这个程序是用来把鼠标的输入和图形联系起来的，用鼠标的左右中键来控制图形的位置，下面这段只是其中的一部分，我用的是Visual Studio 2008的C++，下面这段是老师给的，我看不太懂，switch的作用是什么？是不是把鼠标的位置分别赋给x和y，望高手给解释下，如果可能的最好每句都翻译解释一下，谢谢！！！
void mouseFunc(int button, int state, int x, int y )
  {
     switch( button )
     {
        case GLUT_LEFT_BUTTON:
      
           switch( state )
           {
              case GLUT_DOWN:
                m_LButtonDown = TRUE;
                m_LDownPos.x = x;
                m_LDownPos.y = y;
                break;
              case GLUT_UP:
                m_LButtonDown = FALSE;
                break;
           }
           break;

        case GLUT_RIGHT_BUTTON:
      
           switch( state )
           {
              case GLUT_DOWN:
                m_RButtonDown = TRUE;
                m_RDownPos.x = x;
                m_RDownPos.y = y;
                break;
              case GLUT_UP:
                m_RButtonDown = FALSE;
                break;
           }
           break;

        case GLUT_MIDDLE_BUTTON:

           switch(state)
           {
           case GLUT_DOWN:
               m_MButtonDown = TRUE;
               m_MDownPos.x = x;
               m_MDownPos.y = y;
               break;
           case GLUT_UP:
               m_MButtonDown = FALSE;
               break;
           }
           break;
        

        default:
            break;

     } //End of switch
  }
还有下面这段程序，到底是干什么用的，-=是什么意思？+=是什么意思？x和y前面（double）是什么意思，为什么要放在x和y的前面加上括号？望高手解释下，最好把每句都解释下，谢谢。本科毕业三年后读研中，又用到了C语言，本科本来学的就不太好，现在更是快忘光了！
void mouseMotionFunc(int x, int y)
{
     /* find out what action to perform */
    if (m_LButtonDown)
    {
        m_yRotate -= (m_LDownPos.x - (double)x)/2.0;
        m_xRotate -= (m_LDownPos.y - (double)y)/2.0;
            
        m_LDownPos.x = x;
        m_LDownPos.y = y;
        
        
    }

    if (m_RButtonDown)
    {
         m_transX -= (m_RDownPos.x - (double)x)/10.0f;     
        m_transY += (m_RDownPos.y - (double)y)/10.0f;

        m_RDownPos.x = x;
        m_RDownPos.y = y;

        
    }

        if (m_MButtonDown)
    {
         m_transZ -= (m_MDownPos.y - (double)y)/10.0f;     

        m_MDownPos.x = x;
        m_MDownPos.y = y;

        
    }

  glutSetWindow(win_id);
  glutPostRedisplay();
 
}

完全看不懂啊