Question

draw polyline with mouse interactive

Asked by: valleytech

please help me on drawing polyline with mouse click interactive on window. whenever a new click, a new line will be drawn from previous click to new one. Thank a lot.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
 
#include <GL/glut.h>
 
 
  int ViewportXFirst  , ViewportYFirst ;
  int ViewportXSecond , ViewportYSecond;
 
  int flag =0; // count 2 clicks at a time.
               // flag = 0 : first time. flag =1 : second time
 
 
 
void init (void)
{
      glClearColor (1.0,1.0,1.0,0.0);
 
      glMatrixMode( GL_PROJECTION);
      gluOrtho2D(0.0,200.0,0.0,150.0);
 
	   glMatrixMode(GL_MODELVIEW);
}
 
void lineSegment (void)
{
      glClear (GL_COLOR_BUFFER_BIT);
 
      glColor3f(1.0,0.0,0.0);
      glBegin(GL_LINE_STRIP);
            glVertex2i(ViewportXFirst,ViewportYFirst);
            glVertex2i(ViewportXSecond,ViewportYSecond);
      glEnd();
      
      glFlush();
}
 
void processMouse(int button, int state, int x, int y) {
 
      if (state == GLUT_DOWN)
      {      
            if(flag ==0)
            {
                  printf("flag is %d\n",flag);
                  ViewportXFirst = x;
                  ViewportYFirst = 512- y;
 
                  flag ++;
            }
            else
            {
				if(flag ==1)
				{
                  printf("flag is %d\n",flag);
 
                  ViewportXSecond = x;
                  ViewportYSecond = 512- y;
				}
				else
				{
					ViewportXFirst = ViewportXSecond;  // swap second point to first point 
					ViewportYFirst = ViewportYSecond;  //before obtain new point to second point
 
					ViewportXSecond = x;
					ViewportYSecond = 512- y;
 
				}
				flag++;
                  
            }
 
		
				printf("ViewportXFirst is %d   ",ViewportXFirst);
				printf("ViewportYFirst is %d",ViewportYFirst);
				printf("\n");
 
				printf("ViewportXSecond is %d   ",ViewportXSecond);
				printf("ViewportYSecond is %d",ViewportYSecond);
				printf("\n"); 
      }
	
	  if(flag >=2)
	  {
		glutDisplayFunc(lineSegment);
 
	  }
 
  
}
 
void main(int argc, char ** argv)
{
      glutInit(&argc, argv);
      glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
      glutInitWindowPosition(50,100);
      glutInitWindowSize(512,512);
      glutCreateWindow("An example openGL sample");
 
      init();
     
 
 
      //adding here the mouse processing callbacks
      glutMouseFunc(processMouse);
 
	  glutDisplayFunc(lineSegment);
	  
      
      //glutMotionFunc(processMouseActiveMotion);
      //glutPassiveMotionFunc(processMousePassiveMotion);
      //glutEntryFunc(processMouseEntry);
 
      glutMainLoop();
}

                                  
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Asked On
2009-08-28 at 09:29:10ID24690352
Topic

OpenGL Graphics & Game Programming

Participating Experts
1
Points
500
Comments
23

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Answers

 

by: ikeworkPosted on 2009-08-28 at 10:19:57ID: 25209723

Hey valleytech,

What you will need is a container for the line points. I suggest you make a structure for the points, that stores the x- and y-component.

struct point
{
    int x;
    int y;
};

Then you need an array, or better a vector (it has a dynamic size), which stores the points that the user clicked at:

#include <vector>
// global vector of all the points, that the user clicked
std::vector<point> points;

When the user clicks a point, just add it to your vector:

point p;
p.x = ...
p.y = ...
points.push_back(p);

Then in your render-func you iterate through the vector and render the lines

glBegin(GL_LINE_STRIP);
    for(int i = 0; i < points.size(); ++i)
    {
        glVertex2i(points[i].x, points[i].y);
    }
glEnd();

You have to setup the viewport with glOrtho to the same size as the window is, then your coordinates directly map to a point on the screen.

Put that into your code and if you encounter problems feel free to ask :)


ike

 

by: valleytechPosted on 2009-08-28 at 13:05:24ID: 25211167

let me try to implement your guide. Thanks.

 

by: valleytechPosted on 2009-08-28 at 13:15:26ID: 25211221

i just wonder

glBegin(GL_LINE_STRIP);
    for(int i = 0; i < points.size(); ++i)
    {
        glVertex2i(points[i].x, points[i].y);
    }
glEnd();

--> so each time when user clicks a new point, do i have to start to render from beginning? can u just render a line between last click point and new one? I think of this issue because just in case we have one object with such as 500000points, and whenever it move in a game, we have to render again and again some many points?

