Display Lists, Menus, and Picking

Tom Kelliher, CS 320

Feb. 23, 2011

Administrivia

Announcements

Assignment

Read polygon.c, Section 3.9.

From Last Time

Input devices and interaction introduction.

Outline

  1. Display lists.

  2. Menus.

  3. Pick selection.

Coming Up

polygon.c

Display Lists, Menus, and Picking

Display lists and distributed computing.

Text Display

Idea:

  1. Specify starting location of text (world coordinates).

  2. Start writing, specifying font and character.

Example: 

void renderString(GLdouble x, GLdouble y, void *font, char *text)
{
   glRasterPos2d(x, y);

   while (text)
   {
      glutBitmapCharacter(font, *text);
      ++text;
   }
}
See man page for glutBitmapCharacter for list of available bitmap fonts. Example: 
renderString(0.0, 0.0, GLUT_BITMAP_9_BY_15, "OpenGL rocks!");

Idea similar to font cache: display lists. Program example: 

base = glGenLists(128);

for(i=0;i<128;i++)
{
   glNewList(base+i, GL_COMPILE);
   glutBitmapCharacter(GLUT_BITMAP_9_BY_15, i);
   glEndList();
}

glListBase(base);
Use: 
// Dump time string into out.

glRasterPos2i(ww-80, wh-15);  // Window units = world units.
glColor3f(0.0,0.0,0.0);
glBegin(GL_QUADS);            // Erase current time.
   glVertex2i(ww-80, wh-15);
   glVertex2i(ww, wh-15);
   glVertex2i(ww, wh);
   glVertex2i(ww-80, wh);
glEnd();
glColor3f(1.0,1.0,1.0);
glCallLists(strlen(out), GL_BYTE, out);
Another example: 
list = glGenLists(1)

glNewList(list, GL_COMPILE);
   glBegin(GL_POLYGON);
      glVertex2f(...);
      ...
   glEnd();
glEndList();

...

glCallList(list);

Menus and Sub-Menus

Why do we use menus? -- What's the alternative? 

int main(int argc, char** argv)
{
   int c_menu;

   /* ... */

   c_menu = glutCreateMenu(color_menu);
   glutAddMenuEntry("Black",0);
   glutAddMenuEntry("Red",1);
   glutAddMenuEntry("Green",2);
   glutAddMenuEntry("Blue",3);
   glutAddMenuEntry("Cyan",4);
   glutAddMenuEntry("Magenta",5);
   glutAddMenuEntry("Yellow",6);
   glutAddMenuEntry("White",7);

   glutCreateMenu(main_menu);
   glutAddMenuEntry("new polygon", 1);
   glutAddMenuEntry("end polygon", 2);
   glutAddMenuEntry("delete polygon", 3);
   glutAddMenuEntry("move polygon", 4);
   glutAddMenuEntry("quit",5);
   glutAddSubMenu("Colors", c_menu);
   glutAttachMenu(GLUT_MIDDLE_BUTTON);

   /* ... */
}

Polygon representation: 

typedef struct polygon
{ 
   int color; /* color index */
   bool used; /* TRUE if polygon exists */
   int xmin, xmax, ymin, ymax; /* bounding box */
   float xc, yc; /* center of polygon */
   int nvertices; /* number of vertices */
   int x[MAX_VERTICES]; /* vertices */
   int y[MAX_VERTICES];
} polygon;

Setting a polygon's color: 

GLfloat colors[8][3]={{0.0, 0.0, 0.0}, {1.0, 0.0, 0.0}, {0.0, 1.0, 0.0},
                      {0.0, 0.0, 1.0}, {0.0, 1.0, 1.0}, {1.0, 0.0, 1.0},
                      {1.0, 1.0, 0.0}, {1.0, 1.0, 1.0}};

void color_menu(int index)
{
   present_color =  index;
   if(in_polygon>=0) polygons[in_polygon].color = index;
}

