Double buffer/single buffer switching

Double buffer/single buffer switching

Post by Mark Kilga » Sat, 01 Jun 1996 04:00:00



GLUT programmers,

I've gotten a few email questions about how to write a GLUT
program that switches between double buffering and single
buffering.  The typical reason for this is that many machines
support less color resolution when double buffering, so for
static scenes, you might want to switch to single buffering
for higher color resolution.

The sb2db.c program below shows how to do single/double buffer
switching with GLUT.  The basic idea is to hide and show a
double buffer GLUT subwindow that fully obscures a single buffered
toplevel window.  If you have GLUT, compile it like:

  cc -o sb2db sb2db.c -lglut -lGLU -lGL -lXmu -lXext -lX11 -lm

If you want info on GLUT, see:

  http://reality.sgi.com/mjk_asd/glut3/glut3.html

I hope this helps.

- Mark

/* Copyright (c) Mark J. Kilgard, 1994, 1996. */

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/* sb2db.c - This program demonstrates switching between single buffered
   and double buffered windows when using GLUT.  Use the pop-up menu to
   change the buffering style used.  On machine that split the screen's
   color resolution in half when double buffering, you should notice better
   coloration (less or no dithering) in single buffer mode (but flicker on
   redraws, particularly when rotation is toggled on).  */

/* This program is based on the GLUT scene.c program. */

#include <stdlib.h>
#include <GL/glut.h>

int sbwin, dbwin;
int angle;

/*  Initialize material property and light source.  */
void
myinit(void)
{
  GLfloat light_ambient[] =
  {0.0, 0.0, 0.0, 1.0};
  GLfloat light_diffuse[] =
  {1.0, 0.0, 0.0, 1.0};
  GLfloat light_specular[] =
  {1.0, 1.0, 1.0, 1.0};
  /* light_position is NOT default value */
  GLfloat light_position[] =
  {1.0, 1.0, 1.0, 0.0};

  glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);
  glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
  glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);
  glLightfv(GL_LIGHT0, GL_POSITION, light_position);

  glEnable(GL_LIGHT0);
  glDepthFunc(GL_LESS);
  glEnable(GL_DEPTH_TEST);
  glEnable(GL_LIGHTING);

}

void
display(void)
{
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  glPushMatrix();
  glRotatef(angle, 1.0, 0.0, 0.0);
  glScalef(1.3, 1.3, 1.3);
  glRotatef(20.0, 1.0, 0.0, 0.0);

  glPushMatrix();
  glTranslatef(-0.75, 0.5, 0.0);
  glRotatef(90.0, 1.0, 0.0, 0.0);
  glutSolidTorus(0.275, 0.85, 10, 15);
  glPopMatrix();

  glPushMatrix();
  glTranslatef(-0.75, -0.5, 0.0);
  glRotatef(270.0, 1.0, 0.0, 0.0);
  glutSolidTetrahedron();
  glPopMatrix();

  glPushMatrix();
  glTranslatef(0.75, 0.0, -1.0);
  glutSolidIcosahedron();
  glPopMatrix();

  glPopMatrix();

  if (glutGetWindow() == sbwin) {
    glFlush();
  } else {
    glutSwapBuffers();
  }

}

/* Used by both windows, this routine setups the OpenGL context's
   projection matrix correctly.  Note that we call this routine for
   both contexts to keep them in sync after reshapes. */
void
reshapeOpenGLState(int w, int h)
{
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  if (w <= h)
    glOrtho(-2.5, 2.5, -2.5 * (GLfloat) h / (GLfloat) w,
      2.5 * (GLfloat) h / (GLfloat) w, -10.0, 10.0);
  else
    glOrtho(-2.5 * (GLfloat) w / (GLfloat) h,
      2.5 * (GLfloat) w / (GLfloat) h, -2.5, 2.5, -10.0, 10.0);
  glMatrixMode(GL_MODELVIEW);

}

/* When the single buffered (ie, the top window) gets resized, we
   need to resize the child double buffered window as well.  Hence
   the glutReshapeWindow on the child.  NOTE:  You want a separate
   resize callback for the double buffered window to set the viewport
   since the window's size won't really be changed until the double buffered
   gets its dbReshape callback.  Otherwise, you could trick OpenGL intop
   clipping based on the old window size. */
void
sbReshape(int w, int h)
{
  glutSetWindow(sbwin);
  glViewport(0, 0, w, h);
  reshapeOpenGLState(w, h);
  glutSetWindow(dbwin);
  glutReshapeWindow(w, h);

}

void
dbReshape(int w, int h)
{
  glViewport(0, 0, w, h);
  reshapeOpenGLState(w, h);