 

by: ikeworkPosted on 2009-08-29 at 02:23:17ID: 25213460

>> so each time when user clicks a new point, do i have to start to render from beginning?
>> and whenever it move in a game, we have to render again and again some many points?

Yes, that is how it is done in games. Actually you try to render as often as you can, to get a high frame-rate. Usually the frame rate in games goes from 20 frames per second (fps) up to 60fps.

But you dont need such a high frame rate in your app, because the data in your case can only change, when the user presses a button.

However, you should render the scene again, whenever the window changes, the position or the size.

You can pass a callback-function to glut for that case. Such a callback function looks like this:

void resize(int width, int height)
{
    glViewport( 0, 0, (GLint)width, (GLint)height );

    glMatrixMode( GL_PROJECTION);
    gluOrtho2D(0.0, width,0.0, height);
 
    glMatrixMode(GL_MODELVIEW);
           
}

In main you have to pass the callback to glut like this:

  glutReshapeFunc(resize);


When ever the user clicks on the window and adds a new point, you must call glutPostRedisplay() to render the scene again.

  glutPostRedisplay();
 

 

by: valleytechPosted on 2009-08-29 at 12:19:18ID: 25215004

let me try to code it now. I will get back when i have bugs. Thanks.

 

by: valleytechPosted on 2009-08-29 at 13:10:31ID: 25215182

i guess i have bug at lineSegment because i succeeded to display all points user have clicked. however, when i enable lineSegment, the program is closed. Thanks for your help.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
 
#include <GL/glut.h>
#include <vector>
 
struct point
{
	int x;
	int y;
};
 
	std::vector <point> points;
	point OnePoint;  
 
 
void init (void)
{
      glClearColor (1.0,1.0,1.0,0.0);
 
      glMatrixMode( GL_PROJECTION);
      gluOrtho2D(0.0,200.0,0.0,150.0);
 
	  glMatrixMode(GL_MODELVIEW);
}
 
void lineSegment (void)
{
      glClear (GL_COLOR_BUFFER_BIT);
 
      glColor3f(1.0,0.0,0.0);
 
	   glutPostRedisplay();
 
      glBegin(GL_LINE_STRIP);
		for(int i =0; i <= points.size();i++)
		{
			glVertex2i(points[i].x, points[i].y);
		}
           
      glEnd();
      
      glFlush();
}
 
void processMouse(int button, int state, int x, int y) {
 
      if (state == GLUT_DOWN)
      {      
		  OnePoint.x= x;
		  OnePoint.y= y; 
		  points.push_back(OnePoint);
 
		  for (int i =0; i <= points.size() -1; i++)
		  {
			  printf("OnePoint is %d  %d\n",points[i].x, points[i].y);
		  }
		  printf("*****\n");
 
      }
}
 
void resize(int width, int height)
{
    glViewport( 0, 0, (GLint)width, (GLint)height );
 
    glMatrixMode( GL_PROJECTION);
    gluOrtho2D(0.0, width,0.0, height);
 
    glMatrixMode(GL_MODELVIEW);
           
}
 
void main(int argc, char ** argv)
{
      glutInit(&argc, argv);
      glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
      glutInitWindowPosition(50,100);
      glutInitWindowSize(512,512);
      glutCreateWindow("Drawing polyline with mouse interactive");
 
      init();
     
 
      //adding here the mouse processing callbacks
      glutMouseFunc(processMouse);
 
	  glutDisplayFunc(lineSegment);
 
	  glutReshapeFunc(resize);
	  
      
      //glutMotionFunc(processMouseActiveMotion);
      //glutPassiveMotionFunc(processMousePassiveMotion);
      //glutEntryFunc(processMouseEntry);
 
      glutMainLoop();
}
 
  
                                              
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by: ikeworkPosted on 2009-08-29 at 15:14:46ID: 25215599

glutPostRedisplay() calls lineSegment. Dons put it into line_segement, otherwise you hane an infinite loop.

 

by: ikeworkPosted on 2009-08-29 at 15:17:19ID: 25215610

glutPostRedisplay() must be in processMouse, because after the mouse was pressed, the scene must be rendered again.

 

by: valleytechPosted on 2009-08-29 at 15:23:59ID: 25215645

glutPostRedisplay() doesn't have any parameter. how can i invoke lineSegment from it?

