2.0 3.0 > comp > gb.opengl > gl > fogf
Previous  Next  Edit  Rename  Undo  Search  Administration
 Documentation
Gl.Fogf (gb.opengl)
`Static Sub Fogf ( Pname As Integer, Param As Float )`

Specify fog parameters.

### 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.

### 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 Gl.Enable and Gl.Disable with argument Gl.FOG.

Gl.Fog 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, $\mathit{f}$. 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 $\mathit{density}$, 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 $\mathit{start}$, 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 $\mathit{end}$, 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 ${\mathit{i}}_{\mathit{f}}$, the fog color index. The initial fog index is 0.

Gl.FOG_COLOR

params contains four integer or floating-point values that specify ${\mathit{C}}_{\mathit{f}}$, 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 $\left[0,1\right]$. 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 $\mathit{f}$. Factor $\mathit{f}$ is computed in one of three ways, depending on the fog mode. Let $\mathit{c}$ 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 $\mathit{f}=\frac{\mathit{end}-\mathit{c}}{\mathit{end}-\mathit{start}}$

The equation for Gl.EXP fog is $\mathit{f}={\mathit{e}}^{-\left(\mathit{density}·\mathit{c}\right)}$

The equation for Gl.EXP2 fog is $\mathit{f}={\mathit{e}}^{{-\left(\mathit{density}·\mathit{c}\right)}^{2}}$

Regardless of the fog mode, $\mathit{f}$ is clamped to the range $\left[0,1\right]$ after it is computed. Then, if the GL is in RGBA color mode, the fragment's red, green, and blue colors, represented by ${\mathit{C}}_{\mathit{r}}$, are replaced by

${{\mathit{C}}_{\mathit{r}}}^{″}=\mathit{f}×{\mathit{C}}_{\mathit{r}}+\left(1-\mathit{f}\right)×{\mathit{C}}_{\mathit{f}}$

Fog does not affect a fragment's alpha Komponenty.

In color index mode, the fragment's color index ${\mathit{i}}_{\mathit{r}}$ is replaced by

${{\mathit{i}}_{\mathit{r}}}^{″}={\mathit{i}}_{\mathit{r}}+\left(1-\mathit{f}\right)×{\mathit{i}}_{\mathit{f}}$

### 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 Gl.Fog is executed between the execution of Gl.Begin and the corresponding execution of Gl.End.

### Associated Gets

Gl.IsEnabled with argument Gl.FOG

Gl.Get with argument Gl.FOG_COLOR

Gl.Get with argument Gl.FOG_INDEX

Gl.Get with argument Gl.FOG_DENSITY

Gl.Get with argument Gl.FOG_START

Gl.Get with argument Gl.FOG_END

Gl.Get with argument Gl.FOG_MODE