CODEWITHSUNDEEP
c#.netgdi+xna-4.0
ChP
ChP

Reputation: 486

2D particle system with random velocities

I have recently used Visual Basic .Net to write a particle system which emits particles with random velocities in the x and y direction and is affected by gravity. I switched to C# .Net and used the XNA Game Studio which makes the graphics handling much more convenient than GDI+.

The problem I have with C# is that the random numbers are not "random enough". My particle system has 2500 particles but you can clearly see that the particles are distributed in a grid-like fashion about 100 pixels apart and I did not have that problem with Visual Basic's Rnd() function.

What does Visual Basic do which C# does not, and how can I get the same results in C#?

I have tried to re-initialise my random numbers at different stages of the game loop but I end up either with my particles staying at one position or emitting just in a constant stream in one direction.

This is my C# code: LoadContent is called first thing after the program has started. I'm using the millisecond as a seed just so that I start each time with a different configuration.

The next time I re-seed is after all the calculations are done on the system just before rendering. The other alternative I tried is to re-seed after every 100 particles have been calculated but with no difference.

        protected override void LoadContent()
    {
        // Create a new SpriteBatch, which can be used to draw textures.
        spriteBatch = new SpriteBatch(GraphicsDevice);
        mousePos = Content.Load<Texture2D>("Point");
        spriteTex = Content.Load<Texture2D >("Point");
        rnd = new Random(a.Millisecond);
        for (int i = 0; i < spritePos .Length; i++)
        {
            newPos[i] = new Vector2(800, 450);
            spritePos[i] = newPos[i] ;
            Scale[i] = rnd.Next(100,500);
            renderCol[i] = new Color(rnd.Next(255), rnd.Next(255), rnd.Next(255), 1);
            spriteVelocity[i] = new Vector2((rnd.Next(2000)-1000)/100, -rnd.Next(500,1500)/100);
            Life[i] = rnd.Next(60);
            Rotate[i] = (rnd.Next(1000)-500) * 0.001f;
            RotateSpeed[i] = (rnd.Next(1000)-500) * 0.0001f;

        }

    }

This is my VB code, the only place where I use the rnd function:

            For i = x To x + 1000
                ptc(i) = New particle(New Vector((Rnd() * 200) - 200 * Rnd(), Rnd() * -100 - 200 * Rnd() - 200), New Vector(e.X, e.Y), 500, Color.FromArgb(Rnd() * 255, 255, 0))
                x += 1
            Next

In my VB code there is no place where I call the randomize function, I have noticed that my particles have the same pattern-like behaviour if I do. Excuse all the strange arithmetic, it's all just experimentation.

Upvotes: 1

Views: 1826

Answers (3)

John McDonald
John McDonald

Reputation: 1799

Ok, what you do to fix it Charl, is to create a single Random object (which you are) with no seed, and use it over and over again. So something like:

const double max = 1000.0;
Random rand = new Random();     // Like mentioned, don't provide a seed, .NET already picks a great seed for you
for(int iParticle = 0; iParticle < 2500; iParticle++)
{
    double x = rand.NextDouble() * max;     // Will generate a random number between 0 and max
    double y = rand.NextDouble() * max;
}

To get a random floating point value (float or double) between a lower and upper bound, you can use something like:

double x = (rand.NextDouble() * (max - min)) + min;

EDIT: And make sure to use double or float. Ints are whole numbers only, doubles and floats can store real numbers and is probably what VB was using.

Upvotes: 3

Henk Holterman
Henk Holterman

Reputation: 273274

If you had posted some code we would probably have been able to point out where you are creating a new Random() object for each call .

Like in, for example, Random number in a loop

After seeing the code,

are you aware that (rnd.Next(2000)-1000)/100 is an integer only expression? The result will be converted to float but always end in ##.0.

In VB I / J yields a double.

Upvotes: 1

Vladimir
Vladimir

Reputation: 3689

Random class ensures pseudo-randomness, if that is what you are using. Have a look at RNGCryptoServiceProvider.

Upvotes: -1

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