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# docs.GL

OpenGL 3
OpenGL 2

# glFog

## Name

glFog — specify fog parameters

## C Specification

`void glFogf(` | GLenum | pname,
| GLfloat | param`)`;

`void glFogi(` | GLenum | pname,
| GLint | param`)`;

## Parameters

`pname`

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

`param`

  Specifies the value that `pname` will be set to.

## C Specification

`void glFogfv(` | GLenum | pname,
| const GLfloat * | params`)`;

`void glFogiv(` | GLenum | pname,
| const GLint * | params`)`;

## Parameters

`pname`

  Specifies a fog parameter. `GL_FOG_MODE`, `GL_FOG_DENSITY`, `GL_FOG_START`, `GL_FOG_END`, `GL_FOG_INDEX`, `GL_FOG_COLOR`, and `GL_FOG_COORD_SRC` 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

Fog is initially disabled. While 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`.

`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 integer 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 integer 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 integer 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 integer or floating-point value that specifies end, the far distance used in the linear fog equation. The initial far distance is 1.

`GL_FOG_INDEX`

  `params` is a single integer or floating-point value that specifies i f , the fog color index. The initial fog index is 0.

`GL_FOG_COLOR`

  `params` contains four integer or floating-point values that specify C f , the fog color. Integer values are mapped linearly such that the most positive representable value maps to 1.0, and the most negative representable value maps to -1.0 . 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).

`GL_FOG_COORD_SRC`

  `params` contains either of the following symbolic constants: `GL_FOG_COORD` or `GL_FRAGMENT_DEPTH`. `GL_FOG_COORD` specifies that the current fog coordinate should be used as distance value in the fog color computation. `GL_FRAGMENT_DEPTH` specifies that the current fragment depth should be used as distance value in the fog computation.

Fog blends a fog color with each rasterized pixel fragment's post-texturing color using a blending factor f. Factor f is computed in one of three ways, depending on the fog mode. Let c be either the distance in eye coordinate from the origin (in the case that the `GL_FOG_COORD_SRC` is `GL_FRAGMENT_DEPTH`) or the current fog coordinate (in the case that `GL_FOG_COORD_SRC` is `GL_FOG_COORD`). The equation for `GL_LINEAR` fog is

f = end - c end - start

The equation for `GL_EXP` fog is

f = e - density · c

The equation for `GL_EXP2` fog is

f = e - density · c 2

Regardless of the fog mode, f is clamped to the range 0 1 after it is computed. Then, if the GL is in RGBA color mode, the fragment's red, green, and blue colors, represented by C r , are replaced by

C r ″ = f × C r + 1 - f × C f

Fog does not affect a fragment's alpha component.

In color index mode, the fragment's color index i r is replaced by

i r ″ = i r + 1 - f × i f

## Notes

`GL_FOG_COORD_SRC` is available only if the GL version is 1.4 or greater.

## 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.

`GL_INVALID_OPERATION` is generated if `glFog` is executed between the execution of glBegin and the corresponding execution of glEnd.

## Associated Gets

glIsEnabled with argument `GL_FOG`

glGet with argument `GL_FOG_COLOR`

glGet with argument `GL_FOG_INDEX`

glGet with argument `GL_FOG_DENSITY`

glGet with argument `GL_FOG_START`

glGet with argument `GL_FOG_END`

glGet with argument `GL_FOG_MODE`

## See Also

glEnable

## Copyright

Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/.

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