I have a model rotated by a quaternion. I can only set the rotation, I can't add or subtract from anything. I need to get the value of an axis, and than add an angle to it (maybe a degree or radian?) and than re-add the modified quaternion.
How can I do this? (answer on each axis).
You can multiply two quaternions together to produce a third quaternion that is the result of the two rotations. Note that quaternion multiplication is not commutative, meaning order matters (if you do this in your head a few times, you can see why).
You can produce a quaternion that represents a rotation by a given angle around a particular axis with something like this (excuse the fact that it is c++, not java):
Quaternion Quaternion::create_from_axis_angle(const double &xx, const double &yy, const double &zz, const double &a)
{
// Here we calculate the sin( theta / 2) once for optimization
double factor = sin( a / 2.0 );
// Calculate the x, y and z of the quaternion
double x = xx * factor;
double y = yy * factor;
double z = zz * factor;
// Calcualte the w value by cos( theta / 2 )
double w = cos( a / 2.0 );
return Quaternion(x, y, z, w).normalize();
}
So to rotate around the x axis for example, you could create a quaternion with createFromAxisAngle(1, 0, 0, M_PI/2) and multiply it by the current rotation quaternion of your model.
x and your model quaternion y, you would normally set the total rotation using x * y (depending on how you implement multiplication of course).
Made a runable code from sje397's post for testing other details later, thought I would share. Eventually using C the reason for no Quaternion class.
#include <iostream>
#include <math.h>
using namespace std;
struct float4{
float x;
float y;
float z;
float w;
};
float4 make_float4(float x, float y, float z, float w){
float4 quat = {x,y,z,w};
return quat;
}
float dot(float4 a)
{
return (((a.x * a.x) + (a.y * a.y)) + (a.z * a.z)) + (a.w * a.w);
}
float4 normalize(float4 q)
{
float num = dot(q);
float inv = 1.0f / (sqrtf(num));
return make_float4(q.x * inv, q.y * inv, q.z * inv, q.w * inv);
}
float4 create_from_axis_angle(const float &xx, const float &yy, const float &zz, const float &a)
{
// Here we calculate the sin( theta / 2) once for optimization
float factor = sinf( a / 2.0f );
float4 quat;
// Calculate the x, y and z of the quaternion
quat.x = xx * factor;
quat.y = yy * factor;
quat.z = zz * factor;
// Calcualte the w value by cos( theta / 2 )
quat.w = cosf( a / 2.0f );
return normalize(quat);
}
int main()
{
float degrees = 10.0f;
float4 quat = create_from_axis_angle(1, 0, 0, degrees*(3.14159f/180.0f));
cout << "> (" << quat.x << ", " <<quat.y << ", " <<quat.z << ", " <<quat.w << ")" << endl;
return 0;
}
Output
(0.0871557, 0, 0, 0.996195)
