1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/gfx/angle/extensions/EXT_draw_buffers.txt Wed Dec 31 06:09:35 2014 +0100
1.3 @@ -0,0 +1,542 @@
1.4 +Name
1.5 +
1.6 + EXT_draw_buffers
1.7 +
1.8 +Name Strings
1.9 +
1.10 + GL_EXT_draw_buffers
1.11 +
1.12 +Contributors
1.13 +
1.14 + Contributors to GL_NV_draw_buffers
1.15 + Contributors to GL_NV_fbo_color_attachments
1.16 + Contributors to the OpenGL ES 2.0 specification
1.17 + Contributors to the OpenGLSL ES 1.0.17 specification
1.18 + Contributors to the OpenGL ES 3.0 specification
1.19 + Nicolas Capens, TransGaming Inc.
1.20 + Daniel Koch, TransGaming Inc.
1.21 + Alastair Patrick, Google Inc.
1.22 + Kenneth Russell, Google Inc.
1.23 + Greg Roth, NVIDIA Corporation
1.24 + Ben Bowman, Imagination Technologies
1.25 + Members of the WebGL and OpenGL ES working groups
1.26 +
1.27 +Contact
1.28 +
1.29 + Daniel Koch (daniel 'at' transgaming.com)
1.30 +
1.31 +Status
1.32 +
1.33 + Draft Complete
1.34 +
1.35 +Version
1.36 +
1.37 + Last Modified Date: January 30, 2013
1.38 + Revision: #7
1.39 +
1.40 +Number
1.41 +
1.42 + TBD
1.43 +
1.44 +Dependencies
1.45 +
1.46 + OpenGL ES 2.0 is required.
1.47 +
1.48 + The extension is written against the OpenGL ES 2.0 specification.
1.49 +
1.50 + ANGLE_framebuffer_blit affects the definition of this extension.
1.51 + APPLE_framebuffer_multisample affects the definitin of this extension.
1.52 +
1.53 +Overview
1.54 +
1.55 + This extension increases the number of available framebuffer object
1.56 + color attachment points, extends OpenGL ES 2.0 to allow multiple output
1.57 + colors, and provides a mechanism for directing those outputs to
1.58 + multiple color buffers.
1.59 +
1.60 + This extension is similar to the combination of the GL_NV_draw_buffers
1.61 + and GL_NV_fbo_color_attachments extensions, but imposes certain
1.62 + restrictions informed by the OpenGL ES 3.0 API.
1.63 +
1.64 +New Procedures and Functions
1.65 +
1.66 + void DrawBuffersEXT(sizei n, const enum *bufs);
1.67 +
1.68 +New Tokens
1.69 +
1.70 + Accepted by the <pname> parameter of GetIntegerv:
1.71 +
1.72 + MAX_COLOR_ATTACHMENTS_EXT 0x8CDF
1.73 +
1.74 + Accepted by the <pname> parameters of GetIntegerv and GetFloatv:
1.75 +
1.76 + MAX_DRAW_BUFFERS_EXT 0x8824
1.77 + DRAW_BUFFER0_EXT 0x8825
1.78 + DRAW_BUFFER1_EXT 0x8826
1.79 + DRAW_BUFFER2_EXT 0x8827
1.80 + DRAW_BUFFER3_EXT 0x8828
1.81 + DRAW_BUFFER4_EXT 0x8829
1.82 + DRAW_BUFFER5_EXT 0x882A
1.83 + DRAW_BUFFER6_EXT 0x882B
1.84 + DRAW_BUFFER7_EXT 0x882C
1.85 + DRAW_BUFFER8_EXT 0x882D
1.86 + DRAW_BUFFER9_EXT 0x882E
1.87 + DRAW_BUFFER10_EXT 0x882F
1.88 + DRAW_BUFFER11_EXT 0x8830
1.89 + DRAW_BUFFER12_EXT 0x8831
1.90 + DRAW_BUFFER13_EXT 0x8832
1.91 + DRAW_BUFFER14_EXT 0x8833
1.92 + DRAW_BUFFER15_EXT 0x8834
1.93 +
1.94 + Accepted by the <attachment> parameter of FramebufferRenderbuffer,
1.95 + FramebufferTexture2D and GetFramebufferAttachmentParameteriv, and by
1.96 + the <bufs> parameter of DrawBuffersEXT:
1.97 +
1.98 + COLOR_ATTACHMENT0_EXT 0x8CE0
1.99 + COLOR_ATTACHMENT1_EXT 0x8CE1
1.100 + COLOR_ATTACHMENT2_EXT 0x8CE2
1.101 + COLOR_ATTACHMENT3_EXT 0x8CE3
1.102 + COLOR_ATTACHMENT4_EXT 0x8CE4
