WebGLRenderingContext: vertexAttribPointer() method
The
WebGLRenderingContext.vertexAttribPointer()
method of the
WebGL API binds the buffer currently bound
to gl.ARRAY_BUFFER
to a generic vertex attribute of the current vertex
buffer object and specifies its layout.
Syntax
js
vertexAttribPointer(index, size, type, normalized, stride, offset)
Parameters
index
-
A
GLuint
specifying the index of the vertex attribute that is to be modified. size
-
A
GLint
specifying the number of components per vertex attribute. Must be 1, 2, 3, or 4. type
-
A
GLenum
specifying the data type of each component in the array. Possible values:gl.BYTE
: signed 8-bit integer, with values in [-128, 127]gl.SHORT
: signed 16-bit integer, with values in [-32768, 32767]gl.UNSIGNED_BYTE
: unsigned 8-bit integer, with values in [0, 255]gl.UNSIGNED_SHORT
: unsigned 16-bit integer, with values in [0,65535]gl.FLOAT
: 32-bit IEEE floating point number
When using a WebGL 2 context, the following values are available additionally:
gl.HALF_FLOAT
: 16-bit IEEE floating point numbergl.INT
: 32-bit signed binary integergl.UNSIGNED_INT
: 32-bit unsigned binary integergl.INT_2_10_10_10_REV
: 32-bit signed integer with values in [-512, 511]gl.UNSIGNED_INT_2_10_10_10_REV
: 32-bit unsigned integer with values in [0, 1023]
normalized
-
A
GLboolean
specifying whether integer data values should be normalized into a certain range when being cast to a float.-
For types
gl.BYTE
andgl.SHORT
, normalizes the values to [-1, 1] if true. -
For types
gl.UNSIGNED_BYTE
andgl.UNSIGNED_SHORT
, normalizes the values to [0, 1] if true. -
For types
gl.FLOAT
andgl.HALF_FLOAT
, this parameter has no effect.
-
For types
stride
-
A
GLsizei
specifying the offset in bytes between the beginning of consecutive vertex attributes. Cannot be negative or larger than 255. If stride is 0, the attribute is assumed to be tightly packed, that is, the attributes are not interleaved but each attribute is in a separate block, and the next vertex' attribute follows immediately after the current vertex. offset
-
A
GLintptr
specifying an offset in bytes of the first component in the vertex attribute array. Must be a multiple of the byte length oftype
.
Return value
None (undefined
).
Exceptions
- A
gl.INVALID_VALUE
error is thrown ifstride
oroffset
are negative. -
A
gl.INVALID_OPERATION
error is thrown ifstride
andoffset
are not multiples of the size of the data type. -
A
gl.INVALID_OPERATION
error is thrown if no WebGLBuffer is bound to the ARRAY_BUFFER target. -
When using a WebGL 2 context, a
gl.INVALID_OPERATION
error is thrown if this vertex attribute is defined as an integer in the vertex shader (e.g.uvec4
orivec4
, instead ofvec4
).
Description
Let's assume we want to render some 3D geometry, and for that we will need to supply
our vertices to the Vertex Shader. Each vertex has a few attributes, like position,
normal vector, or texture coordinate, that are defined in an ArrayBuffer
and will be supplied to the Vertex Buffer Object (VBO). First, we need to bind the
WebGLBuffer
we want to use to gl.ARRAY_BUFFER
, then, with
this method, gl.vertexAttribPointer()
, we specify in what order the
attributes are stored, and what data type they are in. In addition, we need to include
the stride, which is the total byte length of all attributes for one vertex. Also, we
have to call gl.enableVertexAttribArray()
to tell WebGL that this attribute should be filled
with data from our array buffer.
Usually, your 3D geometry is already in a certain binary format, so you need to read
the specification of that specific format to figure out the memory layout. However, if
you are designing the format yourself, or your geometry is in text files (like Wavefront .obj files) and
must be converted into an ArrayBuffer
at runtime, you have free choice on
how to structure the memory. For highest performance, interleave the attributes
and use the smallest data type that still accurately represents your geometry.
