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 or GL_TEXTURE_ENV_COLOR .
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 .
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,
L sub t indicates that single component.
A two-component image uses L sub t and A sub t.
A three-component image has only a color value, C sub t.
A four-component image has both a color value C sub t
and an alpha value A sub t.
.ne
.TS
center box tab(:) ;
ci || ci s s s
ci || c c c c
c || c | c | c | c.
Base internal:Texture functions
format:GL_MODULATE :GL_DECAL :GL_BLEND :GL_REPLACE
=
GL_ALPHA :C sub v = C sub f:undefined:C sub v = C sub f:C sub v = C sub f
\^ :A sub v = A sub f A sub t:\^:A sub v = A sub f:A sub v = A sub t
_
GL_LUMINANCE :C sub v = L sub t C sub f:undefined:C sub v = ( 1 - L sub t ) C sub f:C sub v = L sub t
1: : :+ L sub t C sub c:
: : : :
: A sub v = A sub f:\^: A sub v = A sub f:A sub v = A sub f
_
GL_LUMINANCE :C sub v = L sub t C sub f:undefined:C sub v = ( 1 - L sub t ) C sub f :C sub v = L sub t
\_ALPHA : : : + L sub t C sub c
2: : : :
:A sub v = A sub t A sub f:\^:A sub v = A sub t A sub f:A sub v = A sub t
_
GL_INTENSITY :C sub v = C sub f I sub t:undefined:C sub v = ( 1 - I sub t ) C sub f :C sub v = I sub t
: : :+ I sub t C sub c
: : : :
\^ :A sub v = A sub f I sub t:\^:A sub v = ( 1 - I sub t ) A sub f :A sub v = I sub t
: : :+ I sub t A sub c:
_
GL_RGB :C sub v = C sub t C sub f:C sub v = C sub t:C sub v = (1 - C sub t) C sub f :C sub v = C sub t
3: : : + C sub t C sub c
: : : :
:A sub v = A sub f:A sub v = A sub f:A sub v = A sub f:A sub v = A sub f
_
GL_RGBA :C sub v = C sub t C sub f:C sub v = ( 1 - A sub t ) C sub f :C sub v = (1 - C sub t) C sub f :C sub v = C sub t
4: :+ A sub t C sub t: + C sub t C sub c
: : : :
:A sub v = A sub t A sub f:A sub v = A sub f:A sub v = A sub t A sub f:A sub v = A sub t
.TE
.sp
If pname is GL_TEXTURE_ENV_COLOR ,
params is a pointer to an array that holds an RGBA color consisting of four
values.
Integer color components are interpreted linearly such that the most
positive integer maps to 1.0,
and the most negative integer maps to -1.0.
The values are clamped to the range [0,1] when they are specified.
C sub c takes these four values.
GL_TEXTURE_ENV_MODE defaults to GL_MODULATE and
GL_TEXTURE_ENV_COLOR defaults to (0, 0, 0, 0).
GL.glTexCoord()