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<?xml version="1.0"?>
<ratified href="WEBGL_draw_buffers/">
<name>WEBGL_draw_buffers</name>
<contact>
<a href="https://www.khronos.org/webgl/public-mailing-list/">WebGL working group</a> (public_webgl 'at' khronos.org)
</contact>
<contributors>
<contributor>Members of the WebGL working group</contributor>
</contributors>
<number>18</number>
<depends>
<api version="1.0"/>
<core version="2.0">
<glsl version="300 es"/>
</core>
</depends>
<overview>
<mirrors href="https://www.khronos.org/registry/gles/extensions/EXT/EXT_draw_buffers.txt" name="EXT_draw_buffers">
<addendum>
The implementation must support a minimum of 4 color attachments.
</addendum>
<addendum>
The value of the <code>MAX_COLOR_ATTACHMENTS_WEBGL</code> parameter must be greater than or
equal to that of the <code>MAX_DRAW_BUFFERS_WEBGL</code> parameter.
</addendum>
<addendum>
If:
<ul>
<li> A framebuffer's color attachments are all textures allocated with format <code>RGBA</code>
and type <code>UNSIGNED_BYTE</code>, and </li>
<li> The framebuffer has either:
<ul>
<li> No depth or stencil attachment </li>
<li> A valid <code>DEPTH</code> or <code>DEPTH_STENCIL</code> attachment </li>
</ul></li>
</ul>
Then a call to <code>checkFramebufferStatus</code> against this framebuffer must not return
<code>FRAMEBUFFER_UNSUPPORTED</code>. (In other words, the implementation must support the
use of <code>RGBA/UNSIGNED_BYTE</code> textures as color attachments, plus either a
<code>DEPTH</code> or <code>DEPTH_STENCIL</code> attachment.)
</addendum>
<addendum>
Attaching <code>n</code> consecutive color attachments starting at COLOR_ATTACHMENT0_WEBGL,
where <code>n</code> is between 1 and <code>MAX_DRAW_BUFFERS_WEBGL</code>, must not return
<code>FRAMEBUFFER_UNSUPPORTED</code> from a call to <code>checkFramebufferStatus</code>. In
other words, if <code>MAX_DRAW_BUFFERS_WEBGL</code> is 4, then the implementation is
required to support the following combinations of color attachments:
<ul>
<li> <code>COLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTE</code></li>
<li> <code>COLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTE</code><br/>
<code>COLOR_ATTACHMENT1_WEBGL = RGBA/UNSIGNED_BYTE</code><br/></li>
<li> <code>COLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTE</code><br/>
<code>COLOR_ATTACHMENT1_WEBGL = RGBA/UNSIGNED_BYTE</code><br/>
<code>COLOR_ATTACHMENT2_WEBGL = RGBA/UNSIGNED_BYTE</code><br/></li>
<li> <code>COLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTE</code><br/>
<code>COLOR_ATTACHMENT1_WEBGL = RGBA/UNSIGNED_BYTE</code><br/>
<code>COLOR_ATTACHMENT2_WEBGL = RGBA/UNSIGNED_BYTE</code><br/>
<code>COLOR_ATTACHMENT3_WEBGL = RGBA/UNSIGNED_BYTE</code><br/></li>
</ul>
</addendum>
<addendum>
Although the extension name is prefixed with WEBGL the extension must be enabled with the
<code>#extension GL_EXT_draw_buffers</code> directive, as shown in the sample code, to use
the extension in a shader.
Likewise the shading language preprocessor <code>#define GL_EXT_draw_buffers</code>, will be defined to 1 if the extension is supported.
</addendum>
<addendum>
The value of <code>gl_MaxDrawBuffers</code> must match <code>MAX_DRAW_BUFFERS_WEBGL</code> from the API if the extension is enabled in a WebGL context; otherwise, the value must be 1. Whether or not the extension is enabled with the <code>#extension GL_EXT_draw_buffers</code> directive in a shader does not affect the value of <code>gl_MaxDrawBuffers</code>. The value of <code>gl_MaxDrawBuffers</code> is a constant in the shader, and is guaranteed to be frozen at program link time. It is implementation-dependent whether it is frozen at shader compile time. (A consequence is that if a program is linked, and later the WEBGL_draw_buffers extension is enabled, the value of <code>gl_MaxDrawBuffers</code> seen by that program will still be 1.)
</addendum>
<addendum>
If the WEBGL_draw_buffers extension is enabled, but the fragment shader does not contain the <code>#extension GL_EXT_draw_buffers</code> directive to enable it, then writes to <code>gl_FragColor</code> are only written to <code>COLOR_ATTACHMENT0_WEBGL</code>, and not broadcast to all color attachments. In this scenario, other color attachments are guaranteed to remain untouched.
</addendum>
<addendum>
If a fragment shader writes to neither <code>gl_FragColor</code> nor <code>gl_FragData</code>, the values of
the fragment colors following shader execution are untouched.
If a fragment shader contains the <code>#extension GL_EXT_draw_buffers</code> directive, all
<code>gl_FragData</code> variables (from <code>gl_FragData[0]</code> to <code>gl_FragData[MAX_DRAW_BUFFERS_WEBGL - 1]</code>)
default to zero if no values are written to them during a shader execution.