Rendering polygons: 

void myDisplay()
{

   /* display all active polygons */

   int i, j;

   glClear(GL_COLOR_BUFFER_BIT);
   for(i=0; i<MAX_POLYGONS; i++)
   {
      if(polygons[i].used)
      {
         glColor3fv(colors[polygons[i].color]);
         glBegin(GL_POLYGON);
         for(j=0; j<polygons[i].nvertices; j++)
            glVertex2i(polygons[i].x[j], polygons[i].y[j]);
         glEnd();
      }
   }
   glFlush();
}

Coordinate systems

Recall:

  1. Rendering origin for OpenGL.

  2. Window and mouse coordinate origin for the window system.

  3. Translating.

Pick Determination and Drawing States

What's the problem here?

  1. Mouse function called into play here.

  2. Note that middle button taken out of action. 

int pick_polygon(int x, int y)
{

   /* find first polygon in which we are in bounding box */

   int i;

   for(i=0; i<MAX_POLYGONS; i++)
   {
      if(polygons[i].used)
         if((x>=polygons[i].xmin) && (x<=polygons[i].xmax) &&
            (y>=polygons[i].ymin)&&(y<polygons[i].ymax))
         {
            in_polygon = i;
            moving = TRUE;
            return(i);
         }
      printf("not in a polygon\n");
      return(-1);
   }
}


void main_menu(int index)
{
   int i;
   switch(index)
   {
   case(1):  /* create a new polygon */
      {

         moving = FALSE;
   		for(i=0; i<MAX_POLYGONS; i++) if(polygons[i].used == FALSE) break;
    		if(i == MAX_POLYGONS)
    		{
            printf("exceeeded maximum number of polygons\n");
            exit(0);
    		}
         polygons[i].color = present_color;
         polygons[i].used = TRUE;
    		polygons[i].nvertices = 0;
    		in_polygon = i;
         picking = FALSE;
         break;
     	}
	case(2):   /* end polygon and find bounding box and center */
      {
         moving = FALSE;
         if(in_polygon>=0)
         {
            polygons[in_polygon].xmax =
               polygons[in_polygon].xmin =
               polygons[in_polygon].x[0];

            polygons[in_polygon].ymax =
               polygons[in_polygon].ymin =
               polygons[in_polygon].y[0];

            polygons[in_polygon].xc = polygons[in_polygon].x[0];
            polygons[in_polygon].yc = polygons[in_polygon].y[0];

            for(i=1;i<polygons[in_polygon].nvertices;i++)
            {
               if(polygons[in_polygon].x[i]<polygons[in_polygon].xmin)
                  polygons[in_polygon].xmin =
                     polygons[in_polygon].x[i];
               else if(polygons[in_polygon].x[i] >
                       polygons[in_polygon].xmax)
                  polygons[in_polygon].xmax =
                     polygons[in_polygon].x[i];

               if(polygons[in_polygon].y[i]<polygons[in_polygon].ymin)
                  polygons[in_polygon].ymin =
                     polygons[in_polygon].y[i];
               else if(polygons[in_polygon].y[i] >
                       polygons[in_polygon].ymax)
                  polygons[in_polygon].ymax =
                     polygons[in_polygon].y[i];

               polygons[in_polygon].xc += polygons[in_polygon].x[i];
               polygons[in_polygon].yc += polygons[in_polygon].y[i];
            }

            polygons[in_polygon].xc =
               polygons[in_polygon].xc/polygons[in_polygon].nvertices;

            polygons[in_polygon].yc =
               polygons[in_polygon].yc/polygons[in_polygon].nvertices;
         }
         in_polygon = -1;
         glutPostRedisplay();
         break;
      }
	case(3):  /* set picking mode */
      {
         picking = TRUE;
         moving = FALSE;
         break;
      }
   case(4):  /* set moving mode */
      {
         moving = TRUE;
         break;
      }
	case(5):  /* exit */
      {
         exit(0);
         break;
      }
   }
}


Thomas P. Kelliher 2011-02-22
Tom Kelliher