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WEBGL draw buffers

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.

Availability: 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.

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:

var ext = gl.getExtension('WEBGL_draw_buffers');

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

var 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:

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:

<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	Status	Comment
WEBGL_draw_buffers The definition of 'WEBGL_draw_buffers' in that specification.	Recommendation	Initial definition.

Browser compatibility

Feature	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Servo
Basic Support	?	?	28.0¹	?	?	?	?

Feature	Android	Chrome for Android	Edge Mobile	Firefox for Android	IE Mobile	Opera Mobile	Safari Mobile
Basic Support	?	?	?	?	?	?	?