Porting Depth Cueing and Fog Commands

When porting depth-cueing and fog commands, keep the following points in mind:

• The IRIS GL call, fogvertex, sets a mode and the parameters affecting that mode. In OpenGL, you call glFog once to set the mode, and then again twice or more to set various parameters.

• In OpenGL, depth cueing is not a separate feature. Use linear fog instead of depth cueing. (This section gives an example of how to do this.) The following IRIS GL functions have no direct OpenGL equivalent:

  depthcue

  lRGBrange

  lshaderange

  getdcm

• To adjust fog quality, use glHint( GL_FOG_HINT ).

The following table lists the IRIS GL functions for managing fog and their equivalent OpenGL functions.

IRIS GL function	OpenGL function	Meaning
fogvertex	glFog	Sets various fog parameters.
fogvertex( FG_ON )	glEnable ( GL_FOG )	Turns on fog.
fogvertex( FG_OFF )	glDisable ( GL_FOG )	Turns off fog.
depthcue	glFog ( GL_FOG_MOD, GL_LINEAR )	Uses linear fog for depth cueing.

The following table lists the parameters you can pass to glFog.

Fog parameter	Meaning	Default
GL_FOG_DENSITY	Fog density.	1.0
GL_FOG_START	Near distance for linear fog.	0.0
GL_FOG_END	Far distance for linear fog.	1.0
GL_FOG_INDEX	Fog color index.	0.0
GL_FOG_COLOR	Fog RGBA color.	(0, 0, 0, 0)
GL_FOG_MODE	Fog mode.	See the following table.

The fog-density parameter of OpenGL differs from the one in IRIS GL. They are related as follows:

• if fogMode = EXP2

  openGLfogDensity = (irisGLfogDensity ) ( sqrt ( log ( 1 / 255 ) ))

• if fogMode = EXP

  openGLfogDensity = (irisGLfogDensity ) ( log ( 1 / 255 ) )

where sqrt is the square root operation, log is the natural logarithm, irisGLfogDensity is the IRIS GL fog density, and openGLfogDensity is the OpenGL fog density.

To switch between calculating fog in per-pixel mode and per-vertex mode, use glHint( GL_FOG_HINT, hintMode). Two hint modes are available:

• GL_NICEST per-pixel fog calculation
• GL_FASTEST per-vertex fog calculation

The following table lists the IRIS GL fog modes and their OpenGL equivalents.

IRIS GL fog mode	OpenGL fog mode	Hint mode	Meaning
FG_VTX_EXP,FG_PIX_EXP	GL_EXP	GL_FASTEST,GL_NICEST	Heavy fog mode (default).
FG_VTX_EXP2,FG_PIX_EXP2	GL_EXP2	GL_FASTEST,GL_NICEST	Haze mode.
FG_VTX_LIN,FG_PIX_LIN	GL_LINEAR	GL_FASTEST,GL_NICEST	Linear fog mode. (Use for depth cueing.)

The following code example demonstrates depth cueing in OpenGL:

/* 
 *  depthcue.c 
 *  This program draws a wire frame model, which uses 
 *  intensity (brightness) to give clues to distance 
 *  Fog is used to achieve this effect 
 */ 
#include <GL/gl.h> 
#include <GL/glu.h> 
#include "aux.h" 
 
/*  Initialize linear fog for depth cueing 
 */ 
void myinit(void) 
{ 
    GLfloat fogColor[4] = {0.0, 0.0, 0.0, 1.0}; 
 
    glEnable(GL_FOG); 
    glFogi (GL_FOG_MODE, GL_LINEAR); 
    glHint (GL_FOG_HINT, GL_NICEST);  /*  per pixel  */ 
    glFogf (GL_FOG_START, 3.0); 
    glFogf (GL_FOG_END, 5.0); 
    glFogfv (GL_FOG_COLOR, fogColor); 
    glClearColor(0.0, 0.0, 0.0, 1.0); 
 
    glDepthFunc(GL_LEQUAL); 
    glEnable(GL_DEPTH_TEST); 
    glShadeModel(GL_FLAT); 
} 
 
/*  display() draws an icosahedron 
 */ 
void display(void) 
{ 
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); 
    glColor3f (1.0, 1.0, 1.0); 
    auxWireIcosahedron(1.0); 
    glFlush(); 
} 
 
void myReshape(GLsizei w, GLsizei h) 
{ 
    glViewport(0, 0, w, h); 
    glMatrixMode(GL_PROJECTION); 
    glLoadIdentity(); 
    gluPerspective (45.0, (GLfloat) w/(GLfloat) h, 3.0, 5.0); 
    glMatrixMode(GL_MODELVIEW); 
    glLoadIdentity (); 
    glTranslatef (0.0, 0.0, -4.0); /*move object into view*/ 
} 
/*  Main Loop 
 */ 
int main(int argc, char** argv) 
{ 
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA | AUX_DEPTH); 
    auxInitPosition (0, 0, 400, 400); 
    auxInitWindow (argv[0]); 
    myinit(); 
    auxReshapeFunc (myReshape); 
    auxMainLoop(display); 
}