help with glutPassiveMotionFunc

See more:

OpenGL

Hello! it's my first post here and I'm a rookie with OpenGL

I use the function "glutPassiveMotionFunc" to track my mouse.But it seems to have an initial position in (0,0).
Here is short program that can demonstrate my problem:

#include<GL/glut.h>

GLint viewWidth = 400, viewHeight = 400;
int centerX = 200, centerY = 200;
int tempX, tempY;

void drawLine( int x, int y)
{
	glBegin( GL_LINES);
		glVertex2i( centerX, centerY);
		glVertex2i( x, y);
	glEnd();
}

void init()
{
	glClearColor( 0.0, 0.0, 1.0, 1.0);
	glMatrixMode( GL_PROJECTION);
	gluOrtho2D( 0.0, 400.0, 0.0, 400.0);
}

void myDisplay()
{
	glClear( GL_COLOR_BUFFER_BIT);
	glColor3f( 0.0, 0.0, 0.0);

	drawLine( tempX, tempY);

	glutSwapBuffers();
}

void myMouseMove( int x, int y)
{
	tempX = x;
	tempY = viewHeight - y;

	glutPostRedisplay();
}

void main( int argc, char ** argv)
{
	glutInit( &argc, argv);
	glutInitDisplayMode( GLUT_DOUBLE| GLUT_RGB);
	glutInitWindowPosition( 100, 100);
	glutInitWindowSize( viewWidth, viewHeight);
	glutCreateWindow( "problem demonstration");

	init();
	glutDisplayFunc( myDisplay);
	glutPassiveMotionFunc( myMouseMove);

	glutMainLoop();
}

You may run this program with the pointer outside the program window. And before you move you mouse into the program window, you can already see a black line which is the problem that confuses me.

Thanks in advance!