Stretching a Line

[ComputerUser43]

Hi.

1. I'm creating a program where the user creates a line.

2. The user clicks on the first point and then drags the mouse to the next point.

3. There is a line from the first point to wherever the mouse moves to, sort of like dragging a piece of string.

4. When the user clicks on the second point the line is created and it can't be moved around.

5. I'm having trouble getting a line from the first point to the end of the mouse.

6. I used the glutMotionFunc but that didn't do anything.

7. Can anybody help me out?

Thanks

[nexusone]

It is called a rubber band effect, a search on that should bring something up.

Also here is a piece of code showing how to the effect in glut.
I have another version of this code that does just a line, but don't have it handy.

// GlutDrawWindow.c
// By Eric Stringer 2002
// Simple examples of OpenGL and Glut usage.
// Keyboard input
// 'v' = view ortho/perspective
// 'l' = lighting on/off
 
 
#include <windows.h>  // This header file will be needed for some windows compilers
//#include <GL/gl.h>   // gl.h and glu.h also maybe needed for some compilers
//#include <GL/glu.h>
#include <GL/glut.h>  // glut (gl utility toolkit) basic windows functions, keyboard, mouse.
#include <stdio.h>    // standard (I/O library)
#include <stdlib.h>   // standard library (set of standard C functions
#include <math.h>     // Math library (Higher math functions )
 
int window_1, window_2;
 
// lighting
GLfloat LightAmbient[]= { 0.2f, 0.2f, 0.2f, 1.0f };
GLfloat LightDiffuse[]= { 0.2f, 0.2f, 0.2f, 1.0f };
GLfloat LightPosition[]= { 5.0f, 25.0f, 15.0f, 1.0f };
GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 };
 
typedef struct POINT_3D
{
GLfloat x, y ,z;
}v3D;
 
// Define temp line variables
int line_start;
 
POINT_3D line_temp[2];
 
// Storage for our lines
POINT_3D line_array[100];
static int line_index = 0;
 
 
static int view_state = 0, light_state = 0;
 
static float Mouse_x, Mouse_y, Win_x, Win_y;
 
int spin;
 
// I use this to put text on the screen
void Sprint( int x, int y, char *st)
{
	int l,i;
 
	l=strlen( st ); // see how many characters are in text string.
	glRasterPos2i( x, y); // location to start printing text
	for( i=0; i < l; i++)  // loop until i is greater then l
		{
		glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, st[i]); // Print a character on the screen
	}
 
}
 
 
// This creates the spinning of the cube.
static void TimeEvent(int te)
{
 
    spin++;  // increase cube rotation by 1
	if (spin > 360) spin = 0; // if over 360 degress, start back at zero.
    glutSetWindow(window_1);
	glutPostRedisplay();  // Update screen with new rotation data
    glutSetWindow(window_2);
    glutPostRedisplay();
	glutTimerFunc( 100, TimeEvent, 1);  // Reset our timmer.
}
 
 
// Setup our Opengl world, called once at startup.
void init(void)
{
   glClearColor (0.0, 0.0, 0.0, 0.0);  // When screen cleared, use black.
   glShadeModel (GL_SMOOTH);  // How the object color will be rendered smooth or flat
   glEnable(GL_DEPTH_TEST);   // Check depth when rendering
   // Lighting is added to scene
   glLightfv(GL_LIGHT1 ,GL_AMBIENT, LightAmbient);
   glLightfv(GL_LIGHT1 ,GL_DIFFUSE, LightDiffuse);
   glLightfv(GL_LIGHT1 ,GL_POSITION, LightPosition);
   glEnable(GL_LIGHTING);  // Turn on lighting
   glEnable(GL_LIGHT1);    // Turn on light 1
}
 
// Draw our world
void display_1(void)
{
int i;
 
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);  //Clear the screen
 
glMatrixMode (GL_PROJECTION);  // Tell opengl that we are doing project matrix work
glLoadIdentity();  // Clear the matrix
glOrtho(-8.0, 8.0, -8.0, 8.0, 0.0, 30.0);  // Setup an Ortho view
glMatrixMode(GL_MODELVIEW);  // Tell opengl that we are doing model matrix work. (drawing)
glLoadIdentity(); // Clear the model matrix
// print view state on screen
glColor3f( 1.0, 1.0, 1.0);
Sprint(-2, 4, "Ortho view");
 
 
// Lighting on/off
if (light_state == 1)
	{
	glDisable(GL_LIGHTING);
    glDisable(GL_COLOR_MATERIAL);
    }else
	{
    glEnable(GL_LIGHTING);
    glEnable(GL_COLOR_MATERIAL);
    }
 
if (line_start == 1)
    {
    glColor3f( 1.0, 0.0, 0.0);  // Line Color
    //glBegin(GL_LINES);
    //glVertex2f(line_temp[0].x, line_temp[0].y);
    //glVertex2f(line_temp[1].x, line_temp[1].y);
    //glEnd();
    glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
    glLineWidth( 5.0 );
    glBegin(GL_QUADS);
    glVertex2f(line_temp[0].x, line_temp[0].y);
    glVertex2f(line_temp[1].x, line_temp[0].y);
    glVertex2f(line_temp[1].x, line_temp[1].y);
    glVertex2f(line_temp[0].x, line_temp[1].y);
    glEnd();
 
