glGetActiveUniform — Returns information about an active uniform variable for the specified program object
void glGetActiveUniform( | GLuint | program, |
GLuint | index, | |
GLsizei | bufSize, | |
GLsizei * | length, | |
GLint * | size, | |
GLenum * | type, | |
GLchar * | name) ; |
program
Specifies the program object to be queried.
index
Specifies the index of the uniform variable to be queried.
bufSize
Specifies the maximum number of characters
OpenGL is allowed to write in the character buffer
indicated by name
.
length
Returns the number of characters actually
written by OpenGL in the string indicated by
name
(excluding the null
terminator) if a value other than
NULL
is passed.
size
Returns the size of the uniform variable.
type
Returns the data type of the uniform variable.
name
Returns a null terminated string containing the name of the uniform variable.
glGetActiveUniform
returns
information about an active uniform variable in the program
object specified by program
. The number
of active uniform variables can be obtained by calling
glGetProgram
with the value GL_ACTIVE_UNIFORMS
. A value
of 0 for index
selects the first active
uniform variable. Permissible values for
index
range from 0 to the number of
active uniform variables minus 1.
Shaders may use either built-in uniform variables,
user-defined uniform variables, or both. Built-in uniform
variables have a prefix of "gl_" and reference
existing OpenGL state or values derived from such state (e.g.,
gl_Fog
,
gl_ModelViewMatrix
, etc., see the OpenGL
Shading Language specification for a complete list.)
User-defined uniform variables have arbitrary names and obtain
their values from the application through calls to
glUniform.
A uniform variable (either built-in or user-defined) is
considered active if it is determined during the link operation
that it may be accessed during program execution. Therefore,
program
should have previously been the
target of a call to
glLinkProgram,
but it is not necessary for it to have been linked
successfully.
The size of the character buffer required to store the
longest uniform variable name in program
can be obtained by calling
glGetProgram
with the value
GL_ACTIVE_UNIFORM_MAX_LENGTH
. This value
should be used to allocate a buffer of sufficient size to store
the returned uniform variable name. The size of this character
buffer is passed in bufSize
, and a
pointer to this character buffer is passed in
name.
glGetActiveUniform
returns the name
of the uniform variable indicated by
index
, storing it in the character buffer
specified by name
. The string returned
will be null terminated. The actual number of characters written
into this buffer is returned in length
,
and this count does not include the null termination character.
If the length of the returned string is not required, a value of
NULL
can be passed in the
length
argument.
The type
argument will return a pointer to the uniform variable's data
type. The symbolic constants
GL_FLOAT
,
GL_FLOAT_VEC2
,
GL_FLOAT_VEC3
,
GL_FLOAT_VEC4
,
GL_INT
,
GL_INT_VEC2
,
GL_INT_VEC3
,
GL_INT_VEC4
,
GL_BOOL
,
GL_BOOL_VEC2
,
GL_BOOL_VEC3
,
GL_BOOL_VEC4
,
GL_FLOAT_MAT2
,
GL_FLOAT_MAT3
,
GL_FLOAT_MAT4
,
GL_FLOAT_MAT2x3
,
GL_FLOAT_MAT2x4
,
GL_FLOAT_MAT3x2
,
GL_FLOAT_MAT3x4
,
GL_FLOAT_MAT4x2
,
GL_FLOAT_MAT4x3
,
GL_SAMPLER_1D
,
GL_SAMPLER_2D
,
GL_SAMPLER_3D
,
GL_SAMPLER_CUBE
,
GL_SAMPLER_1D_SHADOW
, or
GL_SAMPLER_2D_SHADOW
may be returned.
If one or more elements of an array are active, the name
of the array is returned in name
, the
type is returned in type
, and the
size
parameter returns the highest array
element index used, plus one, as determined by the compiler
and/or linker. Only one active uniform variable will be reported
for a uniform array.
Uniform variables that are declared as structures or arrays of structures will not be returned directly by this function. Instead, each of these uniform variables will be reduced to its fundamental components containing the "." and "[]" operators such that each of the names is valid as an argument to glGetUniformLocation. Each of these reduced uniform variables is counted as one active uniform variable and is assigned an index. A valid name cannot be a structure, an array of structures, or a subcomponent of a vector or matrix.
The size of the uniform variable will be returned in
size
. Uniform variables other than arrays
will have a size of 1. Structures and arrays of structures will
be reduced as described earlier, such that each of the names
returned will be a data type in the earlier list. If this
reduction results in an array, the size returned will be as
described for uniform arrays; otherwise, the size returned will
be 1.
The list of active uniform variables may include both built-in uniform variables (which begin with the prefix "gl_") as well as user-defined uniform variable names.
This function will return as much information as it can
about the specified active uniform variable. If no information
is available, length
will be 0, and
name
will be an empty string. This
situation could occur if this function is called after a link
operation that failed. If an error occurs, the return values
length
, size
,
type
, and name
will be unmodified.
glGetActiveUniform
is available only
if the GL version is 2.0 or greater.
GL_FLOAT_MAT2x3
,
GL_FLOAT_MAT2x4
,
GL_FLOAT_MAT3x2
,
GL_FLOAT_MAT3x4
,
GL_FLOAT_MAT4x2
, and
GL_FLOAT_MAT4x3
will only be returned as a type
if the GL version is 2.1 or greater.
GL_INVALID_VALUE
is generated if
program
is not a value generated by
OpenGL.
GL_INVALID_OPERATION
is generated if
program
is not a program object.
GL_INVALID_VALUE
is generated if
index
is greater than or equal to the
number of active uniform variables in
program
.
GL_INVALID_OPERATION
is generated if
glGetActiveUniform
is executed between the
execution of
glBegin
and the corresponding execution of
glEnd.
GL_INVALID_VALUE
is generated if
bufSize
is less than 0.
glGet
with argument GL_MAX_VERTEX_UNIFORM_COMPONENTS
or
GL_MAX_FRAGMENT_UNIFORM_COMPONENTS
.
glGetProgram
with argument GL_ACTIVE_UNIFORMS
or
GL_ACTIVE_UNIFORM_MAX_LENGTH
.
Copyright © 2003-2005 3Dlabs Inc. Ltd. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. http://opencontent.org/openpub/.