ball reflection angles Xna c#

Question

I'm trying to find a way to handle reflections for a breakout clone.

I would upload an image to the post instead of the following paragraph, however i have not yet gained the privilege of that yet.

If the ball intersects the left hand side i want it to bounce off to the left. if the ball intersects the right hand side i want it to bounce off to the right. if the ball intersects the middle section i want it to bounce up the way. i want to learn how to make it bounce in a varying direction dependant on what side of the left, right, or middle section was intersected

I would like to not use three separate rectangles for this, i want to learn how to do it with one.

I use a Vector2 for ball velocity, projVel.

It's position is projPos.

A rectangle for the paddle lightRect.

The reason I use proj.collRect for the beginning of the if is because I cannot use the intersect method with Vector2.

This is my makeshift collision handler at present, which does work but the speed changes to an extent which renders the game unplayable. The speed clamp i have only slightly slows it down i think. i have a variable for projSpeed i cannot clamp that or it will never be able to stop.

    if (proj.collRect.Intersects(lightSaber.lightRect))
    {
                proj.projPos.Y = lightSaber.lightRect.Y - proj.projTxr.Height;
                proj.projVel.Y *= -1;

        proj.projVel.X = 10 * (proj.projPos.X - lightSaber.lightRect.Center.X) / (lightSaber.lightRect.Center.X);
    }

            proj.projVel.X = Math.Max(-4, Math.Min(proj.projVel.X, 4));
            proj.projVel.Y = Math.Max(-4, Math.Min(proj.projVel.Y, 4));

Help me by showing me how I could do this, maybe in the Math. method, or even an alternative to .Intersects so I can use projPos instead of collRect.

I really am not sure where to start, if there is another way I could do it an example would be great.

If you don't get an answer here, the guys over at Game Development stack exchange may be able to help! — Lex Webb, Jan 17 '17 at 17:26

RogerN · Accepted Answer · 2017-01-17T19:43:58.860

0

Instead of manipulating X and Y velocities independently, I recommend that you calculate a reflection angle based on the position and then derive the velocity from the angle and the speed prior to impact.

Example:

// NOTE: this code assumes that positive Y is down
if (proj.collRect.Intersects(lightSaber.lightRect) && projPos.projVel.Y > 0.0f) // only bounce if projectile is moving downward
{
    // remember current speed for when we calculate new velocity
    var projSpeed = projVel.Length(); 

    // make sure the projectile no longer intersects the bar
    proj.projPos = lightRect.Y - proj.projTxr.Height;

    // interpolate reflection angle
    var t = (proj.projPos.X - lightSaber.lightRect.X) / lightSaber.lightRect.Width;
    var reflectDegrees = 150.0f - t * 120f; // straight up +/- 60 degrees
    var reflectRadians = reflectDegrees * (float)Math.PI / 180.0f;

    // final velocity determined by angle and original projectile speed
    proj.projVel = new Vector2((float)Math.Cos(reflectRadians) * projSpeed, -(float)Math.Sin(reflectRadians) * projSpeed);
}

edited Jan 17 '17 at 19:43

answered Jan 17 '17 at 18:03

RogerN

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That is an interesting way of doing it, however this doesnt fully work. Once it comes back down after launching it gets stuck on top of the paddle moving slightly to the right. when i move the paddle along with it the proj basically looks like a 2D airliner taking off. My lecturer refuses to tell me how to do it, and have ended up spending well over 15 hours trying to get it right. I have tried to fix what you have shown me but i cannot do it. To ensure that the proj does not intersect the bar i had to put .Y at the end of it or i wouldnt be able to complete it – Lonchenzo Jan 17 '17 at 18:48
Sorry, I think I made a mistake in the velocity calculation since positive Y is down. The Y component should be negated (the sine of 90 degrees is 1, but we want to get -1). I will edit my answer to reflect this. – RogerN Jan 17 '17 at 19:14
Thank you for your time by the way. When the proj intersects the paddle on the left side it travels along towards the right end of the paddle before reflecting. I think there is an issue with the var t line of code. However i cannot be sure. – Lonchenzo Jan 17 '17 at 19:28
@LawrenceB I made another adjustment to that line of code and the line below it. That's what I get for just trying to wing it without testing this stuff. If it's still not reflecting, then my assumption about the direction of your Y axis might be incorrect. – RogerN Jan 17 '17 at 19:45
It is all working perfectly now. thank you much appreciated. I have not been taught to use var and any at all of the Math. method. What you have provided will help me alot – Lonchenzo Jan 17 '17 at 19:50

ball reflection angles Xna c#

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