WEBGL_draw_buffers extension

The WEBGL_draw_buffers extension is part of the WebGL API and enables a fragment shader to write to several textures, which is useful for deferred shading, for example.

WebGL extensions are available using the WebGLRenderingContext.getExtension() method. For more information, see also Using Extensions in the WebGL tutorial.

Note: This extension is only available to WebGL1 contexts. In WebGL2, the functionality of this extension is available on the WebGL2 context by default. In WebGL 2, the constants are available without the "WEBGL" suffix and the new GLSL built-ins require GLSL #version 300 es.

Constants

This extension exposes new constants, which can be used in the gl.framebufferRenderbuffer(), gl.framebufferTexture2D(), gl.getFramebufferAttachmentParameter() ext.drawBuffersWEBGL(), and gl.getParameter() methods.

ext.COLOR_ATTACHMENT0_WEBGL ext.COLOR_ATTACHMENT1_WEBGL ext.COLOR_ATTACHMENT2_WEBGL ext.COLOR_ATTACHMENT3_WEBGL ext.COLOR_ATTACHMENT4_WEBGL ext.COLOR_ATTACHMENT5_WEBGL ext.COLOR_ATTACHMENT6_WEBGL ext.COLOR_ATTACHMENT7_WEBGL ext.COLOR_ATTACHMENT8_WEBGL ext.COLOR_ATTACHMENT9_WEBGL ext.COLOR_ATTACHMENT10_WEBGL ext.COLOR_ATTACHMENT11_WEBGL ext.COLOR_ATTACHMENT12_WEBGL ext.COLOR_ATTACHMENT13_WEBGL ext.COLOR_ATTACHMENT14_WEBGL ext.COLOR_ATTACHMENT15_WEBGL

A GLenum specifying a color buffer.

ext.DRAW_BUFFER0_WEBGL ext.DRAW_BUFFER1_WEBGL ext.DRAW_BUFFER2_WEBGL ext.DRAW_BUFFER3_WEBGL ext.DRAW_BUFFER4_WEBGL ext.DRAW_BUFFER5_WEBGL ext.DRAW_BUFFER6_WEBGL ext.DRAW_BUFFER7_WEBGL ext.DRAW_BUFFER8_WEBGL ext.DRAW_BUFFER9_WEBGL ext.DRAW_BUFFER10_WEBGL ext.DRAW_BUFFER11_WEBGL ext.DRAW_BUFFER12_WEBGL ext.DRAW_BUFFER13_WEBGL ext.DRAW_BUFFER14_WEBGL ext.DRAW_BUFFER15_WEBGL

A GLenum returning a draw buffer.

ext.MAX_COLOR_ATTACHMENTS_WEBGL

A GLint indicating the maximum number of framebuffer color attachment points.

ext.MAX_DRAW_BUFFERS_WEBGL

A GLint indicating the maximum number of draw buffers.

Instance methods

This extension exposes one new method.

ext.drawBuffersWEBGL()

Defines the draw buffers to which all fragment colors are written. (When using WebGL2, this method is available as gl.drawBuffers() by default).

Examples

Enabling the extension:

js

const ext = gl.getExtension("WEBGL_draw_buffers");

Binding multiple textures (to a tx[] array) to different framebuffer color attachments:

js

const fb = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, fb);
gl.framebufferTexture2D(
  gl.FRAMEBUFFER,
  ext.COLOR_ATTACHMENT0_WEBGL,
  gl.TEXTURE_2D,
  tx[0],
  0
);
gl.framebufferTexture2D(
  gl.FRAMEBUFFER,
  ext.COLOR_ATTACHMENT1_WEBGL,
  gl.TEXTURE_2D,
  tx[1],
  0
);
gl.framebufferTexture2D(
  gl.FRAMEBUFFER,
  ext.COLOR_ATTACHMENT2_WEBGL,
  gl.TEXTURE_2D,
  tx[2],
  0
);
gl.framebufferTexture2D(
  gl.FRAMEBUFFER,
  ext.COLOR_ATTACHMENT3_WEBGL,
  gl.TEXTURE_2D,
  tx[3],
  0
);

Mapping the color attachments to draw buffer slots that the fragment shader will write to using gl_FragData:

js

ext.drawBuffersWEBGL([
  ext.COLOR_ATTACHMENT0_WEBGL, // gl_FragData[0]
  ext.COLOR_ATTACHMENT1_WEBGL, // gl_FragData[1]
  ext.COLOR_ATTACHMENT2_WEBGL, // gl_FragData[2]
  ext.COLOR_ATTACHMENT3_WEBGL, // gl_FragData[3]
]);

Shader code that writes to multiple textures:

html

<script type="x-shader/x-fragment">
  #extension GL_EXT_draw_buffers : require

  precision highp float;

  void main(void) {
    gl_FragData[0] = vec4(0.25);
    gl_FragData[1] = vec4(0.5);
    gl_FragData[2] = vec4(0.75);
    gl_FragData[3] = vec4(1.0);
  }
</script>

Specifications

Specification
WebGL WEBGL_draw_buffers Khronos Ratified Extension Specification

Browser compatibility

BCD tables only load in the browser

See also