    glBegin(GL_TRIANGLES);
    glVertex2f(line_temp[0].x + fabs(line_temp[0].x - line_temp[1].x)* 0.5, line_temp[0].y);
    //glVertex2f(line_temp[1].x, line_temp[0].y);
    glVertex2f(line_temp[1].x, line_temp[1].y);
    glVertex2f(line_temp[0].x, line_temp[1].y);
    glEnd();
 
    glEnd();
    }
 
// Process lines drawn
for(i = 0; i < line_index; i+=2)
   {
    glColor3f(1.0, 1.0, 1.0);
    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
    glLineWidth( 1.0 );
    //glBegin(GL_LINES);
    //glVertex2f(line_array[i].x, line_array[i].y);
    //glVertex2f(line_array[i+1].x, line_array[i+1].y);
    //glEnd();
    //glBegin(GL_QUADS);
    //glVertex2f(line_array[i].x, line_array[i].y);
    //glVertex2f(line_array[i+1].x, line_array[i].y);
    //glVertex2f(line_array[i+1].x, line_array[i+1].y);
    //glVertex2f(line_array[i].x, line_array[i+1].y);
    //glEnd();
 
    glBegin(GL_TRIANGLES);
    glVertex2f(line_array[i].x + fabs(line_array[i].x - line_array[i+1].x) * 0.5, line_array[i].y);
    //glVertex2f(line_array[1].x, array_array[0].y);
    glVertex2f(line_array[i+1].x, line_array[i+1].y);
    glVertex2f(line_array[i].x, line_array[i+1].y);
    glEnd();
 
    }
 
glutSwapBuffers();
}
 
// This is called when the window has been resized.
void reshape_1 (int w, int h)
{
   Win_x = w;
   Win_y = h;
   glViewport (0, 0, (GLsizei) w, (GLsizei) h);
   glMatrixMode (GL_PROJECTION);
   glLoadIdentity ();
}
 
// Read the keyboard
void keyboard (unsigned char key, int x, int y)
{
   switch (key)
   {
 
	  case 'v':
	  case 'V':
		  view_state = abs(view_state -1);
		  break;
	  case 'l':
	  case 'L':
		  light_state = abs(light_state -1);
		  break;
 	  case 27:
         exit(0); // exit program when [ESC] key presseed
         break;
      default:
         break;
   }
 
 
}
 
void mouse(int button, int state, int x, int y)
{
 
Mouse_x = -8 + 16 * (x/Win_x);
Mouse_y = 8 - 16 * (y/Win_y);
 
 
if ((button == GLUT_LEFT_BUTTON) &amp;&amp; (state == GLUT_DOWN))
{
line_start = 1;
line_temp[0].x = Mouse_x;
line_temp[0].y = Mouse_y;
}
 
if ((button == GLUT_LEFT_BUTTON) &amp;&amp; (state == GLUT_UP))
   {
   line_start = 0;
   line_temp[1].x = Mouse_x;
   line_temp[1].y = Mouse_y;
   line_array[line_index] = line_temp[0];
   line_array[line_index+1] = line_temp[1];
   line_index += 2;
}
 
}
 
 
void mouse_motion( int x, int y)
{
 
Mouse_x = -8 + 16 * (x/Win_x);
Mouse_y = 8 - 16 * (y/Win_y);
if (line_start == 1)
   {
   line_temp[1].x = Mouse_x;
   line_temp[1].y = Mouse_y;
   }
}
 
 
// Main program
int main(int argc, char** argv)
{
   glutInit(&amp;argc, argv);
   glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
   glutTimerFunc( 10, TimeEvent, 1);
   glutInitWindowSize (500, 500);
   glutInitWindowPosition (10, 10);
   window_1 = glutCreateWindow (argv[0]);
   glutSetWindowTitle("GlutDraw");
   glutDisplayFunc(display_1);
   glutReshapeFunc(reshape_1);
   glutKeyboardFunc(keyboard);
   glutMouseFunc(mouse);
   glutMotionFunc(mouse_motion);
   init ();
 
   glutMainLoop();
   return 0;
}

[This message has been edited by nexusone (edited 06-15-2003).]

[ComputerUser43]

Hi nexusone

1. Thanks for the reply. I've got one question for you.

2. I had inserted the rubberbanding code in the function I pass to glutPassiveMotionFunc(). Does that code belong in the Display() function?

Thanks.
Raja

[nexusone]

I am not sure what you mean in your question #2.

But look at how my code works:

I use state variables to know when to draw the rubber band effect.

The state variable is set in the mouse routines, I use both mouse routines, mousemotion is only used to durring the rubber band effect to get current mouse position.
The other mouse routine, I get the the start and end of line.


Now in the display function is all drawing code with variables passed from the mouse functions.

Originally posted by ComputerUser43:
Hi nexusone

1. Thanks for the reply. I've got one question for you.

2. I had inserted the rubberbanding code in the function I pass to glutPassiveMotionFunc(). Does that code belong in the Display() function?

Thanks.
Raja

[ComputerUser43]

Hi nexusone.

Even if you were unsure of my question you still answered it.

The rubberbanding works great now.

Thanks.
Raja