The Tor Browser / file revision

 Name

     EXT_draw_buffers

 Name Strings

     GL_EXT_draw_buffers

 Contributors

     Contributors to GL_NV_draw_buffers

     Contributors to GL_NV_fbo_color_attachments

     Contributors to the OpenGL ES 2.0 specification

     Contributors to the OpenGLSL ES 1.0.17 specification

     Contributors to the OpenGL ES 3.0 specification

     Nicolas Capens, TransGaming Inc.

     Daniel Koch, TransGaming Inc.

     Alastair Patrick, Google Inc.

     Kenneth Russell, Google Inc.

     Greg Roth, NVIDIA Corporation

     Ben Bowman, Imagination Technologies

     Members of the WebGL and OpenGL ES working groups

 Contact

     Daniel Koch (daniel 'at' transgaming.com)

 Status

     Draft Complete 

 Version

     Last Modified Date: January 30, 2013

     Revision: #7

 Number

     TBD

 Dependencies

     OpenGL ES 2.0 is required.

     The extension is written against the OpenGL ES 2.0 specification.

     ANGLE_framebuffer_blit affects the definition of this extension.

     APPLE_framebuffer_multisample affects the definitin of this extension.

 Overview

     This extension increases the number of available framebuffer object

     color attachment points, extends OpenGL ES 2.0 to allow multiple output

     colors, and provides a mechanism for directing those outputs to

     multiple color buffers.

     This extension is similar to the combination of the GL_NV_draw_buffers

     and GL_NV_fbo_color_attachments extensions, but imposes certain

     restrictions informed by the OpenGL ES 3.0 API.

 New Procedures and Functions

       void DrawBuffersEXT(sizei n, const enum *bufs);

 New Tokens

     Accepted by the <pname> parameter of GetIntegerv:

         MAX_COLOR_ATTACHMENTS_EXT             0x8CDF

     Accepted by the <pname> parameters of GetIntegerv and GetFloatv:

         MAX_DRAW_BUFFERS_EXT                  0x8824

         DRAW_BUFFER0_EXT                      0x8825

         DRAW_BUFFER1_EXT                      0x8826

         DRAW_BUFFER2_EXT                      0x8827

         DRAW_BUFFER3_EXT                      0x8828

         DRAW_BUFFER4_EXT                      0x8829

         DRAW_BUFFER5_EXT                      0x882A

         DRAW_BUFFER6_EXT                      0x882B

         DRAW_BUFFER7_EXT                      0x882C

         DRAW_BUFFER8_EXT                      0x882D

         DRAW_BUFFER9_EXT                      0x882E

         DRAW_BUFFER10_EXT                     0x882F

         DRAW_BUFFER11_EXT                     0x8830

         DRAW_BUFFER12_EXT                     0x8831

         DRAW_BUFFER13_EXT                     0x8832

         DRAW_BUFFER14_EXT                     0x8833

         DRAW_BUFFER15_EXT                     0x8834

     Accepted by the <attachment> parameter of FramebufferRenderbuffer,

     FramebufferTexture2D and GetFramebufferAttachmentParameteriv, and by

     the <bufs> parameter of DrawBuffersEXT:

         COLOR_ATTACHMENT0_EXT                      0x8CE0

         COLOR_ATTACHMENT1_EXT                      0x8CE1

         COLOR_ATTACHMENT2_EXT                      0x8CE2

         COLOR_ATTACHMENT3_EXT                      0x8CE3

         COLOR_ATTACHMENT4_EXT                      0x8CE4

         COLOR_ATTACHMENT5_EXT                      0x8CE5

         COLOR_ATTACHMENT6_EXT                      0x8CE6

         COLOR_ATTACHMENT7_EXT                      0x8CE7

         COLOR_ATTACHMENT8_EXT                      0x8CE8

         COLOR_ATTACHMENT9_EXT                      0x8CE9

         COLOR_ATTACHMENT10_EXT                     0x8CEA

         COLOR_ATTACHMENT11_EXT                     0x8CEB

         COLOR_ATTACHMENT12_EXT                     0x8CEC

         COLOR_ATTACHMENT13_EXT                     0x8CED

         COLOR_ATTACHMENT14_EXT                     0x8CEE

         COLOR_ATTACHMENT15_EXT                     0x8CEF

     The COLOR_ATTACHMENT0_EXT constant is equal to the

     COLOR_ATTACHMENT0 constant.

