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.

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

Browser compatibility

[2] This extension was prefixed with MOZ_ or disabled behind the webgl.enable-draft-extensions preference in prior versions.

See also

• WebGLRenderingContext.getExtension()
• WebGL2RenderingContext.drawBuffers()
• WebGL deferred shading - Mozilla Hacks blog