glFogf

NAME

glFogf, glFogi, glFogfv, glFogiv - 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, and GL_FOG_INDEX 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, 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

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 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. Four symbolic constants are accepted: GL_LINEAR, GL_EXP, GL_EXP2, and GL_FOG_FUNC_EXT. 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, 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).

GL_FOG_OFFSET_VALUE_EXT
params contains four integer or floating-point values. The first three specify a reference point (x, y, z) in eye coordinates. The fourth value specifies an offset in eye coordinates. The offset is used to modify z in the fog equations described below, so that bright objects (such as emissive objects representing light sources) will have a more realistic appearance in heavily-fogged environments. The significance of the reference point is discussed below. 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. The default value is (0,0,0,0).
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 four ways, depending on the fog mode. Let z be the distance in eye coordinates from the origin to the fragment being fogged. If GL_FOG_OFFSET_EXT is enabled, the offset specified by GL_FOG_OFFSET_VALUE_EXT is subtracted from z and the result is clamped to 0.0 if it is less than zero.

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

The equation for GL_FOG_FUNC_EXT fog is determined by interpolating a set of application-defined control points. glFogFuncEXT is used to specify the control points. Each control point consists of a pair of floating-point numbers. The first number of the pair specifies a value of z and the second number of the pair specifies a value of f.

A curve is computed through the control points. This curve may be piecewise linear or it may be smoothed, but it will pass through the control points exactly (limited only by the resolution of the implementation). If a given z is less than or equal to the z of the first control point, the resulting value of f is the f of the first control point. If z is greater than or equal to the z of the last control point, the resulting value of f is the f of the last control point. Otherwise, f is determined by finding the point on the curve that corresponds to the given z.

Regardless of the fog mode, f is clamped to the range [0,1] after it is computed. If the GL is in RGBA color mode, the fragment's color Cr is replaced by

Cr' = f * Cr + (1 - f) * Cr

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

ir' = ir + (1 - f) * ir

NOTES

A reference point is required by the GL_FOG_OFFSET_VALUE_EXT extension in perspective projections for some machines. An offset in eye coordinates will map to different offsets in window coordinates at various points in the range of Z. Some machines calculate fogging using the window z coordinate. For best fog accuracy in perspective projections, this reference point must be close to the objects being fogged. For parallel projections or machines that calculate fog using the eye z coordinate, the reference point is ignored. Wildcat architectures that support the GL_FOG_OFFSET_VALUE_EXT extension calculate fogging using eye z coordinates. Therefore, the reference point is not used.

EXTENSIONS

Some routines and constants for this function are part of an extension, not part of the core GL command set. The following extension names must be present in the string returned by glGetString when called with argument GL_EXTENSIONS to use these routines and constants.
GL_EXT_fog_function
GL_FOG_FUNC_EXT is part of the EXT_fog_function extension.

GL_EXT_fog_offset
GL_FOG_OFFSET_EXT and GL_FOG_OFFSET_VALUE_EXT are part of the EXT_fog_offset extension.

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, glFogFuncEXT