旋转功能奇怪的行为

Question

I've been working on a rotation function. The points move sligthly to the rotation center with each cycle. It depends on a function to get the distance between the center and the point to rotate:

struct Vector3F
{
    float x, y, z;
};

inline float GetDistance3FX_inline(Vector3F* v1, Vector3F* v2) 
{ 
    return v1->x - v2->x; 
}

inline float GetDistance3FZ_inline(Vector3F* v1, Vector3F* v2) 
{ 
    return v1->z - v2->z; 
}

The actual rotation function, that does work:

void Rotate3FY(Vector3F* point, Vector3F* center, float rad)
{
    float x = GetDistance3FX_inline(point, center);
    float z = GetDistance3FZ_inline(point, center);
    point->x = x * cos(rad) - z * sin(rad) + center->x;
    point->z = z * cos(rad) + x * sin(rad) + center->z;
}

The function that does not work:

void Rotate3FY(Vector3F* point, Vector3F* center, float rad)
{
    point->x = GetDistance3FX_inline(point, center) * cos(rad) - GetDistance3FZ_inline(point, center) * sin(rad) + center->x;
    point->z = GetDistance3FZ_inline(point, center) * cos(rad) + GetDistance3FX_inline(point, center) * sin(rad) + center->z;
}

I have no clue what logical difference there is between these two. Thank your for any advice.

Answer 1

In your second function when you update point->x you are changing the properties of point . Therefore when you call the same function again later during setting of point->z , the value of point->x has already changed.

void Rotate3FY(Vector3F* point, Vector3F* center, float rad)
{
    point->x = GetDistance3FX_inline(point, center) * cos(rad) - GetDistance3FZ_inline(point, center) * sin(rad) + center->x;
    // point is now different to what it was at the start because you just 
    // changed is x member!!!!!!!
    point->z = GetDistance3FZ_inline(point, center) * cos(rad) + GetDistance3FX_inline(point, center) * sin(rad) + center->z;
}

In the first function, you store the values you require in separate variables before modifying point .