     Each COLOR_ATTACHMENT<i>_EXT adheres to COLOR_ATTACHMENT<i>_EXT

     = COLOR_ATTACHMENT0_EXT + <i>.

 Changes to Chapter 3 of the OpenGL ES 2.0 Specification (Rasterization)

     Section 3.2, (Multisampling). Replace the second paragraph:

     An additional buffer, called the multisample buffer, is added to the

     window system-provided framebuffer. Pixel sample values, including

     color, depth, and stencil values, are stored in this buffer. Samples

     contain separate color values for each fragment color. When the

     window system-provided framebuffer includes a multisample buffer, it

     does not include depth or stencil buffers, even if the multisample

     buffer does not store depth or stencil values. Color buffers do

     coexist with the multisample buffer, however.

     Section 3.8.2, (Shader Execution) Replace subsection "Shader

     Outputs":

     The OpenGL ES Shading Language specification describes the values

     that may be output by a fragment shader. These are gl_FragColor and

     gl_FragData[n].  The final fragment color values or the final

     fragment data values written by a fragment shader are clamped to the

     range [0, 1] and then converted to fixed-point as described in

     section 2.1.2 for framebuffer color components.

     Writing to gl_FragColor specifies the fragment color (color number

     zero) that will be used by subsequent stages of the pipeline.

     Writing to gl_FragData[n] specifies the value of fragment color

     number n. Any colors, or color components, associated with a

     fragment that are not written by the fragment shader are undefined.

     A fragment shader may not statically assign values to both

     gl_FragColor and gl_FragData. In this case, a compile or link error

     will result. A shader statically assigns a value to a variable if,

     after preprocessing, it contains a statement that would write to the

     variable, whether or not run-time flow of control will cause that

     statement to be executed.

 Changes to Chapter 4 of the OpenGL ES 2.0 Specification (Per-Fragment

 Operations and the Frame Buffer)

     Modify the overview of Chapter 4 and replace the sentences

     of the fifth paragraph which read: 

     "The name of the color buffer of an application-created framebuffer

     object is COLOR_ATTACHMENT0. The names of the depth and stencil buffers

     are DEPTH_ATTACHMENT and STENCIL_ATTACHMENT." 

     With the following:

     "A framebuffer object has an array of color buffer attachment points,

     numbered zero through <n>, a depth buffer attachment point, and a

     stencil buffer attachment point." 

     Insert Table 4.3 to Section 4.2.1 (and renumber subsequent tables):

     Symbolic Constant                       Meaning

     -----------------                       ---------------------

     NONE                                    No buffer

     COLOR_ATTACHMENT<i>_EXT (see caption)   Output fragment color to image 

                                             attached at color attachment

                                             point i

     Table 4.3: Arguments to DrawBuffersEXT when the context is bound to a

     framebuffer object, and the buffers they indicate. <i> in

     COLOR_ATTACHMENT<i>_EXT may range from zero to the value of

     MAX_COLOR_ATTACHMENTS_EXT minus one.

     Replace Section 4.2.1, "Selecting a Buffer for Writing" with the following:

     "By default, color values are written into the front buffer for

     single buffered surfaces or into the back buffer for back buffered

     surfaces as determined when making the context current. To control

     the color buffer into which each of the fragment color values is

     written, DrawBuffersEXT is used.

     The command

       void DrawBuffersEXT(sizei n, const enum *bufs);

     defines the draw buffers to which all fragment colors are written.

