/*

     File: OpenGLTrackball.mm

 Abstract: 

 Utility class for managing a virtual sphere's (trackball) interaction with an OpenGL world.

  Version: 1.2

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 */

//---------------------------------------------------------------------------

//---------------------------------------------------------------------------

//

// The trackball works by assuming that a sphere encloses the 3D view. You

// roll this virtual sphere with the mouse.  For example, if you click on

// the center of the sphere and move the mouse straight to the right, you

// roll the sphere around its Y-axis.  This produces a Y-axis rotation.

// You can click on the "edge" of the sphere and roll it around in a circle

// to get a Z-axis rotation.

//

//---------------------------------------------------------------------------

//---------------------------------------------------------------------------

#import <math.h>

//---------------------------------------------------------------------------

#import "Vector.h"

#import "Quaternion.h"

//---------------------------------------------------------------------------

#import "OpenGLTrackball.h"

//---------------------------------------------------------------------------

//---------------------------------------------------------------------------

#define kPi 3.1415927f // IEEE-754 single precision

//---------------------------------------------------------------------------

//---------------------------------------------------------------------------

static const GLfloat kDeltaTolerance  = 0.001;

static const GLfloat kRadians2Degrees = 180.0f/kPi;

static const GLfloat kDegrees2Radians = kPi/180.0f;

//---------------------------------------------------------------------------

//---------------------------------------------------------------------------

struct OpenGLTrackballData

{

    GLfloat                 radius;

    Math::Vector3<GLfloat>  center;

    Math::Vector3<GLfloat>  start;

    Math::Vector3<GLfloat>  end;

};

typedef struct  OpenGLTrackballData OpenGLTrackballData;

//---------------------------------------------------------------------------

//---------------------------------------------------------------------------

@implementation OpenGLTrackball

//---------------------------------------------------------------------------

- (id) init

{

    self = [super init];

    if( self )

    {

        mpTrackball = (OpenGLTrackballDataRef)calloc(1, sizeof(OpenGLTrackballData));

        if( !mpTrackball )

        {

            NSLog( @">> ERROR: OpenGL Trackball - Can't allocate memory for the backing store!" );

        } // else

    } // if

    return( self );

} // init

//---------------------------------------------------------------------------

- (void) cleanUpTrackball

{

    if( mpTrackball != NULL )

    {

        free( mpTrackball );

        mpTrackball = NULL;

    } // if

} // cleanUpTrackball

//---------------------------------------------------------------------------

- (void) dealloc

{

    [self cleanUpTrackball];

    [super dealloc];

} // dealloc

//---------------------------------------------------------------------------

//---------------------------------------------------------------------------

//

// Start up the trackball.  Inputs are mouse positon and view size.

//

// The math behind the trackball is simple: start with a vector from the

// first mouse-click on the sphere to the center of the 3D view.  At the

// same time, set the radius of the sphere to be the smaller dimension of

// the 3D view.  As you drag the mouse around in the 3D view, a second

// vector is computed from the surface of the sphere to the center.  The

// axis of rotation is the cross product of these two vectors, and the

// angle of rotation is the angle between the two vectors.

//

//---------------------------------------------------------------------------

- (void) start:(const NSPoint *)thePosition

        origin:(const NSPoint *)theOrigin

          size:(const NSSize *)theSize

{

    Math::Vector3<GLfloat> bounds(0.5f * theSize->width, 0.5f * theSize->height);

    Math::Vector3<GLfloat> position(thePosition->x, thePosition->y);

    Math::Vector3<GLfloat> origin(theOrigin->x, theOrigin->y);

    // Compute the center of a view.

    mpTrackball->center = origin + bounds;

    // Compute the starting vector from the sphere surface to its center.

    mpTrackball->start = position - mpTrackball->center;

    if( bounds.x > bounds.y )

    {

        mpTrackball->radius = bounds.y;

    } // if

    else

    {

        mpTrackball->radius = bounds.x;

    } // else

    mpTrackball->start.z = mpTrackball->start.bound( mpTrackball->radius );

} // start

//---------------------------------------------------------------------------

//

// Calculate rotation based on current mouse position.  Output is the

// rotation angle.