1.103 + COLOR_ATTACHMENT5_EXT 0x8CE5
1.104 + COLOR_ATTACHMENT6_EXT 0x8CE6
1.105 + COLOR_ATTACHMENT7_EXT 0x8CE7
1.106 + COLOR_ATTACHMENT8_EXT 0x8CE8
1.107 + COLOR_ATTACHMENT9_EXT 0x8CE9
1.108 + COLOR_ATTACHMENT10_EXT 0x8CEA
1.109 + COLOR_ATTACHMENT11_EXT 0x8CEB
1.110 + COLOR_ATTACHMENT12_EXT 0x8CEC
1.111 + COLOR_ATTACHMENT13_EXT 0x8CED
1.112 + COLOR_ATTACHMENT14_EXT 0x8CEE
1.113 + COLOR_ATTACHMENT15_EXT 0x8CEF
1.114 +
1.115 + The COLOR_ATTACHMENT0_EXT constant is equal to the
1.116 + COLOR_ATTACHMENT0 constant.
1.117 +
1.118 + Each COLOR_ATTACHMENT<i>_EXT adheres to COLOR_ATTACHMENT<i>_EXT
1.119 + = COLOR_ATTACHMENT0_EXT + <i>.
1.120 +
1.121 +Changes to Chapter 3 of the OpenGL ES 2.0 Specification (Rasterization)
1.122 +
1.123 + Section 3.2, (Multisampling). Replace the second paragraph:
1.124 +
1.125 + An additional buffer, called the multisample buffer, is added to the
1.126 + window system-provided framebuffer. Pixel sample values, including
1.127 + color, depth, and stencil values, are stored in this buffer. Samples
1.128 + contain separate color values for each fragment color. When the
1.129 + window system-provided framebuffer includes a multisample buffer, it
1.130 + does not include depth or stencil buffers, even if the multisample
1.131 + buffer does not store depth or stencil values. Color buffers do
1.132 + coexist with the multisample buffer, however.
1.133 +
1.134 + Section 3.8.2, (Shader Execution) Replace subsection "Shader
1.135 + Outputs":
1.136 +
1.137 + The OpenGL ES Shading Language specification describes the values
1.138 + that may be output by a fragment shader. These are gl_FragColor and
1.139 + gl_FragData[n]. The final fragment color values or the final
1.140 + fragment data values written by a fragment shader are clamped to the
1.141 + range [0, 1] and then converted to fixed-point as described in
1.142 + section 2.1.2 for framebuffer color components.
1.143 +
1.144 + Writing to gl_FragColor specifies the fragment color (color number
1.145 + zero) that will be used by subsequent stages of the pipeline.
1.146 + Writing to gl_FragData[n] specifies the value of fragment color
1.147 + number n. Any colors, or color components, associated with a
1.148 + fragment that are not written by the fragment shader are undefined.
1.149 + A fragment shader may not statically assign values to both
1.150 + gl_FragColor and gl_FragData. In this case, a compile or link error
1.151 + will result. A shader statically assigns a value to a variable if,
1.152 + after preprocessing, it contains a statement that would write to the
1.153 + variable, whether or not run-time flow of control will cause that
1.154 + statement to be executed.
1.155 +
1.156 +Changes to Chapter 4 of the OpenGL ES 2.0 Specification (Per-Fragment
1.157 +Operations and the Frame Buffer)
1.158 +
1.159 + Modify the overview of Chapter 4 and replace the sentences
1.160 + of the fifth paragraph which read:
1.161 +
1.162 + "The name of the color buffer of an application-created framebuffer
1.163 + object is COLOR_ATTACHMENT0. The names of the depth and stencil buffers
1.164 + are DEPTH_ATTACHMENT and STENCIL_ATTACHMENT."