 

by: ikeworkPosted on 2009-08-30 at 02:15:06ID: 25216872

glutPostRedisplay() calls the function that you passed to glutDisplayFunc, which is lineSegment. So just call glutPostRedisplay, then line_segment will be called from glut.

I changed your code below. I removed glutPostRedisplay from line_segment and added it in processMouse, when a new point was added:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
 
#include <GL/glut.h>
#include <vector>
 
struct point
{
        int x;
        int y;
};
 
        std::vector <point> points;
        point OnePoint;  
 
 
void init (void)
{
      glClearColor (1.0,1.0,1.0,0.0);
 
      glMatrixMode( GL_PROJECTION);
      gluOrtho2D(0.0,200.0,0.0,150.0);
 
          glMatrixMode(GL_MODELVIEW);
}
 
void lineSegment (void)
{
      glClear (GL_COLOR_BUFFER_BIT);
 
      glColor3f(1.0,0.0,0.0);           
 
      glBegin(GL_LINE_STRIP);
                for(int i =0; i <= points.size();i++)
                {
                        glVertex2i(points[i].x, points[i].y);
                }
           
      glEnd();
      
      glFlush();
}
 
void processMouse(int button, int state, int x, int y) {
 
      if (state == GLUT_DOWN)
      {      
                  OnePoint.x= x;
                  OnePoint.y= y; 
                  points.push_back(OnePoint);
 
                  for (int i =0; i <= points.size() -1; i++)
                  {
                          printf("OnePoint is %d  %d\n",points[i].x, points[i].y);
                  }
                  printf("*****\n");
 
                  glutPostRedisplay();  // << has to be here to render the scene with the new data
      }
}
 
void resize(int width, int height)
{
    glViewport( 0, 0, (GLint)width, (GLint)height );
 
    glMatrixMode( GL_PROJECTION);
    gluOrtho2D(0.0, width,0.0, height);
 
    glMatrixMode(GL_MODELVIEW);
           
}
 
void main(int argc, char ** argv)
{
      glutInit(&argc, argv);
      glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
      glutInitWindowPosition(50,100);
      glutInitWindowSize(512,512);
      glutCreateWindow("Drawing polyline with mouse interactive");
 
      init();
     
 
      //adding here the mouse processing callbacks
      glutMouseFunc(processMouse);
 
          glutDisplayFunc(lineSegment);
 
          glutReshapeFunc(resize);
          
      
      //glutMotionFunc(processMouseActiveMotion);
      //glutPassiveMotionFunc(processMousePassiveMotion);
      //glutEntryFunc(processMouseEntry);
 
      glutMainLoop();
}
 
  

                                              
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by: valleytechPosted on 2009-08-30 at 08:47:24ID: 25218037

it still has bugs in my system. Compile and build has no error. however, when i run, a windows pop up immediately " cpp1.exe has encountered a problem and needs to close. We are sorry for the inconvenience" . I don't know why.
PS: i am using visual c++ 6.0 on windows xp.

 

by: ikeworkPosted on 2009-08-30 at 08:56:57ID: 25218080

Ok, I'm gonna try to run the code here. If you made changes, please post the code again.

Is it an assertation? Is there a button "Retry"?

 

by: ikeworkPosted on 2009-08-30 at 09:00:29ID: 25218088

Ah ok, I see what it is, you loop behind the array, you have:

  for(int i =0; i <= points.size();i++)

but it must be "<" not "<="

Change the line:

  for(int i =0; i < points.size();i++)

Thats it. By the way, that was an assertation, if you press "Retry" in that window, then you can see which line caused the error.

 

by: valleytechPosted on 2009-08-30 at 09:06:20ID: 25218111

for(int i =0; i <= points.size();i++) should be
       for(int i =0; i <= points.size() -1 ;i++)
or    for(int i =0; i < points.size();i++)

I forget it.
It can run now. but i can't see the line drawing.

 

by: ikeworkPosted on 2009-08-30 at 09:18:17ID: 25218174

Right, I had the same result here. You need to clear the Depth-Buffer too in line_segment:

Replace this:

  glClear (GL_COLOR_BUFFER_BIT);

with:

  glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

Then you will find another issue. The windows y- coordinates in process-mouse start at window top with 0 and in window bottom is max y. You told opengl the opposite in glOrtho2D

So either replace:

  gluOrtho2D(0.0, width, 0.0, height);

with:

  gluOrtho2D(0.0, width, height, 0.0);
 
or calculate the proper y from the process_mouse parameter

 

by: ikeworkPosted on 2009-08-30 at 09:27:16ID: 25218207

Btw. Why did you remove th "glLoadIdentity()" in line_segement as shown in your previous q:

http://www.experts-exchange.com/Programming/Game/Game_Graphics/OpenGL/Q_24689072.html

You should put it back, it is important to load the identity-matrix into current matrix in matrix-stack before rendering.

