Can someone give me an explanation about the code below? especially the first line, why we need the negative sign "-" for x? but y, z is positive ?
D3DXVECTOR3 CD3DArcBall::ScreenToVector( float fScreenPtX, float fScreenPtY )
{
// Scale to screen
FLOAT x = -(fScreenPtX - m_Offset.x - m_nWidth/2) / (m_fRadius* m_nWidth/2);
FLOAT y = (fScreenPtY - m_Offset.y - m_nHeight/2) / (m_fRadius* m_nHeight/2);
FLOAT z = 0.0f;
FLOAT mag = x*x + y*y;
if( mag > 1.0f )
{
FLOAT scale = 1.0f/sqrtf(mag);
x *= scale;
y *= scale;
}
else
z = sqrtf( 1.0f - mag );
// Return vector
return D3DXVECTOR3( x, y, z );
}
