Name

glFog — specify fog parameters

C Specification

void fsfuncglFogf(GLenum pname,
 GLfloat param);
 
void fsfuncglFogx(GLenum pname,
 GLfixed param);
 

Parameters

pname

Specifies a single-valued fog parameter. GL_FOG_MODE, GL_FOG_DENSITY, GL_FOG_START, and GL_FOG_END are accepted.

param

Specifies the value that pname will be set to.

C Specification

void fsfuncglFogfv(GLenum pname,
 const GLfloat * params);
 
void fsfuncglFogxv(GLenum pname,
 const GLfixed * params);
 

Parameters

pname

Specifies a fog parameter. GL_FOG_MODE, GL_FOG_DENSITY, GL_FOG_START, GL_FOG_END, and GL_FOG_COLOR are accepted.

params

Specifies the value or values to be assigned to pname. GL_FOG_COLOR requires an array of four values. All other parameters accept an array containing only a single value.

Description

If fog is enabled, fog affects rasterized geometry, bitmaps, and pixel blocks, but not buffer clear operations. To enable and disable fog, call glEnable and glDisable with argument GL_FOG. Fog is initially disabled.

glFog assigns the value or values in params to the fog parameter specified by pname. The following values are accepted for pname:

GL_FOG_MODE

params is a single fixed-point or floating-point value that specifies the equation to be used to compute the fog blend factor f. Three symbolic constants are accepted: GL_LINEAR, GL_EXP, and GL_EXP2. The equations corresponding to these symbolic constants are defined below. The initial fog mode is GL_EXP.

GL_FOG_DENSITY

params is a single fixed-point or floating-point value that specifies density, the fog density used in both exponential fog equations. Only nonnegative densities are accepted. The initial fog density is 1.

GL_FOG_START

params is a single fixed-point or floating-point value that specifies start, the near distance used in the linear fog equation. The initial near distance is 0.

GL_FOG_END

params is a single fixed-point or floating-point value that specifies end, the far distance used in the linear fog equation. The initial far distance is 1.

GL_FOG_COLOR

params contains four fixed-point or floating-point values that specify Cf , the fog color. Both fixed-point and floating-point values are mapped directly. After conversion, all color components are clamped to the range [0, 1]. The initial fog color is (0, 0, 0, 0).

Fog blends a fog color with each rasterized pixel fragment's posttexturing color using a blending factor f. Factor f is computed in one of three ways, depending on the fog mode. Let z be the distance in eye coordinates from the origin to the fragment being fogged. The equation for GL_LINEAR fog is

f= end-z end-start

The equation for GL_EXP fog is

f= e -(density-z)

The equation for GL_EXP2 fog is

f= e -(density-z) 2

Regardless of the fog mode, f is clamped to the range [0, 1] after it is computed. Then, the fragment's red, green, and blue colors, represented by Cr , are replaced by

C'r= f C r + 1-f C f

Fog does not affect a fragment's alpha component.

Errors

GL_INVALID_ENUM is generated if pname is not an accepted value, or if pname is GL_FOG_MODE and params is not an accepted value.

GL_INVALID_VALUE is generated if pname is GL_FOG_DENSITY, and params is negative.

See Also

glEnable

Copyright

Copyright © 2003-2004 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.