The maximum number of vertex attributes depends on the graphics card, and you can call
gl.getParameter(gl.MAX_VERTEX_ATTRIBS)
to get this value. On high-end
graphics cards, the maximum is 16, on lower-end graphics cards, the value will be lower.
Attribute index
For each attribute, you must specify its index. This is independent from the location inside the array buffer, so your attributes can be sent in a different order than how they are stored in the array buffer. You have two options:
-
Either you specify the index yourself. In this case, you call
gl.bindAttribLocation()
to connect a named attribute from the vertex shader to the index you want to use. This must be done before callinggl.linkProgram()
. You can then provide this same index togl.vertexAttribPointer()
. -
Alternatively, you use the index that is assigned by the graphics card when
compiling the vertex shader. Depending on the graphics card, the index will vary, so
you must call
gl.getAttribLocation()
to find out the index, and then provide this index togl.vertexAttribPointer()
. If you are using WebGL 2, you can specify the index yourself in the vertex shader code and override the default used by the graphics card, e.g.layout(location = 3) in vec4 position;
would set the"position"
attribute to index 3.
Integer attributes
While the ArrayBuffer
can be filled with both integers and floats, the
attributes will always be converted to a float when they are sent to the vertex shader.
If you need to use integers in your vertex shader code, you can either cast the float
back to an integer in the vertex shader (e.g. (int) floatNumber
), or use
gl.vertexAttribIPointer()
from WebGL2.
Default attribute values
The vertex shader code may include a number of attributes, but we don't need to specify
the values for each attribute. Instead, we can supply a default value that will be
identical for all vertices. We can call
gl.disableVertexAttribArray()
to tell WebGL to use the default value, while calling
gl.enableVertexAttribArray()
will read the values from the array buffer as
specified with gl.vertexAttribPointer()
.
Similarly, if our vertex shader expects e.g. a 4-component attribute with
vec4
but in our gl.vertexAttribPointer()
call we set the
size
to 2
, then WebGL will set the first two components based
on the array buffer, while the third and fourth components are taken from the default
value.
The default value is vec4(0.0, 0.0, 0.0, 1.0)
by default but we can
specify a different default value with
gl.vertexAttrib[1234]f[v]()
.
For example, your vertex shader may be using a position and a color attribute. Most
meshes have the color specified at a per-vertex level, but some meshes are of a uniform
shade. For those meshes, it is not necessary to place the same color for each vertex
into the array buffer, so you use gl.vertexAttrib4fv()
to set a constant
color.
Querying current settings
You can call gl.getVertexAttrib()
and gl.getVertexAttribOffset()
to get the current parameters for an attribute, e.g. the
data type or whether the attribute should be normalized. Keep in mind that these WebGL
functions have a slow performance and it is better to store the state inside your
JavaScript application. However, these functions are great for debugging a WebGL context
without touching the application code.
Examples
This example shows how to send your vertex attributes to the shader program. We use an imaginary data structure where the attributes of each vertex are stored interleaved with a length of 20 bytes per vertex:
- position: We need to store the X, Y and Z coordinates. For highest precision, we use 32-bit floats; in total this uses 12 bytes.
- normal vector: We need to store the X, Y and Z components of the normal vector, but since precision is not that important, we use 8-bit signed integers. For better performance, we align the data to 32 bits by also storing a fourth zero-valued component, bringing the total size to 4 bytes. Also, we tell WebGL to normalize the values because our normals are always in range [-1, 1].
- texture coordinate: We need to store the U and V coordinates; for this 16-bit unsigned integers offer enough precision, the total size is 4 bytes. We also tell WebGL to normalize the values to [0, 1].