}

void
rotation(void)
{
  angle += 2;
  angle = angle % 360;
  glutPostRedisplay();

}

int animation = 0;  /* Are we doing an animated rotation currently? */

void
main_menu(int value)
{
  switch (value) {
  case 1:
    glutSetWindow(sbwin);
    glutSetWindowTitle("sb2db - single buffer mode");
    glutSetWindow(dbwin);
    glutHideWindow();   /* Hide the double buffered window. */
    break;
  case 2:
    glutSetWindow(sbwin);
    glutSetWindowTitle("sb2db - double buffer mode");
    glutSetWindow(dbwin);
    glutShowWindow();   /* Show the double buffered window. */
    break;
  case 3:
    animation = 1 - animation;  /* Toggle. */
    if (animation)
      glutIdleFunc(rotation);
    else
      glutIdleFunc(NULL);
    break;
  case 4:
    /* Smart toggle rotation ensures we switch to double buffered when
       animating and single buffered when not animating. */
    animation = 1 - animation;  /* Toggle. */
    if (animation) {
      glutIdleFunc(rotation);
      glutSetWindow(sbwin);
      glutSetWindowTitle("sb2db - double buffer mode");
      glutSetWindow(dbwin);
      glutShowWindow();   /* Show the double buffered window. */
    } else {
      glutIdleFunc(NULL);
      glutSetWindow(sbwin);
      glutSetWindowTitle("sb2db - single buffer mode");
      glutSetWindow(dbwin);
      glutHideWindow();   /* Hide the double buffered window. */
    }
    break;
  case 666:
    exit(0);
    break;
  }

}

/* You have to track the visibility of both the single buffered
   and double buffered windows together. */
void
visibility(int state)
{
  static int sbvis = GLUT_NOT_VISIBLE, dbvis = GLUT_NOT_VISIBLE;
  int eithervis;

  if (glutGetWindow() == sbwin) {
    sbvis = state;
  } else {
    dbvis = state;
  }
  eithervis = (sbvis == GLUT_VISIBLE) || (dbvis == GLUT_VISIBLE);
  if (eithervis) {
    /* Resume rotating idle callback if we become visible and
       animation is enabled. */
    if (animation) {
      glutIdleFunc(rotation);
    }
  } else {
    /* Disable animation when both windows are not visible. */
    glutIdleFunc(NULL);
  }

}

int
main(int argc, char **argv)
{
  glutInitWindowSize(500, 500);
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_RGB | GLUT_DEPTH | GLUT_SINGLE);

  /* The top window is single buffered. */
  sbwin = glutCreateWindow(argv[0]);
  glutReshapeFunc(sbReshape);
  glutDisplayFunc(display);
  glutVisibilityFunc(visibility);
  myinit();

  /* The child window is double buffered.  We show this window
     when displaying double buffered and hide it to show the
     single buffered window. */
  glutInitDisplayMode(GLUT_RGB | GLUT_DEPTH | GLUT_DOUBLE);
  dbwin = glutCreateSubWindow(glutGetWindow(),
    0, 0, glutGet(GLUT_WINDOW_WIDTH), glutGet(GLUT_WINDOW_HEIGHT));
  glutDisplayFunc(display);
  glutReshapeFunc(dbReshape);
  glutVisibilityFunc(visibility);

  /* Call myinit for both the single buffered window and the
     double buffered window.  We must mirror the same OpenGL
     state in both window's OpenGL contexts.  If you make this
     program more complicated, remember to keep the window's
     context state in sync. */
  myinit();

  /* Initially hide the double buffered window to start in
     single buffered mode. */
  glutHideWindow();

  glutCreateMenu(main_menu);
  glutAddMenuEntry("Single buffer", 1);
  glutAddMenuEntry("Double buffer", 2);
  glutAddMenuEntry("Toggle rotation", 3);
  glutAddMenuEntry("Smart rotation toggle", 4);
  glutAddMenuEntry("Quit", 666);
  glutAttachMenu(GLUT_RIGHT_BUTTON);
  glutSetWindow(sbwin);
  glutAttachMenu(GLUT_RIGHT_BUTTON);
  glutMainLoop();
  return 0;             /* ANSI C requires main to return int. */

}

 
 
 

1. switching from double buffered to single buffered mode

Does anyone know of a gracefull way to switch back and forth between
single and double buffered mode in a OpenGl drawing widget, i.e. a
widget created by
glwidget = XtVaCreateManagedWidget ( "glwidget",
                                     glwMDrawingAreaWidgetClass,
                                     GLwNdoublebuffer, True, ...);?

Thanks, Hans

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