HP Workstation zx2000 OpenGL 1.1 Reference for HP-UX 11.x - Page 428

T glTexEnv Description A texture environment specifies how texture values are interpreted when a fragment is textured. target must be GL_TEXTURE_ENV. pname can be either GL_TEXTURE_ENV_MODE, GL_TEXTURE_ENV_COLOR or GL_TEXTURE_LIGHTING_MODE_hp (if the extension GL_hp_texture_lighting is supported). If pname is GL_TEXTURE_ENV_MODE, then params is (or points to) the symbolic name of a texture function. Four texture functions may be specified: GL_MODULATE, GL_DECAL, GL_BLEND, and GL_REPLACE. If pname is GL_TEXTURE_LIGHTING_MODE_hp, two possible values for param may be specified: either GL_TEXTURE_PRE_SPECULAR_hp or GL_TEXTURE_POST_SPECULAR_hp. A texture function acts on the fragment to be textured using the texture image value that applies to the fragment (see glTexParameter) and produces an RGBA color for that fragment. The following table shows how the RGBA color is produced for each of the three texture functions that can be chosen. C is a triple of color values (RGB) and A is the associated alpha value. RGBA values extracted from a texture image are in the range [0, 1]. The subscript f refers to the incoming fragment, the subscript t to the texture image, the subscript c to the texture environment color, and subscript v indicates a value produced by the texture function. A texture image can have up to four components per texture element (see glTexImage1D, glTexImage2D, glCopyTexImage1D, and glCopyTexImage2D). In a one-component image, Lt indicates that single component. A two-component image uses Lt and At. A three-component image has only a color value, Ct. A four-component image has both a color value Ct and an alpha value At. Base Internal Format Texture Functions GL_MODULATE GL_DECAL GL_BLEND GL_REPLACE GL_ALPHA GL_LUMINANCE 1 Cv = Cf Av = Af At Cv = Lt Cf Av = Af GL_LUMINANCE_ALPHA Cv = Lt Cf 2 Av = At Af GL_INTENSITY Cv = Cf It Av = Af It (undefined) (undefined) (undefined) (undefined) Cv = Cf Av = Af Cv = (1 - Lt ) Cf + Lt Cc Av = Af Cv = (1 - Lt ) Cf + Lt Cc Av = At Af Cv = (1 - It ) Cf + It Cc Av = (1 - It ) Af + It Ac Cv = Cf Av = At Cv = Lt Av = Af Cv = Lt Av = At Cv = It Av = It 428 Chapter 18