This program draws points&lines placed with mouse clicks.

lines.cpp

/* 
* This program draws points placed with mouse clicks.
* Maximum number of points allowed is currently set at 16.
*
* Usage:
* Left click to place a control point.
* Press escape to exit.
*/
#include <stdlib.h>
#include <GL/glut.h>
#include <stdio.h>
#include <math.h>
#define MaxNumPts 16
float PointArray[MaxNumPts][2];
int NumPts = 0;
// Function prototypes
void myKeyboardFunc( unsigned char key, int x, int y );
void myMouseFunc( int button, int state, int x, int y );
void displayPoints(void);
void addNewPoint( float x, float y );
void initRendering();
void resizeWindow(int w, int h);
// Window size in pixels
int WindowHeight;
int WindowWidth;


// Remove the first point in the list if too many points.
void removeFirstPoint() 
{
	int i;
	if ( NumPts>0 ) 
	{
		// Remove the first point, slide the rest down
		NumPts--;
		for ( i=0; i<NumPts; i++ ) 
		{
			PointArray[i][0] = PointArray[i+1][0];
			PointArray[i][1] = PointArray[i+1][1];
		}
	}
}

void removeLastPoint() 
{
	if ( NumPts>0 ) 
	{
		// Remove the first point, slide the rest down
		NumPts--;
	}
}



void myKeyboardFunc (unsigned char key, int x, int y)
{
	 switch (key) {
		case 27: // Escape key
		exit(0);
		break;
		case 70:
		case 102:
			removeFirstPoint();
			glutPostRedisplay();
			break;
		case 76:
		case 108:
			removeLastPoint();
			glutPostRedisplay();
			break;
	 }
}
// Left button presses place a control point.
void myMouseFunc( int button, int state, int x, int y )
{
	if ( button==GLUT_LEFT_BUTTON && state==GLUT_DOWN ) 
	{
		float xPos = ((float)x)/((float)(WindowWidth-1));
		float yPos = ((float)y)/((float)(WindowHeight-1));
		yPos = 1.0f-yPos; // Flip value since y position is from top row.
		addNewPoint( xPos, yPos );
		glutPostRedisplay();
	}
}

// Add a new point to the end of the list.
// Remove the first point in the list if too many points.
void addNewPoint( float x, float y ) 
{
	if ( NumPts>=MaxNumPts ) 
	{
		removeFirstPoint();
	}
	PointArray[NumPts][0] = x;
	PointArray[NumPts][1] = y;
	NumPts++;
}

void displayPoints(void)
{
	 int i;
	 glClear(GL_COLOR_BUFFER_BIT);
	 glColor4f( 1.0f, 0.0f, 0.0f, 1.0f );
	 // Draw the points
	 if ( NumPts>0 ) 
	 {
		glBegin( GL_POINTS);
		for ( i=0; i<NumPts; i++ ) 
		{
		   glVertex2f( PointArray[i][0], PointArray[i][1] );
		}
		glEnd();

		glBegin( GL_LINES);
		
			glColor3f(0,0,1);
		for ( i=0; i<NumPts-1; i++ ) 
		{
		   glVertex2f( PointArray[i][0], PointArray[i][1] );
		   glVertex2f( PointArray[i+1][0], PointArray[i+1][1] );
		}
		glEnd();

	}
	glFlush();
}
void initRendering() {
	glClearColor( 1.0f, 1.0f, 1.0f, 1.0f );
	// Make big points and wide lines. (Can comment out if desired.)
	glPointSize(8);
	//glLineWidth(5);
	// The following commands should make OpenGL create round points and
	// antialias points and lines. (This is implementation dependent
	// unfortunately, and
	// may slow down rendering considerably.)
	// You may comment these out if you wish.
	glEnable(GL_POINT_SMOOTH); // Make round points, not square points
	glEnable(GL_LINE_SMOOTH);
	glHint(GL_POINT_SMOOTH_HINT, GL_NICEST);
	glHint(GL_LINE_SMOOTH_HINT, GL_NICEST); // Antialias the lines
	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
}
void resizeWindow(int w, int h)
{
	WindowHeight = (h>1) ? h : 2;
	WindowWidth = (w>1) ? w : 2;
	glViewport(0, 0, (GLsizei) w, (GLsizei) h);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluOrtho2D(0.0f, 1.0f, 0.0f, 1.0f); // Always view [0,1]x[0,1].
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
}


int main(int argc, char** argv)
{
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB );
	glutInitWindowSize(500, 500);
	glutInitWindowPosition(100, 100);
	glutCreateWindow(argv[0]);
	initRendering();
	glutDisplayFunc(displayPoints);
	glutReshapeFunc(resizeWindow);
	glutKeyboardFunc(myKeyboardFunc);
	glutMouseFunc(myMouseFunc);
	glutMainLoop();
	return 0; // This line is never reached
}