above and post a quick line breakdown.

    thanks in advance.

  • r seem to be the "speed" of the ball

    ball.x += ball.vx is the same as typing ball.x = ball.x + ball.vx. I've never done this with duplication but I assume it's the same there.

    "The abs() method returns the absolute value of a number."

    Math.abs(7.25) = 7.25

    Math.abs(-7.25) = 7.25

    Math.abs(null) = 0

    Math.abs("Hello") = NaN

    Math.abs(7.25-10) = 2.75

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  • If you need general javascript help it might be better to try a dedicated programming forum or a site like Stackoverflow.

    MDC is also an excellent reference for javascript - check out the doc for Math.abs().

  • [quote:29t9f1ed]r seem to be the "speed" of the ball

    Actually I think it's the radius.

  • [quote:30h1jfze]r seem to be the "speed" of the ball

    Actually I think it's the radius.

    Ah of course. Makes sense

  • Thanks to everybody for your help.

    I managed to find a good tutorial on Object collisions for games and apps and i will checkout the Js websites you have mentioned.

    Heres a link to the files and tutorials if anybody is interested -

    Only (150k) containing - 2 Game physics tutorials (gravity a elasticity)

    The Html/jscript file i that i mention in the post.

  • Hey that's pretty cool.

    Here's a very basic implementation:

    You might recognize this game idea (click the mouse)

    Just needs a cave generator. Also, moon lander anyone?

  • Symbols marked in red:

    ball.x += ball.vx;

    Here's an easier example:

    5 += 3;

    This is 8. It's the same as:

    5 = 5 + 3;

    Same with *=:

    100*= 0.99;

    This will equal 99. It's the same as:

    100 = 100*0.99;

    Multiplying by 0.99 represents 99% of it's value. If you multiplied by 0.01 you would get 1% of it's value.

    ball.vy = -Math.abs(ball.vy);

    Maths.abs() takes the absolute (positive) value of it's input.

    Math.abs(-454) = 454;

    Math.abs(13) = 13 (does nothing, already positive number)

    Doing -Math.abs() will get it's negative value, so it guarantees a negative number.

    Hope that helps

  • (monsharen)

    So thats how you do it!i was only getting a ball that constantly bounced around the screen. i will have a look at your capx. cheers.


    I'm doing doing the old 'crash course in jscript' and it doesn't include descriptions for a lot of the equations, so thanks for your help.

  • No worries Any other questions, just ask.

    Hopefully on the new site I'll be able to spend some time writing some JS tutorials/reference pages. They should be quite useful.

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