1.165 +
1.166 + With the following:
1.167 +
1.168 + "A framebuffer object has an array of color buffer attachment points,
1.169 + numbered zero through <n>, a depth buffer attachment point, and a
1.170 + stencil buffer attachment point."
1.171 +
1.172 + Insert Table 4.3 to Section 4.2.1 (and renumber subsequent tables):
1.173 +
1.174 + Symbolic Constant Meaning
1.175 + ----------------- ---------------------
1.176 + NONE No buffer
1.177 +
1.178 + COLOR_ATTACHMENT<i>_EXT (see caption) Output fragment color to image
1.179 + attached at color attachment
1.180 + point i
1.181 +
1.182 + Table 4.3: Arguments to DrawBuffersEXT when the context is bound to a
1.183 + framebuffer object, and the buffers they indicate. <i> in
1.184 + COLOR_ATTACHMENT<i>_EXT may range from zero to the value of
1.185 + MAX_COLOR_ATTACHMENTS_EXT minus one.
1.186 +
1.187 + Replace Section 4.2.1, "Selecting a Buffer for Writing" with the following:
1.188 +
1.189 + "By default, color values are written into the front buffer for
1.190 + single buffered surfaces or into the back buffer for back buffered
1.191 + surfaces as determined when making the context current. To control
1.192 + the color buffer into which each of the fragment color values is
1.193 + written, DrawBuffersEXT is used.
1.194 +
1.195 + The command
1.196 +
1.197 + void DrawBuffersEXT(sizei n, const enum *bufs);
1.198 +
1.199 + defines the draw buffers to which all fragment colors are written.
1.200 + <n> specifies the number of buffers in <bufs>. <bufs> is a pointer
1.201 + to an array of symbolic constants specifying the buffer to which
1.202 + each fragment color is written.
1.203 +
1.204 + Each buffer listed in <bufs> must be BACK, NONE, or one of the
1.205 + values from table 4.3. Further, acceptable values for the constants
1.206 + in <bufs> depend on whether the GL is using the default framebuffer
1.207 + (i.e., DRAW_FRAMEBUFFER_BINDING is zero), or a framebuffer object
1.208 + (i.e., DRAW_FRAMEBUFFER_BINDING is non-zero). For more information
1.209 + about framebuffer objects, see section 4.4.
1.210 +
1.211 + If the GL is bound to the default framebuffer, then <n> must be 1
1.212 + and the constant must be BACK or NONE. When draw buffer zero is
1.213 + BACK, color values are written into the sole buffer for single-
1.214 + buffered contexts, or into the back buffer for double-buffered
1.215 + contexts. If DrawBuffersEXT is supplied with a constant other than
1.216 + BACK and NONE, the error INVALID_OPERATION is generated.
1.217 +
1.218 + If the GL is bound to a draw framebuffer object, then each of the
1.219 + constants must be one of the values listed in table 4.3.
1.220 +
1.221 + In both cases, the draw buffers being defined correspond in order to
1.222 + the respective fragment colors. The draw buffer for fragment
1.223 + colors beyond <n> is set to NONE.
1.224 +
1.225 + The maximum number of draw buffers is implementation-dependent. The
1.226 + number of draw buffers supported can be queried by calling
1.227 + GetIntegerv with the symbolic constant MAX_DRAW_BUFFERS_EXT. An
1.228 + INVALID_VALUE error is generated if <n> is greater than
1.229 + MAX_DRAW_BUFFERS_EXT.
1.230 +
1.231 + If the GL is bound to a draw framebuffer object, the <i>th buffer listed
1.232 + in <bufs> must be COLOR_ATTACHMENT<i>_EXT or NONE. Specifying a
1.233 + buffer out of order, BACK, or COLOR_ATTACHMENT<m>_EXT where <m> is
1.234 + greater than or equal to the value of MAX_COLOR_ATTACHMENTS_EXT,
1.235 + will generate the error INVALID_OPERATION.
1.236 +
1.237 + If a fragment shader writes to "gl_FragColor", DrawBuffersEXT
1.238 + specifies the set of draw buffers into which the color
1.239 + written to "gl_FragColor" is written. If a fragment shader writes to
1.240 + "gl_FragData", DrawBuffersEXT specifies a set of draw buffers
1.241 + into which each of the multiple output colors defined by these
1.242 + variables are separately written. If a fragment shader writes to
1.243 + neither "gl_FragColor" nor "gl_FragData" the values of the
1.244 + fragment colors following shader execution are undefined, and may
1.245 + differ for each fragment color.
