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# Shervanator

Member Since 05 Oct 2012
Offline Last Active Feb 22 2016 10:47 PM

### Calculating objects direction scene graph

07 September 2014 - 07:04 AM

Hi guys!

So I have a 3D vector representing the direction a object is facing in my game engine, I calculate this direction by multiplying the objects rotation (which is a quaternion) by a vector which represents forwards (in my case: (0, 0, -1)). However the way I have modelled all the objects in my game engine is via a scene graph where each object can have a parent object. Each object has a local transformation which is relative to its parents transformation.

Each frame I recursively go though each object in the scene graph and calculate the transform matrix (posMatrix * rotMatrix * scaleMatrix) and save this for each object. In the case that the object is a child of another objects then i also multiply the transform matrix by the parents transform matrix, and I am able to get the correct hierarchy transformation in world space coordinates.

However I am having issues with calculating the direction a object is facing while maintaining the hierarchy. I found one solution which works but it is not very efficient, this was:

```glm::quat Entity::getRotation(void)
{
if (parentEntity == NULL) {
return transform.getRotation();
} else {
return parentEntity->getRotation() * transform.getRotation();
}
}

glm::vec3 Entity::getDirection(void)
{
if (parentEntity == NULL) {
return transform.getDirection();
} else {
return getRotation() * glm::vec3(0, 0, -1);
}
}
```

I dont like this solution as I feel there must be a more efficient way to just reuse the parents world transformation matrix that I already calculate to transform this direction vector for me.

I have tried something like this, but it doesn't seem to work correctly:

```glm::vec3 Transform::getDirection(void)
{
return getRotation() * glm::vec3(0, 0, -1);
}

glm::vec3 Entity::getDirection(void)
{
if (parentEntity == NULL) {
return transform.getDirection();
} else {
return glm::quat_cast(parentEntity->worldMatrix) * transform.getDirection();
}
}

```

Any help I could get would be very much appreciated!

Thanks!

### Spherical coordinates for orbital motion

05 October 2012 - 08:39 AM

Hi, I've been working on getting some object to orbit around another object by using a spherical coordinate system. This has worked fine when the object is orbiting horizontally and vertically, however if you try to get the object orbiting diagonally it runs into issues with converting the spherical coordinates back to Cartesian coordinates.

Then every frame this information is updated in the following way:

```direction = Vector2.Normalize(direction);
theta += timeDelta * direction.X * orbitSpeed;
phi -= timeDelta * direction.Y * orbitSpeed;

pos.X = radius * (float)Math.Cos(theta) * (float)Math.Sin(phi);
pos.Z = radius * (float)Math.Sin(theta) * (float)Math.Sin(phi);

pos.Normalize();
```

Where direction is the heading direction, radius is the distance away from the centre, timeDelta is the time since the last frame, orbit speed is a speed constant for the orbit, theta is the angle about the y-axis, phi is the angle about the x-axis, and pos is the final position vector for the object.

This works fine when direction = (1, 0) or (0, 1), however when it is made to equal (1, 1) then the object does weird orbits and it seems to be due pos.z not being calculated correctly.

Below is an image of the problem: