Rendering to Multiple Subwindows

Before we start lets recall our callback definitions, as defined when the window and subwindows were created:

• idle function – renderSceneAll
• display func for main window – renderScene
• display func for subwindow 1 – renderScenesw1
• display func for subwindow 2 – renderScenesw2
• display func for subwindow 3 – renderScenesw3

We’ll start by the display functions for each window. The main window is covered with subwindows so we only want to paint it black. Since we are working with multiple windows the first thing we must do is call glutSetWindow with the proper window id. Then we just clear the color buffer with the default color, black.

void renderScene() {
	glutSetWindow(mainWindow);
	glClear(GL_COLOR_BUFFER_BIT);
	glutSwapBuffers();
}

We must define the display function for each subwindow. In our example, the geometry is the same for all windows, the only thing that differs is the viewpoint, or the camera, if you prefer.

The function where the common geometry is rendered is called renderScene2. However, before we call this function we must set the current window to be the respective subwindow, load the identity matrix to clean the MODELVIEW matrix, and set the camera with gluLookAt.

As mentioned before in the initial section covering subwindows we have three subwindows with different perspectives of the same scene. The first subwindow displays the scene from the current point of view, i.e. the main camera. The second displays the same scene from the top, i.e. as if the camera was above the current position looking downwards, and with the same orientation as the line of sigh. The third subwindow behaves like a camera to the right of the current position, and pointing at the current position.

The following code defines the display functions for every window. This code is an extended version of the previous code. If you require more detail just explore the previous sections, namely Moving the Camera II for the keyboard movement, Bitmaps and the Orthogonal View for text display, or Frames per Second for the respective computation.

Note that there are a few differences in the contents of the windows. The top window will display the fps counter using bitmap strings. The two bottom windows will display a red cone at the position of the main camera.

// Common render items for all subwindows
void renderScene2() {

// Draw ground

	glColor3f(0.9f, 0.9f, 0.9f);
	glBegin(GL_QUADS);
		glVertex3f(-100.0f, 0.0f, -100.0f);
		glVertex3f(-100.0f, 0.0f,  100.0f);
		glVertex3f( 100.0f, 0.0f,  100.0f);
		glVertex3f( 100.0f, 0.0f, -100.0f);
	glEnd();

// Draw 36 SnowMen
	char number[3];
	for(int i = -3; i < 3; i++)
		for(int j=-3; j < 3; j++) {

			glPushMatrix();
			glTranslatef(i*10.0f, 0.0f, j * 10.0f);
			drawSnowMan();
			glPopMatrix();
		}
}

// Display func for main window
void renderScene() {
	glutSetWindow(mainWindow);
	glClear(GL_COLOR_BUFFER_BIT);
	glutSwapBuffers();
}

// Display func for sub window 1
void renderScenesw1() {

	glutSetWindow(subWindow1);

	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glLoadIdentity();
	gluLookAt(x, y, z,
		  x + lx,y + ly,z + lz,
		  0.0f,1.0f,0.0f);

	renderScene2();

	// display fps in the top window
 	frame++;

	time=glutGet(GLUT_ELAPSED_TIME);
	if (time - timebase > 1000) {
		sprintf(s,"Lighthouse3D - FPS:%4.2f",
			frame*1000.0/(time-timebase));
		timebase = time;
		frame = 0;
	}

	setOrthographicProjection();

	glPushMatrix();
	glLoadIdentity();
	renderBitmapString(5,30,0,GLUT_BITMAP_HELVETICA_12,s);
	glPopMatrix();

	restorePerspectiveProjection();

	glutSwapBuffers();
}

// Display func for sub window 2
void renderScenesw2() {

	glutSetWindow(subWindow2);

	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glLoadIdentity();
	gluLookAt(x, y+15, z,
		  x ,y - 1,z,
		  lx,0,lz);

	// Draw red cone at the location of the main camera
	glPushMatrix();
	glColor3f(1.0,0.0,0.0);
	glTranslatef(x,y,z);
	glRotatef(180-(angle+deltaAngle)*180.0/3.14,0.0,1.0,0.0);
	glutSolidCone(0.2,0.8f,4,4);
	glPopMatrix();

	renderScene2();

	glutSwapBuffers();
}

// Display func for sub window 3
void renderScenesw3() {

	glutSetWindow(subWindow3);

	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glLoadIdentity();
	gluLookAt(x-lz*10 , y, z+lx*10,
		  x ,y ,z ,
		  0.0f,1.0f,0.0f);

	// Draw red cone at the location of the main camera
	glPushMatrix();
	glColor3f(1.0,0.0,0.0);
	glTranslatef(x,y,z);
	glRotatef(180-(angle+deltaAngle)*180.0/3.14,0.0,1.0,0.0);
	glutSolidCone(0.2,0.8f,4,4);
	glPopMatrix();

	renderScene2();

	glutSwapBuffers();
}

Now all its left to do is to define the global idle function. In our example, this function is renderSceneAll. This function checks if the variables deltaMove or deltaAngle are not zero, and updates the values of the current position, and the line of sight vector.

Afterwards we call the display functions for each of the subwindows. Note that we are not calling the display function for the main window because its contents never change.

// Global idle func
void renderSceneAll() {

	// check for keyboard movement
	if (deltaMove)
		computePos(deltaMove);

	renderScenesw1();
	renderScenesw2();
	renderScenesw3();
}

Thats it. Suggestions, bugs, or requests for more info are welcome. The next page contains the full source code.