For example, the following vertex:
json
{
"position": [1.0, 2.0, 1.5],
"normal": [1.0, 0.0, 0.0],
"texCoord": [0.5, 0.25]
}
Will be stored in the array buffer as follows:
Creating the array buffer
First, we dynamically create the array buffer from JSON data using a
DataView
. Note the use of true
because WebGL expects our
data to be in little-endian.
js
// Load geometry with fetch() and Response.json()
const response = await fetch("assets/geometry.json");
const vertices = await response.json();
// Create array buffer
const buffer = new ArrayBuffer(20 * vertices.length);
// Fill array buffer
const dv = new DataView(buffer);
vertices.forEach((vertex, i) => {
dv.setFloat32(20 * i, vertex.position[0], true);
dv.setFloat32(20 * i + 4, vertex.position[1], true);
dv.setFloat32(20 * i + 8, vertex.position[2], true);
dv.setInt8(20 * i + 12, vertex.normal[0] * 0x7f);
dv.setInt8(20 * i + 13, vertex.normal[1] * 0x7f);
dv.setInt8(20 * i + 14, vertex.normal[2] * 0x7f);
dv.setInt8(20 * i + 15, 0);
dv.setUint16(20 * i + 16, vertex.texCoord[0] * 0xffff, true);
dv.setUint16(20 * i + 18, vertex.texCoord[1] * 0xffff, true);
});
For higher performance, we could also do the previous JSON to ArrayBuffer conversion on the server-side, e.g. with Node.js. Then we could load the binary file and interpret it as an array buffer:
js
const response = await fetch("assets/geometry.bin");
const buffer = await response.arrayBuffer();
Consume array buffer with WebGL
First, we create a new Vertex Buffer Object (VBO) and supply it with our array buffer:
js
//Bind array buffer to a Vertex Buffer Object
const vbo = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vbo);
gl.bufferData(gl.ARRAY_BUFFER, buffer, gl.STATIC_DRAW);
Then, we specify the memory layout of the array buffer, either by setting the index ourselves:
js
//Describe the layout of the buffer:
//1. position, not normalized
gl.vertexAttribPointer(0, 3, gl.FLOAT, false, 20, 0);
gl.enableVertexAttribArray(0);
//2. normal vector, normalized to [-1, 1]
gl.vertexAttribPointer(1, 4, gl.BYTE, true, 20, 12);
gl.enableVertexAttribArray(1);
//3. texture coordinates, normalized to [0, 1]
gl.vertexAttribPointer(2, 2, gl.UNSIGNED_SHORT, true, 20, 16);
gl.enableVertexAttribArray(2);
//Set the attributes in the vertex shader to the same indices
gl.bindAttribLocation(shaderProgram, 0, "position");
gl.bindAttribLocation(shaderProgram, 1, "normal");
gl.bindAttribLocation(shaderProgram, 2, "texUV");
//Since the attribute indices have changed, we must re-link the shader
//Note that this will reset all uniforms that were previously set.
gl.linkProgram(shaderProgram);
Or we can use the index provided by the graphics card instead of setting the index ourselves; this avoids the re-linking of the shader program.
js
const locPosition = gl.getAttribLocation(shaderProgram, "position");
gl.vertexAttribPointer(locPosition, 3, gl.FLOAT, false, 20, 0);
gl.enableVertexAttribArray(locPosition);
const locNormal = gl.getAttribLocation(shaderProgram, "normal");
gl.vertexAttribPointer(locNormal, 4, gl.BYTE, true, 20, 12);
gl.enableVertexAttribArray(locNormal);
const locTexUV = gl.getAttribLocation(shaderProgram, "texUV");
gl.vertexAttribPointer(locTexUV, 2, gl.UNSIGNED_SHORT, true, 20, 16);
gl.enableVertexAttribArray(locTexUV);
Specifications
Specification |
---|
WebGL Specification # 5.14.10 |
Browser compatibility
BCD tables only load in the browser
See also
- Vertex Specification on the OpenGL wiki
WebGL2RenderingContext.vertexAttribIPointer()