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Manual Reference Pages  - GLFOG (3G)

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glFogf, glFogi, glFogfv, glFogiv - specify fog parameters

CONTENTS

C Specification
Parameters
C Specification
Parameters
Description
Errors
Associated Gets
See Also

void glFogf( GLenum pname,

           GLfloat param )

void glFogi( GLenum pname,

           GLint param )

pname	Specifies a single-valued fog parameter.  GL_FOG_MODE,  GL_FOG_DENSITY,  GL_FOG_START,  GL_FOG_END, and  GL_FOG_INDEX are accepted.
param	Specifies the value that pname will be set to.

void glFogfv( GLenum pname,

            const GLfloat *params )

void glFogiv( GLenum pname,

            const GLint *params )

pname	Specifies a fog parameter.  GL_FOG_MODE,  GL_FOG_DENSITY,  GL_FOG_START,  GL_FOG_END,  GL_FOG_INDEX, 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.

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 if, the fog color index. The initial fog index is 0.
GL_FOG_COLOR	params contains four integer or floating-point values that specify Cf, 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).

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, 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

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.

 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

 glEnable(3G)

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