Fog in OpenGL

Fog in OpenGL

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

A Problem:
- Scenes may not appear realistic
One Fix:
- Make objects fade into the distance (i.e., fade objects based on their distance from the viewpoint)

Fog Equations

Interpolation Factor:
- \(f(z) = e^{(-d \cdot z)}\)
- \(f(z) = e^{(-d \cdot z)^2}\)
- \(f(z) = \frac{U-z}{U-L}\)
- where \(d\) is the density, \(U\) is the "end", and \(L\) is the "start"
Color Calculation:
- \(C(z) = f(z) S + [1 - f(z)] F\)
- where \(S\) is the "source" color and \(F\) is the "fog" color

Using Fog in OpenGL

Choose the Fog Color:
- void glFog*v(GLenum pname, TYPE *params)
- where pname is GL_FOG_COLOR and params points to the RGBA values
Choose the Fog Equation:
- void glFog*(GLenum pname, TYPE params)
- where pname is GL_FOG_MODE and TYPE is either GL_EXP, GL_EXP2, or GL_LINEAR
Set the Parameters for the Fog Equation:
- pname can be GL_FOG_DENSITY, GL_FOG_START, or GL_FOG_END
Enable Fog:
- glEnable(GL_FOG);

An Example

Initialization

openglexamples/fog/fog.c

(Fragment: initFog)

  glEnable(GL_FOG);
  {
    glFogi(GL_FOG_MODE, GL_EXP);
    glFogfv(GL_FOG_COLOR, colorGray);
    glFogf(GL_FOG_DENSITY, 0.45);
  }
  glClearColor(colorGray[0], colorGray[1], colorGray[2], colorGray[3]);

An Example (cont.)

Rendering

openglexamples/fog/fog.c

(Fragment: display)

/**
 * Display/render the OpenGL window
 */
void display() {
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  glPushMatrix();
  glTranslatef(-1.0, -0.5, -2.0);
  glutSolidTeapot(0.5f);
  glPopMatrix();

  glPushMatrix();
  glTranslatef(0.0, -0.5, -3.5);
  glutSolidTeapot(0.5f);
  glPopMatrix();

  glPushMatrix();
  glTranslatef(1.0, -0.5, -5.0);
  glutSolidTeapot(0.5f);
  glPopMatrix();

  glFlush();
}