glFog(3G)
glFogf, glFogi, glFogfv, glFogiv - specify fog parameters
As shipped in IRIX 6.5.19. Last changed in IRIX 6.5.15.
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, GL_FOG_COLOR, and GL_FOG_OFFSET_VALUE_SGIX are accepted. params Specifies the value or values to be assigned to pname. GL_FOG_COLOR requires an array of four values to specify the RGBA components of the fog color. GL_FOG_OFFSET_VALUE_SGIX requires an array of four values to specify a reference point in eye-space coordinates, and a Z offset in eye-space coordinates. 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. Four symbolic constants are accepted: GL_LINEAR, GL_EXP, GL_EXP2, and GL_FOG_FUNC_SGIS. 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 f fog index is 0. GL_FOG_COLOR params contains four integer or floating-point values that specify C , the fog color. Integer values are f 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_SGIX params contains four integer or floating-point values. The first three specify a reference point as X, Y, and Z eye coordinates. The fourth specifies a Z 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 initial value 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 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_SGIX is enabled, subtract the Z offset specified by GL_FOG_OFFSET_VALUE_SGIX from z, and clamp the result so that it is greater than or equal to zero. The equation for GL_LINEAR fog is ___________ f = end - start The equation for GL_EXP fog is . -(density z) f = e The equation for GL_EXP2 fog is . 2 -(density z) f = e The blending factor for GL_FOG_FUNC_SGIS fog is determined by interpolating a set of application-defined control points. glFogFuncSGIS 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. The GL fits a curve through all 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, then 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, then 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. Then, if the GL is in RGBA color mode, the fragment's red, green, and blue colors, represented by C , are replaced by r ' C = fC + (1-f)C r r f Fog does not affect a fragment's alpha component. In color index mode, the fragment's color index i is replaced by r ' i = i + (1-f)i r r f NOTES In perspective projections, a Z offset in eye coordinates will map to different Z offsets in window coordinates at various points in the range of Z. Machines that evaluate the fog blending factor by using a window Z coordinate to index a table must therefore compute the window-coordinate Z offset relative to some reference point. This is the purpose of the reference point specified by the first three values in params when pname is GL_FOG_OFFSET_VALUE_SGIX. For best fog accuracy in perspective projections, this reference point must be close to the objects being fogged. In parallel projections the window-coordinate Z offset does not vary with position in the Z range, so the reference point is ignored. GL_FOG_FUNC_SGIS is part of the SGIS_fog_func extension, not part of the core GL command set. If GL_SGIS_fog_func is included in the string returned by glGetString when called with argument GL_EXTENSIONS, extension SGIS_fog_func is supported by the connection. See glIntro for more information about using extensions. GL_FOG_OFFSET_VALUE_SGIX is part of the SGIX_fog_offset extension, and similar considerations apply. 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 glIsEnabled with argument GL_FOG_OFFSET_SGIX glGet with argument GL_FOG_COLOR glGet with argument GL_FOG_FUNC_SGIS glGet with argument GL_MAX_FOG_FUNC_POINTS_SGIS 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 glGet with argument GL_FOG_OFFSET_VALUE_SGIX MACHINE DEPENDENCIES Fog mode GL_FOG_FUNC_SGIS is supported only on InfiniteReality systems. The fog offset capability is supported only on InfiniteReality systems. On InfiniteReality systems, fog cannot be used with color-index visuals that are 2 or 4 bits deep. Accordingly, these visuals are marked as non-conforming. SEE ALSO glEnable, glFogFuncSGIS