3 replies to this topic

[Garold]

I am having some success aligning a particle to a direction and then rotating it along its axis. At the moment I am just constantly rotating it. How do I calculate the correct rotation so it always faces the camera?

 

Please have a look at this video: Quad Slowly Rotating Along Axis

 

I am using DPSF. I have modified the "FlyingSparks" demo.

 

To calculate the orientation Quaternion I run this code:

 

 

private void UpdateOrientation(DefaultTextureQuadTextureCoordinatesParticle cparticle, float felapsedtimeinseconds)
{
   var normalized = cparticle.Velocity;
   normalized.Normalize();
   cparticle.Orientation = Quaternion.Identity;
   cparticle.Right = -normalized;
   _rotation += 0.03f;
   cparticle.Orientation = Quaternion.CreateFromAxisAngle(normalized, _rotation) * cparticle.Orientation;
}

 

After some trial and error the above code produces what you see in the video.

 

Line 5: resets the orientation.

Line 6: aligns the quad to the velocity direction.

Line 7: increments the rotation.

Line 8: further rotates the quad along its axis by the rotation.

 

What I am asking is how do I calculate the rotation?

 

I am not a maths expert. I can guess that I need the normal of the particle, calculated within line 6 and then use that to determine it's difference from the camera direction. I have looked at some billboarding techniques but they don't offer exactly what I want.

 

I am very close to finishing my game and this is the last hurdle, any help or pointers would be greatly appreciated, thank you for taking the time to look at this

[RobTheBloke]

Basically, stop rotating it!

Get the transform matrix of your camera:

 

X = [ a b c 0 ]

Y = [ d e f 0 ]

Z = [ g h i 0 ]

 

The camera *looks* down Z, and the X/Y axes are left/right and up/down respectively. Given the width & height of your quad, you can compute the points directly:

 

P0 =  X * (width/2) + Y * (height/2)

P1 = -X * (width/2) + Y * (height/2)

P2 = -X * (width/2) - Y * (height/2)

P3 =  X * (width/2) - Y * (height/2)

[Garold]

Thanks Rob.

 

I found a solution within the library, after learning some basic math.

 

 Orientation3D.SetUpDirection(ref cparticle.Orientation, Vector3.Cross(cparticle.Velocity, CameraPosition - cparticle.Position));

 

I'll try to see if I can use your method to edit the vertices directly, but it looks a little like the function it calls is doing as you suggest anyway.

 

        public static Quaternion GetRotationTo(Vector3 CurrentDirection, Vector3 DesiredDirection, Vector3 sFallbackAxis)
        {
            Quaternion quaternion = new Quaternion();
            Vector3 vector = CurrentDirection;
            Vector3 vector2 = DesiredDirection;
            vector.Normalize();
            vector2.Normalize();
            float num = Vector3.Dot(vector, vector2);
            if (num >= 1f)
            {
                return Quaternion.Identity;
            }
            if (num <= -0.999999f)
            {
                if (sFallbackAxis != Vector3.Zero)
                {
                    sFallbackAxis.Normalize();
                    return Quaternion.CreateFromAxisAngle(sFallbackAxis, 3.141593f);
                }
                Vector3 axis = Vector3.Cross(Vector3.UnitX, vector);
                if (axis.LengthSquared() < 0.999998f)
                {
                    axis = Vector3.Cross(Vector3.UnitY, vector);
                }
                axis.Normalize();
                return Quaternion.CreateFromAxisAngle(axis, 3.141593f);
            }
            Vector3 vector4 = Vector3.Cross(vector, vector2);
            double num2 = Math.Sqrt((double) ((1f + num) * 2f));
            double num3 = 1.0 / num2;
            quaternion.X = (float) (vector4.X * num3);
            quaternion.Y = (float) (vector4.Y * num3);
            quaternion.Z = (float) (vector4.Z * num3);
            quaternion.W = (float) (num2 * 0.5);
            quaternion.Normalize();
            return quaternion;
        }

[Garold]

Your method seems way simpler