     <n> specifies the number of buffers in <bufs>. <bufs> is a pointer

     to an array of symbolic constants specifying the buffer to which

     each fragment color is written.

     Each buffer listed in <bufs> must be BACK, NONE, or one of the

     values from table 4.3. Further, acceptable values for the constants

     in <bufs> depend on whether the GL is using the default framebuffer

     (i.e., DRAW_FRAMEBUFFER_BINDING is zero), or a framebuffer object

     (i.e., DRAW_FRAMEBUFFER_BINDING is non-zero). For more information

     about framebuffer objects, see section 4.4.

     If the GL is bound to the default framebuffer, then <n> must be 1

     and the constant must be BACK or NONE. When draw buffer zero is

     BACK, color values are written into the sole buffer for single-

     buffered contexts, or into the back buffer for double-buffered

     contexts. If DrawBuffersEXT is supplied with a constant other than

     BACK and NONE, the error INVALID_OPERATION is generated.

     If the GL is bound to a draw framebuffer object, then each of the

     constants must be one of the values listed in table 4.3.

     In both cases, the draw buffers being defined correspond in order to

     the respective fragment colors. The draw buffer for fragment

     colors beyond <n> is set to NONE.

     The maximum number of draw buffers is implementation-dependent. The

     number of draw buffers supported can be queried by calling

     GetIntegerv with the symbolic constant MAX_DRAW_BUFFERS_EXT. An

     INVALID_VALUE error is generated if <n> is greater than

     MAX_DRAW_BUFFERS_EXT.

     If the GL is bound to a draw framebuffer object, the <i>th buffer listed

     in <bufs> must be COLOR_ATTACHMENT<i>_EXT or NONE. Specifying a

     buffer out of order, BACK, or COLOR_ATTACHMENT<m>_EXT where <m> is

     greater than or equal to the value of MAX_COLOR_ATTACHMENTS_EXT,

     will generate the error INVALID_OPERATION.

     If a fragment shader writes to "gl_FragColor", DrawBuffersEXT 

     specifies the set of draw buffers into which the color 

     written to "gl_FragColor" is written. If a fragment shader writes to

     "gl_FragData", DrawBuffersEXT specifies a set of draw buffers

     into which each of the multiple output colors defined by these

     variables are separately written. If a fragment shader writes to

     neither "gl_FragColor" nor "gl_FragData" the values of the

     fragment colors following shader execution are undefined, and may

     differ for each fragment color.

     Indicating a buffer or buffers using DrawBuffersEXT causes

     subsequent pixel color value writes to affect the indicated

     buffers. If the GL is bound to a draw framebuffer object and a draw

     buffer selects an attachment that has no image attached, then that

     fragment color is not written.

     Specifying NONE as the draw buffer for a fragment color will inhibit

     that fragment color from being written.

     The state required to handle color buffer selection for each

     framebuffer is an integer for each supported fragment color. For the

     default framebuffer, in the initial state the draw buffer for

     fragment color zero is BACK if there is a default framebuffer

     associated with the context, otherwise NONE. For framebuffer

     objects, in the initial state the draw buffer for fragment color

     zero is COLOR_ATTACHMENT0_EXT.

     For both the default framebuffer and framebuffer objects, the

     initial state of draw buffers for fragment colors other than zero is

     NONE.

     The value of the draw buffer selected for fragment color <i> can be

     queried by calling GetIntegerv with the symbolic constant

     DRAW_BUFFER<i>_EXT."

     Modify Section 4.2.3 (Clearing the Buffers) and replace the first

     two paragraphs with the following:

     "The GL provides a means for setting portions of every pixel in a

     particular buffer to the same value.  The argument to

         void Clear(bitfield buf);

     is the bitwise OR of a number of values indicating which buffers are

     to be cleared. The values are COLOR_BUFFER_BIT, DEPTH_BUFFER_BIT, and

     STENCIL_BUFFER_BIT, indicating the buffers currently enabled for color

     writing, the depth buffer, and the stencil buffer (see below), 

     respectively. The value to which each buffer is cleared depends on

     the setting of the clear value for that buffer.  If the mask is not a

     bitwise OR of the specified values, then the error INVALID_VALUE is

     generated.

         void ClearColor(clampf r, clampf, g, clampf b, clampf a);

     sets the clear value for fixed-point color buffers.  Each of the

     specified components is clamped to [0, 1] and converted to fixed-point

     as described in section 2.1.2 for framebuffer color components."

