I'm trying to get quarter of the circle based on a certain direction (velocity)

The number that I want should be between -D3DX_PI and D3DX_PI, How do I determine it based on the velocity direction?

Example:

Lets say I have the velocity of: (X: 0, Y: 10, Z: 0), in this case the velocity is pointing at the upper direction, so the numbers between -D3DX_PI and D3DX_PI that I should get are based on the upper quarter of the circle.

So, I want to determine which quarter of the circle the velocity is pointing at.

Reading between the lines it sounds like maybe you want to extract heading (aka yaw) and elevation (aka pitch) values from your direction vector.

Let me make it more clear

I have this function:

D3DXVECTOR3 GetRandomVector(D3DXVECTOR3 velocity)
{
    D3DXVECTOR3 vVector;

    // Pick a random Z between -1.0f and 1.0f.
    vVector.z = GetRandMinMax( -1.0f, 1.0f );

    // Get radius of this circle
    float radius = (float)sqrt(1 - vVector.z * vVector.z);

    // Pick a random point on a circle.
    float t = GetRandMinMax( -D3DX_PI, D3DX_PI ); // I THINK THIS LINE SHOULD BE MODIFIED TO PICK A POINT LIMITED TO QUARTER OF THE CIRCLE (According to the direction of D3DXVECTOR3 velocity)

    // Compute matching X and Y for our Z.
    vVector.x = (float)cosf(t) * radius;
    vVector.y = (float)sinf(t) * radius;

    return vVector;
}

In the above method, the random vector is anywhere on the circle between (-D3DX_PI, D3DX_PI) I want to limit the random number to be only quarter of the circle based on the direction of D3DXVECTOR3 velocity.

If I give a velocity of (0, 10, 0) I want to get A and B accordingly.

0, 10, 0 direction is UP so the result should be the upper quarter.

[attachment=15796:circle.png]

I should change the following line:

float t = GetRandMinMax( -D3DX_PI, D3DX_PI );

To:

float t = GetRandMinMax( A, B );

How do I calculate A and B based on the velocity (direction)?

I'm trying to create random vector that points at the velocity (for particles)

So I have:

D3DXVECTOR3 velocity;

float velocity_var;

According to the above variables I want to make the particles generate to move according to the "velocity", however, velocity_var should determine how much the particles will spread.

Example 1:

Velocity: 0.0f, 10.0f, 0.0f

velocity_var: 4.0f

End Point

\ /

\ /

\ /

\ /

Start

Example 2:

Velocity: 0.0f, 10.0f, 0.0f

velocity_var: 0.0f

End Point

|

|

|

|

Start

Assuming this is a continuation of http://www.gamedev.net/topic/643188-sparks-animation/#entry5062421 then the solution is purely vector based. The code I provided before had a bias as I didn't do the full proper solution, but in general it still works, you just have to change the manner of calculation to remove the biasing. Using the following as reference:

[attachment=15797:Reference.jpg]

In that image the white line is your chosen axis of emission, the blue line is a reference "up" (or side, just something at a right angle) vector and the red circle is the radius of the cone of emission. What you want is to make an offset vector which falls within the red circle like the grey radius line, but you want to adjust the length of the offset and direction to be anywhere in that circle. The easiest method of doing this is to generate two values; an angle and a radius.

Assuming that the right angle vector is 1 in length and the plane of the red circle is at +1 along the emission axis it becomes simple math to do the conversions. First, to compute the radius, is very simple, tan( angle ). Second, take the right angle vector (blue) and scale it to the resulting radius times the randomly generated radius. Rotate the result about the emission vector by 2pi times the generated 0-1 angle. Add that to the emission vector and normalize. You have a nice little random vector around the emission vector.

There are likely more ways to do this, possibly even easier methods. Hopefully though this gives you a reasonably understandable starting point.

Can you provide a little bit of code so I can understand you better?

Here is how I generate a new particle:

void generateNewParticle()
{
      // ...
      D3DXVECTOR3 randVec = GetRandomVector();
      particle.velocity += randVec * velocity_var;
      particle.position = particle.velocity;
      // Code to add the particle to the list here...
}

D3DXVECTOR3 GetRandomVector()
{
    D3DXVECTOR3 vVector;

    // Pick a random Z between -1.0f and 1.0f.
    vVector.z = GetRandMinMax( -1.0f, 1.0f );

    // Get radius of this circle
    float radius = (float)sqrt(1 - vVector.z * vVector.z);

    // Pick a random point on a circle.
    float t = GetRandMinMax( -D3DX_PI, D3DX_PI );

    // Compute matching X and Y for our Z.
    vVector.x = (float)cosf(t) * radius;
    vVector.y = (float)sinf(t) * radius;

    return vVector;
}

Here is how I draw the sparks every frame:

RenderSingleLine( particle.position, particle.position - particle.velocity * lineLength );

I'm not really good in mathematics.

Now, I'm trying to change the code to make the spark lines look at the moving direction instantly after they get generated.