Open GL Super Bible:Visual Effects:nBlending and Fog















To access the contents, click the chapter and section titles.


Open GL Super Bible


(Publisher: Macmillan Computer Publishing)

Author(s): Waite group Press

ISBN: 1571690735

Publication Date: 08/01/96

 




Previous
Table of Contents
Next




Fog
OpenGL provides depth-cueing (shading based upon distance) and atmospheric effects through the glFog function. Essentially, fog provides a way of adding (mixing) a predefined color with each vertex or texture image based upon the distance from the user. Fog is often used in flight simulators and animation packages to provide the final real-world look to computer graphics.
OpenGL supports three kinds of fog: GL_LINEAR for depth-cueing, GL_EXP for heavy fog or clouds, and GL_EXP2 for smoke and weather haze. Figure 16-3 shows GL_LINEAR fog; later, in Figure 16-5, you can see the effect of GL_EXP fog.

Figure 16-3  Depth-cued teapots using glFog
You choose the type of fog (or fog mode) using glFogi:


glFogi(GL_FOG_MODE, GL_LINEAR);

glFogi(GL_FOG_MODE, GL_EXP);

glFogi(GL_FOG_MODE, GL_EXP2);

Once you have chosen the fog type, you must choose a fog color that will be mixed with your scene using the glFogfv or glFogiv functions:


GLfloat fog_color[4] = { r, g, b, a };
glFogfv(GL_FOG_COLOR, fog_color);

GLint fog_color[4] = { r, g, b, a };
glFogiv(GL_FOG_COLOR, fog_color);

For depth-cueing, youÅ‚ll generally want to make the fog color the same as the background (black, in Figure 16-3). This will make the depth-cueing look “correct" to the eyethat is, objects farther away will appear to fade into the background. For some applications, you might want to give the fog a bright color such as yellow, instead, so that things stand out more against the background.

Drawing Depth-Cued Teapots
Listing 16-3 draws two teapots using depth-cueing. The draw_scene function handles all graphics drawing and starts by setting the fog color to black and the fog mode to GL_LINEAR.


static float fog_color[4] = { 0.0, 0.0, 0.0, 0.0 };

glEnable(GL_FOG);
glFogf(GL_FOG_MODE, GL_LINEAR);
glFogfv(GL_FOG_COLOR, fog_color);

Finally, it draws both teapots at different distances from the viewer. The results are visibly obvious.

Listing 16-3 FOGPOT.C: Depth-cued teapots using glFog

#include <GL/glaux.h>

/*
* These #define constants are provided for compatibility between MS
* Windows and the rest of the world.
*
* CALLBACK and APIENTRY are function modifiers under MS Windows.
*/

#ifndef WIN32
# define CALLBACK
# define APIENTRY
#endif /* !WIN32 */

GLfloat rotation = 0.0;

/*
* 'reshape_scene()' - Change the size of the scene
*/

void CALLBACK
reshape_scene(GLsizei width, /* I - Width of the window in pixels */
GLsizei height) /* I - Height of the window in pixels */
{
/*
* Reset the current viewport and perspective transformation
*/

glViewport(0, 0, width, height);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(22.5, (float)width / (float)height, 0.1, 1000.0);

glMatrixMode(GL_MODELVIEW);
}

/*
* 'draw_scene()' - Draw a scene containing a cube with a sphere in front
* of it.
*/

void CALLBACK
draw_scene(void)
{
static float red_light[4] = { 1.0, 0.0, 0.0, 1.0 };
static float red_pos[4] = { 1.0, 1.0, 1.0, 0.0 };
static float blue_light[4] = { 0.0, 0.0, 1.0, 1.0 };
static float blue_pos[4] = { -1.0, -1.0, -1.0, 0.0 };
static float fog_color[4] = { 0.0, 0.0, 0.0, 0.0 };

/*
* Enable drawing features that we need
*/

glEnable(GL_DEPTH_TEST);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
glEnable(GL_LIGHT1);

glShadeModel(GL_SMOOTH);

/*
* Clear the color and depth buffers
*/

glClearColor(0.0, 0.0, 0.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

/*
* Draw the cube and sphere in different colors
*
* We have positioned two lights in this scene. The first is red and
* located above, to the right, and behind the viewer. The second is
* blue and located below, to the left, and in front of the viewer.
*/

glLightfv(GL_LIGHT0, GL_DIFFUSE, red_light);
glLightfv(GL_LIGHT0, GL_POSITION, red_pos);

glLightfv(GL_LIGHT1, GL_DIFFUSE, blue_light);
glLightfv(GL_LIGHT1, GL_POSITION, blue_pos);

glEnable(GL_COLOR_MATERIAL);

glEnable(GL_FOG);
glFogf(GL_FOG_MODE, GL_LINEAR);
glFogfv(GL_FOG_COLOR, fog_color);

glPushMatrix();
glTranslatef(-1.0, 0.0, -15.0);
glRotatef(-rotation, 0.0, 1.0, 0.0);

glColor3f(1.0, 1.0, 0.0);
auxSolidTeapot(1.0);
glPopMatrix();

glPushMatrix();
glTranslatef(1.0, 0.0, -10.0);
glRotatef(rotation, 0.0, 1.0, 0.0);

glColor3f(0.0, 1.0, 1.0);
auxSolidTeapot(1.0);
glPopMatrix();

auxSwapBuffers();
}

/*
* 'rotate_objects()' - Rotate while we are idle
*/

void CALLBACK
rotate_objects(void)
{
rotation += 2.0;
if (rotation >= 360.0)
rotation -= 360.0;

draw_scene();
}

/*
* 'main()' - Initialize the window and display the scene until the user
* presses the ESCape key.
*/

void
main(void)
{
auxInitDisplayMode(AUX_RGB | AUX_DOUBLE | AUX_DEPTH);
auxInitWindow("Fogged Teapots");

auxReshapeFunc(reshape_scene);
auxIdleFunc(rotate_objects);

auxMainLoop(draw_scene);
}





Previous
Table of Contents
Next














 


Use of this site is subject to certain Terms & Conditions, Copyright © 1996-2000 EarthWeb Inc.
All rights reserved. Reproduction whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Read EarthWeb's privacy statement.