1.246 +
1.247 + Indicating a buffer or buffers using DrawBuffersEXT causes
1.248 + subsequent pixel color value writes to affect the indicated
1.249 + buffers. If the GL is bound to a draw framebuffer object and a draw
1.250 + buffer selects an attachment that has no image attached, then that
1.251 + fragment color is not written.
1.252 +
1.253 + Specifying NONE as the draw buffer for a fragment color will inhibit
1.254 + that fragment color from being written.
1.255 +
1.256 + The state required to handle color buffer selection for each
1.257 + framebuffer is an integer for each supported fragment color. For the
1.258 + default framebuffer, in the initial state the draw buffer for
1.259 + fragment color zero is BACK if there is a default framebuffer
1.260 + associated with the context, otherwise NONE. For framebuffer
1.261 + objects, in the initial state the draw buffer for fragment color
1.262 + zero is COLOR_ATTACHMENT0_EXT.
1.263 +
1.264 + For both the default framebuffer and framebuffer objects, the
1.265 + initial state of draw buffers for fragment colors other than zero is
1.266 + NONE.
1.267 +
1.268 + The value of the draw buffer selected for fragment color <i> can be
1.269 + queried by calling GetIntegerv with the symbolic constant
1.270 + DRAW_BUFFER<i>_EXT."
1.271 +
1.272 + Modify Section 4.2.3 (Clearing the Buffers) and replace the first
1.273 + two paragraphs with the following:
1.274 +
1.275 + "The GL provides a means for setting portions of every pixel in a
1.276 + particular buffer to the same value. The argument to
1.277 +
1.278 + void Clear(bitfield buf);
1.279 +
1.280 + is the bitwise OR of a number of values indicating which buffers are
1.281 + to be cleared. The values are COLOR_BUFFER_BIT, DEPTH_BUFFER_BIT, and
1.282 + STENCIL_BUFFER_BIT, indicating the buffers currently enabled for color
1.283 + writing, the depth buffer, and the stencil buffer (see below),
1.284 + respectively. The value to which each buffer is cleared depends on
1.285 + the setting of the clear value for that buffer. If the mask is not a
1.286 + bitwise OR of the specified values, then the error INVALID_VALUE is
1.287 + generated.
1.288 +
1.289 + void ClearColor(clampf r, clampf, g, clampf b, clampf a);
1.290 +
1.291 + sets the clear value for fixed-point color buffers. Each of the
1.292 + specified components is clamped to [0, 1] and converted to fixed-point
1.293 + as described in section 2.1.2 for framebuffer color components."
1.294 +
1.295 + Replace the second paragraph of Section 4.4.1 (Binding and Managing
1.296 + Framebuffer Objects) with the following:
1.297 +
1.298 + "The namespace for framebuffer objects is the unsigned integers, with
1.299 + zero reserved by OpenGL ES to refer to the default framebuffer. A
1.300 + framebuffer object is created by binding an unused name to the
1.301 + target FRAMEBUFFER, DRAW_FRAMEBUFFER, or READ_FRAMEBUFFER. The binding
1.302 + is effected by calling
1.303 +
1.304 + void BindFramebuffer(enum target, uint framebuffer);
1.305 +
1.306 + with <target> set the desired framebuffer target and <framebuffer> set
1.307 + to the unused name. The resulting framebuffer object is a new state
1.308 + vector. There is a number of color attachment points, plus one each
1.309 + for the depth and stencil attachment points. The number of color attachment
1.310 + points is equal to the value of MAX_COLOR_ATTACHMENTS_EXT."
1.311 +
1.312 + Replace the third item in the bulleted list in Section 4.4.1 (Binding
1.313 + and Managing Framebuffer Objects) with the following:
1.314 +
1.315 + " * The only color buffer bitplanes are the ones defined by the
1.316 + framebuffer attachments points named COLOR_ATTACHMENT0_EXT through
1.317 + COLOR_ATTACHMENT<n>_EXT."