     Replace the second paragraph of Section 4.4.1 (Binding and Managing

     Framebuffer Objects) with the following:

     "The namespace for framebuffer objects is the unsigned integers, with

     zero reserved by OpenGL ES to refer to the default framebuffer. A

     framebuffer object is created by binding an unused name to the

     target FRAMEBUFFER, DRAW_FRAMEBUFFER, or READ_FRAMEBUFFER. The binding

     is effected by calling

         void BindFramebuffer(enum target, uint framebuffer);

     with <target> set the desired framebuffer target and <framebuffer> set 

     to the unused name. The resulting framebuffer object is a new state 

     vector. There is a number of color attachment points, plus one each

     for the depth and stencil attachment points. The number of color attachment

     points is equal to the value of MAX_COLOR_ATTACHMENTS_EXT."

     Replace the third item in the bulleted list in Section 4.4.1 (Binding

     and Managing Framebuffer Objects) with the following:

     " * The only color buffer bitplanes are the ones defined by the

     framebuffer attachments points named COLOR_ATTACHMENT0_EXT through

     COLOR_ATTACHMENT<n>_EXT."

     Modify Section 4.4.3 (Renderbuffer Objects) in the 

     "Attaching Renderbuffer Images to a Framebuffer" subsection as follows:

     Insert the following table:

     Name of attachment

     ---------------------------------------

     COLOR_ATTACHMENT<i>_EXT (see caption)

     DEPTH_ATTACHMENT

     STENCIL_ATTACHMENT

     Table 4.x: Framebuffer attachment points. <i> in COLOR_ATTACHMENT<i>_EXT

     may range from zero to the value of MAX_COLOR_ATTACHMENTS_EXT minus 1.

     Modify the third sentence of the paragraph following the definition of

     FramebufferRenderbuffer to be as follows:

     "<attachment> should be set to one of the attachment points of the 

     framebuffer listed in Table 4.x." 

     Modify Section 4.4.3 (Renderbuffer Objects) in the "Attaching Texture

     Images to a Framebuffer" subsection as follows:

     Modify the last sentence of the paragraph following the definition of

     FramebufferTexture2D to be as follows:

     "<attachment> must be one of the attachment points of the framebuffer

     listed in Table 4.x."

     Modify Section 4.4.5 (Framebuffer Completeness) and replace the 3rd

     item in the bulleted list in the "Framebuffer Attachment Completeness"

     subsection with the following:

     " * If <attachment> is COLOR_ATTACHMENT<i>_EXT, then <image> must

     have a color-renderable internal format."

 Changes to Chapter 6 of the OpenGL ES 2.0 Specification (State and

 State Requests)

     In section 6.1.3 (Enumerated Queries) modify the third sentence in

     the definition of GetFramebufferAttachmentParameteriv to be as follows:

     "<attachment> must be one of the attachment points of the framebuffer

     listed in Table 4.x."

 Changes to Chapter 3 of the OpenGL ES Shading Language 1.0.17 Specification (Basics)

     Add a new section:

     3.4.1 GL_EXT_draw_buffers Extension

     To use the GL_EXT_draw_buffers extension in a shader it

     must be enabled using the #extension directive.

     The shading language preprocessor #define

     GL_EXT_draw_buffers will be defined to 1, if the

     GL_EXT_draw_buffers extension is supported.

