HP Workstation zx2000 OpenGL 1.1 Reference for HP-UX 11.x - Page 156

F glFog 156 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 post texturing 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 color Cr is replaced by Cr' = f Cr + (1- f) Cf In color index mode, the fragment's color index ir is replaced by ir' = ir + (1 - f) if 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. Chapter 6