# C++ Rotation function works in 2D perspective but fails in 3D

1. Jul 24, 2012

### dot_binary

I'm making a 3D rotation of a poligon.The problem is when I rotate some points in 2D, it works, but if I use the same function to rotate the same points in a 3D perspective , while rotating the vertices move to the center of the object and remain there.
I also wrote the 3D rendering part.At first I thought that my rotation function is not precise, I made it more precise and still it it doesn't work.The 3D rendering function only takes the values stored in the objects array and puts vertices on screen.

The values stored in o[0][0] and o[0][1] represent the X and Y coordonates of the origin
and the ones stored in o[1][0] and o[1][1] are the coordonates of a point.
For now I'm only rotating on X and Y axis.

This is the rotation function:

int rotate(int t,double Ox, double Oy , double Ox2, double Oy2 , double Ub ){
double pi = 3.14159265 ;

if( Ub > 359 ){ Ub = Ub-360; }
if( Ub < 0 ){ Ub = 360-abs(int(Ub)); }

double U = (Ub*pi)/180 ;
double Tx = Ox2 - Ox ;
double Ty = Oy2 - Oy ;
double x = ( Tx *cos( U ) ) - ( Ty *sin( U ) );
double y = ( Ty *cos( U ) ) + ( Tx *sin( U ) );

Ox2 = floor( (Ox + x) + 0.5 ) ;
Oy2 = floor( (Oy + y) + 0.5 ) ;

if ( t == 1 ){
return int(Ox2) ;
}else{
return int(Oy2) ;
}
}

This is how I'm doing the rotation of one point around another point:

o[1][0] = rotate(1,o[0][0],o[0][1],o[1][0],o[1][1],Ub2);
o[1][1] = rotate(2,o[0][0],o[0][1],o[1][0],o[1][1],Ub2);

where Ub2 is the angle of rotation.

2. Jul 24, 2012

### AlephZero

You didn't explain why you are doing all the conversions between int to double so I don't really understand what the routine is doing, but I'll take a guess at the problem.

The first call to rotate() changes the values of o[1][0].
In the second call, should you pass in the original value of o[1][0], not the changed value?

Ignoring the int-to-double conversions, it would probably be better to change the routine to
Code (Text):

void rotate(int t,double Ox, double Oy , double& Ox2, double& Oy2 , double Ub )

and update both of Ox2 and Oy2 with one call

3. Jul 25, 2012

### dot_binary

The first call changed the value of o[1][0] , that was the problem.I overlooked that.
Thank you for pointing that out.

4. Aug 4, 2012

### dot_binary

I still have a problem with my rotation function.

I rewrote the rotation function because the previous one didn't work as I wanted.
Now if I rotate a line in a 2D space, if I give it fixed values linke this:

line(400,400,rotate(1,400,400,400,600,Ub2),rotate(2,400,400,400,600,Ub2));

...it works but if the values come from an array, the line shrinks while rotating.And I'm shure I didn't made the mistake I previously made.
This is how I do the rotation now:

rx = rotate(1,aq[1][1],aq[1][2],aq[2][1],aq[2][2],Ub2) ;
ry = rotate(2,aq[1][1],aq[1][2],aq[2][1],aq[2][2],Ub2);
aq[2][1] = rx ;
aq[2][2] = ry ;
line(aq[1][1],aq[1][2],aq[2][1],aq[2][2]);

The rotation function:

int rotate(int u ,int Ox, int Oy , int Ox2, int Oy2 , float Ub ){
float PI = 3.1415926535;

if( Ub > 359 ){ Ub = Ub-360; }
if( Ub < 0 ){ Ub = 360-abs(int(Ub)); }

int Tx = abs(Ox2 - Ox) ;
int Ty = abs(Oy2 - Oy) ;

// get size of line
int hkj = int( round( sqrt( (Tx*Tx) + (Ty*Ty) ) ) );

if( ( Ub >=0 ) and ( Ub < 90 ) ){
Ox2 = int( sin((Ub*PI)/180) * hkj );
Oy2 = int( cos((Ub*PI)/180) * hkj );
}
if( ( Ub >=90 ) and ( Ub < 180 ) ){
Ox2 = int( cos(((Ub-90)*PI)/180) * hkj );
Oy2 = int( (sin(((Ub-90)*PI)/180) * hkj)* -1);
}
if( ( Ub >=180 ) and ( Ub < 270 ) ){
Ox2 = int( (sin(((Ub-180)*PI)/180) * hkj )* -1);
Oy2 = int( (cos(((Ub-180)*PI)/180) * hkj )* -1);
}
if( ( Ub >=270 ) and ( Ub < 360 ) ){
Ox2 = int( (cos(((Ub-270)*PI)/180) * hkj )* -1 );
Oy2 = int( sin(((Ub-270)*PI)/180) * hkj ) ;
}

if ( u == 1 ){
return Ox+Ox2 ;
}else{
return Oy+Oy2 ;
}
}