1.318 +
1.319 + Modify Section 4.4.3 (Renderbuffer Objects) in the
1.320 + "Attaching Renderbuffer Images to a Framebuffer" subsection as follows:
1.321 +
1.322 + Insert the following table:
1.323 +
1.324 + Name of attachment
1.325 + ---------------------------------------
1.326 + COLOR_ATTACHMENT<i>_EXT (see caption)
1.327 + DEPTH_ATTACHMENT
1.328 + STENCIL_ATTACHMENT
1.329 +
1.330 + Table 4.x: Framebuffer attachment points. <i> in COLOR_ATTACHMENT<i>_EXT
1.331 + may range from zero to the value of MAX_COLOR_ATTACHMENTS_EXT minus 1.
1.332 +
1.333 + Modify the third sentence of the paragraph following the definition of
1.334 + FramebufferRenderbuffer to be as follows:
1.335 +
1.336 + "<attachment> should be set to one of the attachment points of the
1.337 + framebuffer listed in Table 4.x."
1.338 +
1.339 + Modify Section 4.4.3 (Renderbuffer Objects) in the "Attaching Texture
1.340 + Images to a Framebuffer" subsection as follows:
1.341 +
1.342 + Modify the last sentence of the paragraph following the definition of
1.343 + FramebufferTexture2D to be as follows:
1.344 +
1.345 + "<attachment> must be one of the attachment points of the framebuffer
1.346 + listed in Table 4.x."
1.347 +
1.348 + Modify Section 4.4.5 (Framebuffer Completeness) and replace the 3rd
1.349 + item in the bulleted list in the "Framebuffer Attachment Completeness"
1.350 + subsection with the following:
1.351 +
1.352 + " * If <attachment> is COLOR_ATTACHMENT<i>_EXT, then <image> must
1.353 + have a color-renderable internal format."
1.354 +
1.355 +Changes to Chapter 6 of the OpenGL ES 2.0 Specification (State and
1.356 +State Requests)
1.357 +
1.358 + In section 6.1.3 (Enumerated Queries) modify the third sentence in
1.359 + the definition of GetFramebufferAttachmentParameteriv to be as follows:
1.360 +
1.361 + "<attachment> must be one of the attachment points of the framebuffer
1.362 + listed in Table 4.x."
1.363 +
1.364 +Changes to Chapter 3 of the OpenGL ES Shading Language 1.0.17 Specification (Basics)
1.365 +
1.366 + Add a new section:
1.367 +
1.368 + 3.4.1 GL_EXT_draw_buffers Extension
1.369 +
1.370 + To use the GL_EXT_draw_buffers extension in a shader it
1.371 + must be enabled using the #extension directive.
1.372 +
1.373 + The shading language preprocessor #define
1.374 + GL_EXT_draw_buffers will be defined to 1, if the
1.375 + GL_EXT_draw_buffers extension is supported.
1.376 +
1.377 +Dependencies on ANGLE_framebuffer_blit and APPLE_framebuffer_multisample:
1.378 +
1.379 + If neither ANGLE_framebuffer_blit nor APPLE_framebuffer_multisample are
1.380 + supported, then all references to "draw framebuffers" should be replaced
1.381 + with references to "framebuffers". References to DRAW_FRAMEBUFFER_BINDING
1.382 + should be replaced with references to FRAMEBUFFER_BINDING. References to
1.383 + DRAW_FRAMEBUFFER and READ_FRAMEBUFFER should be removed.
1.384 +
1.385 + If ANGLE_framebuffer_blit is supported, DRAW_FRAMEBUFFER_BINDING, DRAW_FRAMEBUFFER
1.386 + and READ_FRAMEBUFFER all refer to corresponding _ANGLE suffixed names
1.387 + (they have the same token values).
1.388 +
1.389 + If APPLE_framebuffer_multisample is supported, DRAW_FRAMEBUFFER_BINDING,
1.390 + DRAW_FRAMEBUFFER and READ_FRAMEBUFFER all refer to the corresponding _APPLE
1.391 + suffixed names (they have the same token values).
1.392 +
1.393 +Errors
1.394 +
1.395 + The INVALID_OPERATION error is generated if DrawBuffersEXT is called
1.396 + when the default framebuffer is bound and any of the following conditions
1.397 + hold:
1.398 + - <n> is greater than 1 and less than MAX_DRAW_BUFFERS_EXT,
1.399 + - <bufs> contains a value other than BACK or NONE.