 

by: valleytechPosted on 2009-08-30 at 09:28:46ID: 25218210

hi ike,
 i found another way to solve the problem. keep those
     glClear (GL_COLOR_BUFFER_BIT);
and gluOrtho2D(0.0, width, 0.0, height);

 However,  in inti()  comment
        gluOrtho2D(0.0,200.0,0.0,150.0);

 Then it can run now.

   Let my try your way.

 

by: ikeworkPosted on 2009-08-30 at 09:31:33ID: 25218223

I dont understand, can you post that code please?

 

by: valleytechPosted on 2009-08-30 at 09:32:51ID: 25218233

here it is.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
 
#include <GL/glut.h>
#include <vector>
 
struct point
{
        int x;
        int y;
};
 
        std::vector <point> points;
        point OnePoint;  
 
void init (void)
{
      glClearColor (1.0,1.0,1.0,0.0);
 
      glMatrixMode( GL_PROJECTION);
  //    gluOrtho2D(0.0,200.0,0.0,150.0);
 
      glMatrixMode(GL_MODELVIEW);
}
 
void lineSegment (void)
{
      glClear (GL_COLOR_BUFFER_BIT);
	  //glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
      glColor3f(1.0,0.0,0.0);           
 
      glBegin(GL_LINE_STRIP);
                for(int i =0; i < points.size();i++)
                {
                        glVertex2i(points[i].x, points[i].y);
                }
           
      glEnd();
      
      glFlush();
}
 
void processMouse(int button, int state, int x, int y) {
 
      if (state == GLUT_DOWN)
      {      
                  OnePoint.x= x;
                  OnePoint.y= y; 
                  points.push_back(OnePoint);
 
                  for (int i =0; i <= points.size() -1; i++)
                  {
                          printf("OnePoint is %d  %d\n",points[i].x, points[i].y);
                  }
                  printf("*****\n");
 
				  
				  glutPostRedisplay();  // << has to be here to render the scene with the new data
			
	  }
}
 
void resize(int width, int height)
{
    glViewport( 0, 0, (GLint)width, (GLint)height );
 
    glMatrixMode( GL_PROJECTION);
    
	//gluOrtho2D(0.0, width,0.0, height);
 
	gluOrtho2D(0.0, width, height, 0.0);
 
    glMatrixMode(GL_MODELVIEW);
           
}
 
void main(int argc, char ** argv)
{
      glutInit(&argc, argv);
      glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
      glutInitWindowPosition(50,100);
      glutInitWindowSize(512,512);
      glutCreateWindow("Drawing polyline with mouse interactive");
 
      init();
     
 
      //adding here the mouse processing callbacks
      glutMouseFunc(processMouse);
 
      glutDisplayFunc(lineSegment);
 
      glutReshapeFunc(resize);
          
      
      //glutMotionFunc(processMouseActiveMotion);
      //glutPassiveMotionFunc(processMousePassiveMotion);
      //glutEntryFunc(processMouseEntry);
 
      glutMainLoop();
}
 

                                              
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by: ikeworkPosted on 2009-08-30 at 09:36:41ID: 25218252

Yes, that works too, actually you can remove everything from init, but the glClearColor, since resize is called on startup, it is all done there.

So you just need:

void init (void)
{
      glClearColor (1.0,1.0,1.0,0.0);
}

About the Depth-Buffer. Do you plan to render more things behind and before the lines? If yes, you should clear the Depth-Buffer each time in line_segment, otherwise that wont work correctly.

 

by: ikeworkPosted on 2009-08-30 at 09:49:14ID: 25218289

So the problem is solved, why dont you close the question?

 

by: valleytechPosted on 2009-08-30 at 09:50:22ID: 25218293

I really like your help. It help me to understand a lot for more advanced topic later on. Let me change it  the Depth-buffer for future reference.

I plan to update this program to make it flexible a little bit more in next question :)

Before i close this, i just want to clarify one thing. The way how openGL works in  program:
     Mouse click -> glutMOuseFun -> glutPostRedisplay-> glutDisplayFunc
Am I right?  

 

by: ikeworkPosted on 2009-08-30 at 09:55:00ID: 25218309

Almost, more precisely its:

Mouse click -> process_mouse -> glutPostRedisplay -> line_segment

20120131-EE-VQP-002

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