GLUT mouse Tracking and Drawing - opengl

Question

I am making a simple GLUT program which tracks the mouse and put points on the path.
Well This is my code:
void init()
{
glClearColor( 0.0, 0.0, 1.0, 1.0);
glMatrixMode( GL_PROJECTION);
gluOrtho2D( 0.0, 400.0, 0.0, 400.0);
for(int i=0;i<5000;i++)
{
arr[i][0]=0;
arr[i][1]=0;
}
glPointSize(10.0);
}
void drawPoints()
{
glBegin( GL_POINTS );
glColor3f( 0.0,0.0,0.0 );
for ( int i = 0; i < z; i++ )
{
glVertex2f( arr[i][0], arr[i][1]);
}
glEnd();
}
void myDisplay()
{
glClear( GL_COLOR_BUFFER_BIT);
drawPoints();
glutSwapBuffers();
glutPostRedisplay();
}
void myMouseMove( int x, int y)
{
arr[z][0]=x;
arr[z++][1]=y;
}
int main( int argc, char ** argv)
{
glutInit( &argc, argv);
glutInitDisplayMode( GLUT_DOUBLE| GLUT_RGB);
glutInitWindowPosition( 100, 100);
glutInitWindowSize(600,600);
glutCreateWindow( "Testing");
init();
glutDisplayFunc( myDisplay);
glutPassiveMotionFunc( myMouseMove);
glutMainLoop();
return 0;
}
However I am having few problems:
Y coordinate runs in opposite direction.
Draws point ahead of cursor position(while moving in a direction).
Is there any better way to do this?

Answer 1

The Y coordinate being flipped is actually expected behavior. Simply correct for it in your code and you should be fine.
If you want to make sure that your rendered image and mouse cursor are completely synchronized, simply have glut hide the mouse cursor, and then render it yourself using OpenGL.

Answer 2

You should be aware that when using a traditional projection matrix in OpenGL: (0,0) is the lower-left corner. Most window systems will map (0,0) to the top-left corner.
In some circumstances, you can make them match up simply by swapping the bottom/top fields in your call to glOrtho (...) or gluOrtho2D (...) - this has other consequences like reversing polygon winding, so it is not always the best approach.
As for "drawing points ahead of the cursor," I think you may be describing input latency (particularly if you are using VSYNC).
When you use a software cursor vs. hardware cursor the position of the mouse may be off by one or more frames. There is a somewhat technical discussion here (see: Idiosyncrasies) on the effect of buffer swap intervals (OpenGL's mechanism for VSYNC) on input latency.

Answer 3

To fix the inversion do 600 - y for setting arr element. Also your gluOrtho2D is on a scale of 400 by 400 while your windows is 600 by 600 so change your gluOrtho2D scale to the same as the window size and you'll be fine.