</addendum>
<addendum>
If an image is attached to more than one color attachment point in a framebuffer, <code>checkFramebufferStatus</code>
returns <code>FRAMEBUFFER_UNSUPPORTED</code>. An image can be an individual mip level, or a face of cube map.
</addendum>
</mirrors>
<features>
<feature>
Adds support for multiple color buffers and color outputs from fragment shaders.
</feature>
</features>
</overview>
<idl xml:space="preserve">
[NoInterfaceObject]
interface WEBGL_draw_buffers {
const GLenum COLOR_ATTACHMENT0_WEBGL = 0x8CE0;
const GLenum COLOR_ATTACHMENT1_WEBGL = 0x8CE1;
const GLenum COLOR_ATTACHMENT2_WEBGL = 0x8CE2;
const GLenum COLOR_ATTACHMENT3_WEBGL = 0x8CE3;
const GLenum COLOR_ATTACHMENT4_WEBGL = 0x8CE4;
const GLenum COLOR_ATTACHMENT5_WEBGL = 0x8CE5;
const GLenum COLOR_ATTACHMENT6_WEBGL = 0x8CE6;
const GLenum COLOR_ATTACHMENT7_WEBGL = 0x8CE7;
const GLenum COLOR_ATTACHMENT8_WEBGL = 0x8CE8;
const GLenum COLOR_ATTACHMENT9_WEBGL = 0x8CE9;
const GLenum COLOR_ATTACHMENT10_WEBGL = 0x8CEA;
const GLenum COLOR_ATTACHMENT11_WEBGL = 0x8CEB;
const GLenum COLOR_ATTACHMENT12_WEBGL = 0x8CEC;
const GLenum COLOR_ATTACHMENT13_WEBGL = 0x8CED;
const GLenum COLOR_ATTACHMENT14_WEBGL = 0x8CEE;
const GLenum COLOR_ATTACHMENT15_WEBGL = 0x8CEF;
const GLenum DRAW_BUFFER0_WEBGL = 0x8825;
const GLenum DRAW_BUFFER1_WEBGL = 0x8826;
const GLenum DRAW_BUFFER2_WEBGL = 0x8827;
const GLenum DRAW_BUFFER3_WEBGL = 0x8828;
const GLenum DRAW_BUFFER4_WEBGL = 0x8829;
const GLenum DRAW_BUFFER5_WEBGL = 0x882A;
const GLenum DRAW_BUFFER6_WEBGL = 0x882B;
const GLenum DRAW_BUFFER7_WEBGL = 0x882C;
const GLenum DRAW_BUFFER8_WEBGL = 0x882D;
const GLenum DRAW_BUFFER9_WEBGL = 0x882E;
const GLenum DRAW_BUFFER10_WEBGL = 0x882F;
const GLenum DRAW_BUFFER11_WEBGL = 0x8830;
const GLenum DRAW_BUFFER12_WEBGL = 0x8831;
const GLenum DRAW_BUFFER13_WEBGL = 0x8832;
const GLenum DRAW_BUFFER14_WEBGL = 0x8833;
const GLenum DRAW_BUFFER15_WEBGL = 0x8834;
const GLenum MAX_COLOR_ATTACHMENTS_WEBGL = 0x8CDF;
const GLenum MAX_DRAW_BUFFERS_WEBGL = 0x8824;
void drawBuffersWEBGL(sequence<GLenum> buffers);
};
</idl>
<samplecode xml:space="preserve">
<pre>
#extension GL_EXT_draw_buffers : require
precision mediump float;
void main() {
gl_FragData[0] = vec4(1.0, 0.0, 0.0, 1.0);
gl_FragData[1] = vec4(0.0, 1.0, 0.0, 1.0);
gl_FragData[2] = vec4(0.0, 0.0, 1.0, 1.0);
gl_FragData[3] = vec4(1.0, 1.0, 1.0, 1.0);
}
</pre>
</samplecode>
<history>
<revision date="2012/10/16">
<change>Initial revision.</change>
</revision>
<revision date="2012/11/05">
<change>Corrected typo in drawBuffersWEBGL. Changed referenced spec version to 1.0 to fix broken link.</change>
</revision>
<revision date="2013/02/02">
<change>Renamed to EXT_draw_buffers per plan of OpenGL ES working group. Moved to draft status. Assigned number.</change>
</revision>
<revision date="2013/03/05">
<change>Renamed to WEBGL_draw_buffers per discussion on public_webgl list. Added guarantees to make it easier for developers to use the extension.</change>
</revision>
<revision date="2013/06/07">
<change>Clarified naming of shader directives.</change>
</revision>
<revision date="2014/07/15">
<change>Added NoInterfaceObject extended attribute.</change>
</revision>
<revision date="2014/08/08">
<change>Ratified by Khronos Board of Promoters.</change>
</revision>
<revision date="2016/06/28">
<change>Revised behavior of gl_MaxDrawBuffers and gl_FragColor broadcasting, to avoid significant performance impact for WebGL 1.0 implementations running on top of the desktop OpenGL API.</change>
</revision>
<revision date="2016/07/08">
<change>Removed undefined behaviors.</change>
</revision>
<revision date="2016/07/11">
<change>Revised user-defined variable behavior to default to zero.</change>
</revision>
<revision date="2016/07/23">
<change>Revised behavior of the same image is attached to more than one color attachment point.</change>
</revision>
</history>
</ratified>
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