Parameters

Parameters

Description

OpenGL defines a number of standard vertex attributes that applications can modify with standard API entry points (color, normal, texture coordinates, etc.). The glVertexAttrib family of entry points allows an application to pass generic vertex attributes in numbered locations.

Generic attributes are defined as four-component values that are organized into an array. The first entry of this array is numbered 0, and the size of the array is specified by the implementation-dependent constant GL_MAX_VERTEX_ATTRIBS. Individual elements of this array can be modified with a glVertexAttrib call that specifies the index of the element to be modified and a value for that element.

These commands can be used to specify one, two, three, or all four components of the generic vertex attribute specified by index. A 1 in the name of the command indicates that only one value is passed, and it will be used to modify the first component of the generic vertex attribute. The second and third components will be set to 0, and the fourth component will be set to 1. Similarly, a 2 in the name of the command indicates that values are provided for the first two components, the third component will be set to 0, and the fourth component will be set to 1. A 3 in the name of the command indicates that values are provided for the first three components and the fourth component will be set to 1, whereas a 4 in the name indicates that values are provided for all four components.

The letters s, f, i, d, ub, us, and ui indicate whether the arguments are of type short, float, int, double, unsigned byte, unsigned short, or unsigned int. When v is appended to the name, the commands can take a pointer to an array of such values. The commands containing N indicate that the arguments will be passed as fixed-point values that are scaled to a normalized range according to the component conversion rules defined by the OpenGL specification. Signed values are understood to represent fixed-point values in the range [-1,1], and unsigned values are understood to represent fixed-point values in the range [0,1].

OpenGL Shading Language attribute variables are allowed to be of type mat2, mat3, or mat4. Attributes of these types may be loaded using the glVertexAttrib entry points. Matrices must be loaded into successive generic attribute slots in column major order, with one column of the matrix in each generic attribute slot.

A user-defined attribute variable declared in a vertex shader can be bound to a generic attribute index by calling glBindAttribLocation. This allows an application to use more descriptive variable names in a vertex shader. A subsequent change to the specified generic vertex attribute will be immediately reflected as a change to the corresponding attribute variable in the vertex shader.

The binding between a generic vertex attribute index and a user-defined attribute variable in a vertex shader is part of the state of a program object, but the current value of the generic vertex attribute is not. The value of each generic vertex attribute is part of current state, just like standard vertex attributes, and it is maintained even if a different program object is used.

An application may freely modify generic vertex attributes that are not bound to a named vertex shader attribute variable. These values are simply maintained as part of current state and will not be accessed by the vertex shader. If a generic vertex attribute bound to an attribute variable in a vertex shader is not updated while the vertex shader is executing, the vertex shader will repeatedly use the current value for the generic vertex attribute.

The generic vertex attribute with index 0 is the same as the vertex position attribute previously defined by OpenGL. A glVertex2, glVertex3, or glVertex4 command is completely equivalent to the corresponding glVertexAttrib command with an index argument of 0. A vertex shader can access generic vertex attribute 0 by using the built-in attribute variable gl_Vertex. There are no current values for generic vertex attribute 0. This is the only generic vertex attribute with this property; calls to set other standard vertex attributes can be freely mixed with calls to set any of the other generic vertex attributes.

Notes

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

Generic vertex attributes can be updated at any time. In particular, glVertexAttrib can be called between a call to glBegin and the corresponding call to glEnd.

It is possible for an application to bind more than one attribute name to the same generic vertex attribute index. This is referred to as aliasing, and it is allowed only if just one of the aliased attribute variables is active in the vertex shader, or if no path through the vertex shader consumes more than one of the attributes aliased to the same location. OpenGL implementations are not required to do error checking to detect aliasing, they are allowed to assume that aliasing will not occur, and they are allowed to employ optimizations that work only in the absence of aliasing.

There is no provision for binding standard vertex attributes; therefore, it is not possible to alias generic attributes with standard attributes.

Errors

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

Associated Gets

glGet with the argument GL_CURRENT_PROGRAM

glGetActiveAttrib with argument program and the index of an active attribute variable

glGetAttribLocation with argument program and an attribute variable name

glGetVertexAttrib with arguments GL_CURRENT_VERTEX_ATTRIB and index

See Also

glBindAttribLocation, glVertex, glVertexAttribPointer

Copyright

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/.