//

//---------------------------------------------------------------------------

- (void) roll:(const NSPoint *)thePosition

           to:(GLfloat *)theRotation

{

    Math::Vector3<GLfloat> position(thePosition->x, thePosition->y);

    mpTrackball->end = position - mpTrackball->center;

    // Has there been a change in the (x,y) componets of our 3-vector?

    Math::Vector3<GLfloat> d = Math::diff(mpTrackball->end, mpTrackball->start);

    if( ( d.x >= kDeltaTolerance ) && ( d.y >= kDeltaTolerance ) )

    {

        // There has been sufficent amount of change in our

        // 3-vector to warrant continuing further.

        GLfloat A  = 0.0f;  // sin( angle )

        GLfloat B  = 0.0f;  // cos( angle )

        GLfloat c  = 0.0f;

        GLfloat ls = 0.0f;

        GLfloat le = 0.0f;

        GLfloat lr = 0.0f;

        // Compute the end-vector from the surface of the sphere to its center.

        mpTrackball->end.z = mpTrackball->end.bound( mpTrackball->radius );

        // ls = 1 / || s ||

        ls = 1.0f / Math::norm(mpTrackball->start);

        // le = 1 / || e ||

        le = 1.0f / Math::norm(mpTrackball->end);

        // c = s * e; interior product

        c = mpTrackball->start * mpTrackball->end;

        // B = cos(a) = (s . e) / (||s|| ||e||)

        B = ls * le * c;

        // r = s ^ e; exterior product

        Math::Vector3<GLfloat> r = mpTrackball->start ^ mpTrackball->end;

        // lr = || r || = ||(s ^ e)||

        lr = Math::norm(r);

        // A = sin(a) = ||(s ^ e)|| / (||s|| ||e||)

        A = ls * le * lr;

        // Normalize the rotation axis.

        lr = 1.0f / lr;

        // GL rotations are in degrees.

        // Use atan for a better angle.  Note that when using acos or asin,

        // you only get half the possible angles, and you can end up with

        // rotations that flip around and near the poles.

        theRotation[0] = kRadians2Degrees * atan2f( A, B );

        theRotation[1] = lr * r.x;

        theRotation[2] = lr * r.y;

        theRotation[3] = lr * r.z;

        // returns rotation

    } // if

} // rollTo

//---------------------------------------------------------------------------

//

// Determine C = A . B

//

// (1) In quaternions: let P <- B, and Q <- A.

// (2) Compute the Hamilton product, R = P ^ Q.

// (3) Then, C = R if R is not an identity rotation.

//

//---------------------------------------------------------------------------

- (void) add:(const GLfloat *)A

          to:(GLfloat *)B

{

    Math::Quaternion<GLfloat> P(B);

    Math::Quaternion<GLfloat> Q(A);

    // Avoid floating point errors by renormalizing quaternions

    // if it was necessary

    P = Math::norml(P);

    Q = Math::norml(Q);

    // Compute the Hamilton product

    Math::Quaternion<GLfloat> R =  P ^ Q;

    // Check for identity rotation.

    // An identity rotation is expressed as rotation by 0

    // about any axis.  The "angle" term in a quaternion

    // is really the cosine of the half-angle. So, if the

    // cosine of the half-angle is one (or, 1.0 within our

    // tolerance), then you have an identity rotation.

    if( __builtin_fabsf( R.t - 1.0f ) < 1.0e-7 )

    {

        // Encountered an identity rotation.

        B[0] = 1.0f;

        B[1] = 0.0f;

        B[2] = 0.0f;

        B[3] = 0.0f;

    } // if

    else

    {

        // Encountered a non-identity rotation, hence, the

        // cosine of the half-angle is non-zero, which implies

        // that the sine of the angle is also non-zero.  As a

        // result, it is safe to divide by sine of the angle

        // theta.

        // Turn the quaternion back into an {angle, {axis}} rotation.

        R.toRotation(B);

    } // else

} // add

//---------------------------------------------------------------------------

@end

//---------------------------------------------------------------------------

//---------------------------------------------------------------------------