 Dependencies on ANGLE_framebuffer_blit and APPLE_framebuffer_multisample:

     If neither ANGLE_framebuffer_blit nor APPLE_framebuffer_multisample are

     supported, then all references to "draw framebuffers" should be replaced

     with references to "framebuffers". References to DRAW_FRAMEBUFFER_BINDING

     should be replaced with references to FRAMEBUFFER_BINDING. References to

     DRAW_FRAMEBUFFER and READ_FRAMEBUFFER should be removed.

     If ANGLE_framebuffer_blit is supported, DRAW_FRAMEBUFFER_BINDING, DRAW_FRAMEBUFFER

     and READ_FRAMEBUFFER all refer to corresponding _ANGLE suffixed names

     (they have the same token values).

     If APPLE_framebuffer_multisample is supported, DRAW_FRAMEBUFFER_BINDING,

     DRAW_FRAMEBUFFER and READ_FRAMEBUFFER all refer to the corresponding _APPLE

     suffixed names (they have the same token values).

 Errors

     The INVALID_OPERATION error is generated if DrawBuffersEXT is called

     when the default framebuffer is bound and any of the following conditions

     hold: 

      - <n> is greater than 1 and less than MAX_DRAW_BUFFERS_EXT,

      - <bufs> contains a value other than BACK or NONE.

     The INVALID_OPERATION error is generated if DrawBuffersEXT is called

     when bound to a draw framebuffer object and any of the following

     conditions hold:

      - the <i>th value in <bufs> is not COLOR_ATTACHMENT<i>_EXT or NONE.

     The INVALID_VALUE error is generated if DrawBuffersEXT is called

     with a value of <n> which is greater than MAX_DRAW_BUFFERS_EXT.

     The INVALID_ENUM error is generated by FramebufferRenderbuffer if

     the <attachment> parameter is not one of the values listed in Table 4.x.

     The INVALID_ENUM error is generated by FramebufferTexture2D if

     the <attachment> parameter is not one of the values listed in Table 4.x.

     The INVALID_ENUM error is generated by GetFramebufferAttachmentParameteriv

     if the <attachment> parameter is not one of the values listed in Table 4.x.

 New State

     Add Table 6.X Framebuffer (State per framebuffer object):

     State               Type Get Command  Initial Value Description 

     ------------------  ---- ------------ ------------- -----------

     DRAW_BUFFER<i>_EXT  Z10* GetIntegerv  see 4.2.1     Draw buffer selected 

 							  for fragment color i

     Add to Table 6.18 (Implementation Dependent Values)

     Get value                 Type Get Cmnd    Minimum Value Description             Sec.

     --------------------      ---- ----------- ------------- -----------             -----

     MAX_DRAW_BUFFERS_EXT      Z+   GetIntegerv 1             Maximum number of       4.2.1

                                                              active draw buffers

     MAX_COLOR_ATTACHMENTS_EXT Z+   GetIntegerv 1             Number of framebuffer   4.4.1

                                                              color attachment points

 Issues

     See ARB_draw_buffers for relevant issues.

   1) What are the differences between this extension and NV_draw_buffers

     + NV_fbo_color_attachments?

     RESOLVED. This extension:

      - adds interactions with blit_framebuffer and the separate 

        draw/read binding points

      - The draw buffer and color attachment limits are global instead

        of per-fbo (see Issue 2)

      - can be used to with default framebuffer to set NONE/BACK (see Issue 4)

      - retains the ordering restrictions on color attachments that are 

        imposed by ES 3.0.

    2) Should the MAX_DRAW_BUFFERS_EXT and MAX_COLOR_ATTACHMENTS_EXT limits

     be per-framebuffer values or implementation dependent constants?

     DISCUSSION: In ARB_draw_buffers this was per-context (see Issue 2).

     EXT_framebuffer_object (and subsequently ARB_framebuffer_object, and GL 3.0

     through GL 4.2) made these queries framebuffer-dependent. 

     However in GL 4.3 and GLES 3.0, these limits were changed from 

     framebuffer-dependent state to implementation-dependent state after

     much discussion (Bug 7990).

