WebGLRenderingContext.enableVertexAttribArray()

The WebGLRenderingContext method enableVertexAttribArray(), part of the WebGL API, turns on the generic vertex attribute array at the specified index into the list of attribute arrays.

You can disable the attribute array by calling disableVertexAttribArray().

In WebGL, values that apply to a specific vertex are stored in attributes. These are only available to the JavaScript code and the vertex shader. Attributes are referenced by an index number into the list of attributes maintained by the GPU. Some vertex attribute indexes may have predefined purposes, depending on the platform and/or the GPU. Others are assigned by the WebGL layer when you create the attributes.

Either way, since attributes cannot be used unless enabled, and are enabled by default, you need to call enableVertexAttribArray() to enable individual attributes so that they can be used. Once that's been done, other methods can be used to access the attribute, including vertexAttribPointer(), vertexAttrib*(), and getVertexAttrib().

Syntax

void gl.enableVertexAttribArray(index);

Parameters

index
A GLuint specifying the index number that uniquely identifies the vertex attribute to enable. If you know the name of the attribute but not its index, you can get the index by calling getAttribLocation().

Return value

undefined.

Errors

To check for errors after calling enableVertexAttribArray(), call getError().

WebGLRenderingContext.INVALID_VALUE
The specified index is invalid; that is, it's greater than or equal to the maximum number of entries permitted in the context's vertex attribute list, as indicated by the value of WebGLRenderingContext.MAX_VERTEX_ATTRIBS.

Example

This code — a snippet taken from the full example A basic 2D WebGL animation example — shows the use of enableVertexArray() to activate the attribute that will be used by the WebGL layer to pass individual vertexes from the vertex buffer into the vertex shader function.

gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);

aVertexPosition =
    gl.getAttribLocation(shaderProgram, "aVertexPosition");

gl.enableVertexAttribArray(aVertexPosition);
gl.vertexAttribPointer(aVertexPosition, vertexNumComponents,
      gl.FLOAT, false, 0, 0);

gl.drawArrays(gl.TRIANGLES, 0, vertexCount);
This code snippet is taken from the function animateScene() in "A basic 2D WebGL animation example." See that article for the full sample and to see the resulting animation in action.

This code sets the buffer of vertexes that will be used to draw the triangles of the shape by calling bindBuffer(). Then the vertex position attribute's index is obtained from the shader program by calling getAttribLocation().

With the index of the vertex position attribute now available in aVertexPosition, we call enableVertexAttribArray() to enable the position attribute so it can be used by the shader program (in particular, by the vertex shader).

Then the vertex buffer is bound to the aVertexPosition attribute by calling vertexAttribPointer(). This step is not obvious, since this binding is almost a side effect. But as a result, accessing aVertexPosition now obtains data from the vertex buffer.

With the association in place between the vertex buffer for our shape and the aVertexPosition attribute used to deliver vertexes one by one into the vertex shader, we're ready to draw the shape by calling drawArrays().

Specifications

Specification Status Comment
WebGL 1.0
The definition of 'enableVertexAttribArray' in that specification.
Recommendation Initial definition.
OpenGL ES 2.0
The definition of 'glEnableVertexAttribArray' in that specification.
Standard Man page of the OpenGL API.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support912411125.1
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support Yes25 Yes Yes11128.1

See also

Document Tags and Contributors

 Contributors to this page: Sheppy, fscholz, teoli
 Last updated by: Sheppy,