1.400 +
1.401 + The INVALID_OPERATION error is generated if DrawBuffersEXT is called
1.402 + when bound to a draw framebuffer object and any of the following
1.403 + conditions hold:
1.404 + - the <i>th value in <bufs> is not COLOR_ATTACHMENT<i>_EXT or NONE.
1.405 +
1.406 + The INVALID_VALUE error is generated if DrawBuffersEXT is called
1.407 + with a value of <n> which is greater than MAX_DRAW_BUFFERS_EXT.
1.408 +
1.409 + The INVALID_ENUM error is generated by FramebufferRenderbuffer if
1.410 + the <attachment> parameter is not one of the values listed in Table 4.x.
1.411 +
1.412 + The INVALID_ENUM error is generated by FramebufferTexture2D if
1.413 + the <attachment> parameter is not one of the values listed in Table 4.x.
1.414 +
1.415 + The INVALID_ENUM error is generated by GetFramebufferAttachmentParameteriv
1.416 + if the <attachment> parameter is not one of the values listed in Table 4.x.
1.417 +
1.418 +New State
1.419 +
1.420 + Add Table 6.X Framebuffer (State per framebuffer object):
1.421 +
1.422 + State Type Get Command Initial Value Description
1.423 + ------------------ ---- ------------ ------------- -----------
1.424 + DRAW_BUFFER<i>_EXT Z10* GetIntegerv see 4.2.1 Draw buffer selected
1.425 + for fragment color i
1.426 +
1.427 + Add to Table 6.18 (Implementation Dependent Values)
1.428 +
1.429 + Get value Type Get Cmnd Minimum Value Description Sec.
1.430 + -------------------- ---- ----------- ------------- ----------- -----
1.431 + MAX_DRAW_BUFFERS_EXT Z+ GetIntegerv 1 Maximum number of 4.2.1
1.432 + active draw buffers
1.433 + MAX_COLOR_ATTACHMENTS_EXT Z+ GetIntegerv 1 Number of framebuffer 4.4.1
1.434 + color attachment points
1.435 +Issues
1.436 +
1.437 + See ARB_draw_buffers for relevant issues.
1.438 +
1.439 + 1) What are the differences between this extension and NV_draw_buffers
1.440 + + NV_fbo_color_attachments?
1.441 +
1.442 + RESOLVED. This extension:
1.443 + - adds interactions with blit_framebuffer and the separate
1.444 + draw/read binding points
1.445 + - The draw buffer and color attachment limits are global instead
1.446 + of per-fbo (see Issue 2)
1.447 + - can be used to with default framebuffer to set NONE/BACK (see Issue 4)
1.448 + - retains the ordering restrictions on color attachments that are
1.449 + imposed by ES 3.0.
1.450 +
1.451 + 2) Should the MAX_DRAW_BUFFERS_EXT and MAX_COLOR_ATTACHMENTS_EXT limits
1.452 + be per-framebuffer values or implementation dependent constants?
1.453 +
1.454 + DISCUSSION: In ARB_draw_buffers this was per-context (see Issue 2).
1.455 + EXT_framebuffer_object (and subsequently ARB_framebuffer_object, and GL 3.0
1.456 + through GL 4.2) made these queries framebuffer-dependent.
1.457 + However in GL 4.3 and GLES 3.0, these limits were changed from
1.458 + framebuffer-dependent state to implementation-dependent state after
1.459 + much discussion (Bug 7990).
1.460 +
1.461 + NV_draw_buffers has MAX_DRAW_BUFFERS listed as per-framebuffer state,
1.462 + but NV_fbo_color_attachments has MAX_COLOR_ATTACHMENTS as an
1.463 + implementation-dependent constant.
1.464 +
1.465 + This is relevant because some implementations are not able to support
1.466 + multisampling in conjuction with multiple color attachments. If the
1.467 + query is per-framebuffer, they can report a maximum of one attachment
1.468 + when there are multisampled attachments, but a higher limit when only
1.469 + single-sampled attachments are present.