     NV_draw_buffers has MAX_DRAW_BUFFERS listed as per-framebuffer state,

     but NV_fbo_color_attachments has MAX_COLOR_ATTACHMENTS as an 

     implementation-dependent constant.

     This is relevant because some implementations are not able to support 

     multisampling in conjuction with multiple color attachments.  If the

     query is per-framebuffer, they can report a maximum of one attachment

     when there are multisampled attachments, but a higher limit when only

     single-sampled attachments are present.

     RESOLVED. Make this global context state as this is most consistent 

     with GLES 3.0 and updated GL drivers. In an implementation cannot

     support multisampling in conjunction with multiple color attachments,

     perhaps such an implementation could report FBO incomplete in this 

     situation, but this is less than desirable.

    3) Should we support broadcast from gl_FragColor to all gl_FragData[x]

     or should it be synonymous with gl_FragData[0]?

     DISCUSSION: With NV_draw_buffers, writing to gl_FragColor writes to all

     the enabled draw buffers (ie broadcast). In OpenGL ES 3.0 when using 

     ESSL 1.0, gl_FragColor is equivalent to writing a single output to 

     gl_FragData[0] and multiple outputs are not possible. When using ESSL 3.0,

     only user-defined out variables may be used.

     If broadcast is supported, some implementations may have to replace

     writes to gl_FragColor with replicated writes to all possible gl_FragData

     locations when this extension is enabled.

     RESOLVED: Writes to gl_FragColor are broadcast to all enabled color

     buffers. ES 3.0 using ESSL 1.0 doesn't support broadcast because

     ESSL 1.0 was not extended to have multiple color outputs (but that is

     what this extension adds). ESSL 3.0 doesn't support the broadcast because

     it doesn't have the gl_FragColor variable at all, and only has user-

     defined out variables. This extension extends ESSL 1.0 to have multiple

     color outputs. Broadcasting from gl_FragColor to all enabled color 

     buffers is the most consistent with existing draw buffer extensions to

     date (both NV_draw_buffers and desktop GL).

    4) Should we allow DrawBuffersEXT to be called when the default FBO is

     bound?

     DISCUSSION: NV_draw_buffers specifies that DrawBuffersNV errors with 

     INVALID_OPERATION when the default FBO is bound. OpenGL ES 3.0 allows

     DrawBuffers to toggle between BACK and NONE on the default FBO.

     An implementation that does not natively support disabling the drawbuffer

     on the default FBO could emulate this by disabling color writes.

     RESOLVED: Allow DrawBuffersEXT to be called for the default FBO. This

     is more forward looking and is compatible with ES 3.0.

    5) What are the requirements on the color attachment sizes and formats?

     RESOLVED: ES 2.0 requires that all color buffers attached to application-

     created framebuffer objects must have the same number of bitplanes

     (Chapter 4 overview p91). ES 2.0 also requires that all attached images

     have the same width and height (Section 4.4.5 Framebuffer Completeness).

     This extension does not lift those requirements, and failing to meet

     them will result in an incomplete FBO.

    6) Does this have any interactions with glClear?

     RESOLVED: Yes. When multiple color buffers are enabled for writing,

     glClear clears all of the color buffers.  Added language clarifying

     that glClear and glClearColor may affect multiple color buffers.

 Revision History

     01/30/2013  dgkoch  add issue 6 and clear interactions

                         renamed to EXT_draw_buffers based on feedback

                         changed resolution of issue 3.

     01/23/2013  dgkoch  add resolutions to issues 2-4.

                         add issue 5.

                         Add Table 4.x and update various explicit

                         references to COLOR_ATTACHMENT0.

                         Add errors.

     11/13/2012  dgkoch  add revision history

                         add text from updated ES 3.0 spec

                         add issues for discussion

     10/16/2012  kbr     update name string

     10/16/2012  kbr     remove restrition requiring draw buffer 0 to be non-NULL

     10/12/2012  kbr     remove references to GetDoublev and ReadBuffer

     10/11/2012  kbr     initial draft extension