glBindAttribLocation

Name

glBindAttribLocation Associates a generic vertex attribute index with a named attribute variable

C Specification

void glBindAttribLocation(GLuint program,
                          GLuint index,
                          const GLchar *name)

Parameters

program	Specifies the handle of the program object in which the association is to be made.
index	Specifies the index of the generic vertex attribute to be bound.
name	Specifies a null terminated string containing the name of the vertex shader attribute variable to which index is to be bound.

Description

glBindAttribLocation associates a user-defined attribute variable in the program object specified by program with a generic vertex attribute index. The name of the user-defined attribute variable is passed as a null terminated string in name. The generic vertex attribute index to be bound to this variable is specified by index. When program is made part of current state, values provided through the generic vertex attribute index modify the value of the user-defined attribute variable specified by name.

If name refers to a matrix attribute variable, index refers to the first column of the matrix. Other matrix columns are then automatically bound to locations index+1 for a matrix of type mat2; index+1 and index+2 for a matrix of type mat3; and index+1, index+2, and index+3 for a matrix of type mat4.

This command makes it possible for vertex shaders to use descriptive names for attribute variables rather than generic variables that are numbered from 0 to GL_MAX_VERTEX_ATTRIBS1. The values sent to each generic attribute index are part of current state, just like standard vertex attributes such as color, normal, and vertex position. If a different program object is made current by calling glUseProgram, the generic vertex attributes are tracked in such a way that the same values will be observed by attributes in the new program object that are also bound to index.

Attribute variable name-to-generic attribute index bindings for a program object can be explicitly assigned at any time with glBindAttribLocation. Attribute bindings do not go into effect until glLinkProgram is called. After a program object has been linked successfully, the index values for generic attributes remain fixed (and their values can be queried) until the next link command occurs.

Applications are not allowed to bind any of the standard OpenGL vertex attributes with this command, because they are bound automatically when needed. Any attribute binding that occurs after the program object has been linked does not take effect until the next time the program object is linked.

Notes

glBindAttribLocation is available only if the GL version is 2.0 or greater.

glBindAttribLocation can be called before any vertex shader objects are bound to the specified program object. It is also permissible to bind a generic attribute index to an attribute variable name that is never used in a vertex shader.

If name was bound previously, that information is lost. Thus, you cannot bind one user-defined attribute variable to multiple indices, but you can bind multiple user-defined attribute variables to the same index.

Applications are allowed to bind more than one user-defined attribute variable to the same generic vertex attribute index. This is called aliasing, and it is allowed only if just one of the aliased attributes is active in the executable program, or if no path through the shader consumes more than one attribute of a set of attributes aliased to the same location. The compiler and linker are allowed to assume that no aliasing is done and are free to employ optimizations that work only in the absence of aliasing. OpenGL implementations are not required to do error checking to detect aliasing. Because there is no way to bind standard attributes, it is not possible to alias generic attributes with conventional ones (except for generic attribute 0).

Active attributes that are not explicitly bound are bound by the linker when glLinkProgram is called. The locations assigned can be queried with glGetAttribLocation.

OpenGL copies the name string when glBindAttribLocation is called, so an application can free its copy of the name string immediately after the function returns.

Errors

GL_INVALID_VALUE is generated if index is greater than or equal to GL_MAX_VERTEX_ATTRIBS.

GL_INVALID_OPERATION is generated if name starts with the reserved prefix "gl_".

GL_INVALID_VALUE is generated if program is not a value generated by OpenGL.

GL_INVALID_OPERATION is generated if program is not of type GL_PROGRAM_OBJECT.

GL_INVALID_OPERATION is generated if glBindAttribLocation is executed between the execution of glBegin and the corresponding execution of glEnd.

Associated Gets

glGet with argument GL_MAX_VERTEX_ATTRIBS

glGetActiveAttrib with argument program

glGetAttribLocation with arguments program and name

glIsProgram

See Also

glDisableVertexAttribArray, glEnableVertexAttribArray, glUseProgram, glVertex-Attrib, glVertexAttribPointer