1.470 +
1.471 + RESOLVED. Make this global context state as this is most consistent
1.472 + with GLES 3.0 and updated GL drivers. In an implementation cannot
1.473 + support multisampling in conjunction with multiple color attachments,
1.474 + perhaps such an implementation could report FBO incomplete in this
1.475 + situation, but this is less than desirable.
1.476 +
1.477 + 3) Should we support broadcast from gl_FragColor to all gl_FragData[x]
1.478 + or should it be synonymous with gl_FragData[0]?
1.479 +
1.480 + DISCUSSION: With NV_draw_buffers, writing to gl_FragColor writes to all
1.481 + the enabled draw buffers (ie broadcast). In OpenGL ES 3.0 when using
1.482 + ESSL 1.0, gl_FragColor is equivalent to writing a single output to
1.483 + gl_FragData[0] and multiple outputs are not possible. When using ESSL 3.0,
1.484 + only user-defined out variables may be used.
1.485 +
1.486 + If broadcast is supported, some implementations may have to replace
1.487 + writes to gl_FragColor with replicated writes to all possible gl_FragData
1.488 + locations when this extension is enabled.
1.489 +
1.490 + RESOLVED: Writes to gl_FragColor are broadcast to all enabled color
1.491 + buffers. ES 3.0 using ESSL 1.0 doesn't support broadcast because
1.492 + ESSL 1.0 was not extended to have multiple color outputs (but that is
1.493 + what this extension adds). ESSL 3.0 doesn't support the broadcast because
1.494 + it doesn't have the gl_FragColor variable at all, and only has user-
1.495 + defined out variables. This extension extends ESSL 1.0 to have multiple
1.496 + color outputs. Broadcasting from gl_FragColor to all enabled color
1.497 + buffers is the most consistent with existing draw buffer extensions to
1.498 + date (both NV_draw_buffers and desktop GL).
1.499 +
1.500 + 4) Should we allow DrawBuffersEXT to be called when the default FBO is
1.501 + bound?
1.502 +
1.503 + DISCUSSION: NV_draw_buffers specifies that DrawBuffersNV errors with
1.504 + INVALID_OPERATION when the default FBO is bound. OpenGL ES 3.0 allows
1.505 + DrawBuffers to toggle between BACK and NONE on the default FBO.
1.506 +
1.507 + An implementation that does not natively support disabling the drawbuffer
1.508 + on the default FBO could emulate this by disabling color writes.
1.509 +
1.510 + RESOLVED: Allow DrawBuffersEXT to be called for the default FBO. This
1.511 + is more forward looking and is compatible with ES 3.0.
1.512 +
1.513 + 5) What are the requirements on the color attachment sizes and formats?
1.514 +
1.515 + RESOLVED: ES 2.0 requires that all color buffers attached to application-
1.516 + created framebuffer objects must have the same number of bitplanes
1.517 + (Chapter 4 overview p91). ES 2.0 also requires that all attached images
1.518 + have the same width and height (Section 4.4.5 Framebuffer Completeness).
1.519 + This extension does not lift those requirements, and failing to meet
1.520 + them will result in an incomplete FBO.
1.521 +
1.522 + 6) Does this have any interactions with glClear?
1.523 +
1.524 + RESOLVED: Yes. When multiple color buffers are enabled for writing,
1.525 + glClear clears all of the color buffers. Added language clarifying
1.526 + that glClear and glClearColor may affect multiple color buffers.
1.527 +
1.528 +Revision History
1.529 +
1.530 + 01/30/2013 dgkoch add issue 6 and clear interactions
1.531 + renamed to EXT_draw_buffers based on feedback
1.532 + changed resolution of issue 3.
1.533 + 01/23/2013 dgkoch add resolutions to issues 2-4.
1.534 + add issue 5.
1.535 + Add Table 4.x and update various explicit
1.536 + references to COLOR_ATTACHMENT0.
1.537 + Add errors.
1.538 + 11/13/2012 dgkoch add revision history
1.539 + add text from updated ES 3.0 spec
1.540 + add issues for discussion
1.541 + 10/16/2012 kbr update name string
1.542 + 10/16/2012 kbr remove restrition requiring draw buffer 0 to be non-NULL
1.543 + 10/12/2012 kbr remove references to GetDoublev and ReadBuffer
1.544 + 10/11/2012 kbr initial draft extension
1.545 +
