extension.xml - mozsearch

<?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&lt;GLenum&gt; 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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