| Name |
| |
| NV_draw_buffers |
| |
| Name Strings |
| |
| GL_NV_draw_buffers |
| |
| Contact |
| |
| Greg Roth, NVIDIA Corporation (groth 'at' nvidia.com) |
| |
| Contributors |
| |
| Benj Lipchak, AMD |
| Bill Licea-Kane, AMD |
| Rob Mace, NVIDIA Corporation |
| James Helferty, NVIDIA Corporation |
| |
| Status |
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| Complete. |
| |
| Version |
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| Last Modified Date: July 11, 2013 |
| NVIDIA Revision: 4.0 |
| |
| Number |
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| OpenGL ES Extension #91 |
| |
| Dependencies |
| |
| Written against the OpenGL ES 2.0 Specification and the OpenGL ES |
| Shader Language 1.0.14 Specification. |
| |
| This extension interacts with the OpenGL ES 3.0 Specification |
| |
| Overview |
| |
| This extension extends OpenGL ES 2.0 to allow multiple output |
| colors, and provides a mechanism for directing those outputs to |
| multiple color buffers. |
| |
| This extension serves a similar purpose to ARB_draw_buffers except |
| that this is for OpenGL ES 2.0. |
| |
| When OpenGL ES 3.0 is present, this extension relaxes the order |
| restriction on color attachments to draw framebuffer objects. |
| |
| IP Status |
| |
| NVIDIA Proprietary |
| |
| New Procedures and Functions |
| |
| void DrawBuffersNV(sizei n, const enum *bufs); |
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| New Tokens |
| |
| Accepted by the <pname> parameters of GetIntegerv, GetFloatv, |
| and GetDoublev: |
| |
| MAX_DRAW_BUFFERS_NV 0x8824 |
| DRAW_BUFFER0_NV 0x8825 |
| DRAW_BUFFER1_NV 0x8826 |
| DRAW_BUFFER2_NV 0x8827 |
| DRAW_BUFFER3_NV 0x8828 |
| DRAW_BUFFER4_NV 0x8829 |
| DRAW_BUFFER5_NV 0x882A |
| DRAW_BUFFER6_NV 0x882B |
| DRAW_BUFFER7_NV 0x882C |
| DRAW_BUFFER8_NV 0x882D |
| DRAW_BUFFER9_NV 0x882E |
| DRAW_BUFFER10_NV 0x882F |
| DRAW_BUFFER11_NV 0x8830 |
| DRAW_BUFFER12_NV 0x8831 |
| DRAW_BUFFER13_NV 0x8832 |
| DRAW_BUFFER14_NV 0x8833 |
| DRAW_BUFFER15_NV 0x8834 |
| |
| |
| Accepted by the <bufs> parameter of DrawBuffersNV: |
| |
| COLOR_ATTACHMENT0_NV 0x8CE0 |
| COLOR_ATTACHMENT1_NV 0x8CE1 |
| COLOR_ATTACHMENT2_NV 0x8CE2 |
| COLOR_ATTACHMENT3_NV 0x8CE3 |
| COLOR_ATTACHMENT4_NV 0x8CE4 |
| COLOR_ATTACHMENT5_NV 0x8CE5 |
| COLOR_ATTACHMENT6_NV 0x8CE6 |
| COLOR_ATTACHMENT7_NV 0x8CE7 |
| COLOR_ATTACHMENT8_NV 0x8CE8 |
| COLOR_ATTACHMENT9_NV 0x8CE9 |
| COLOR_ATTACHMENT10_NV 0x8CEA |
| COLOR_ATTACHMENT11_NV 0x8CEB |
| COLOR_ATTACHMENT12_NV 0x8CEC |
| COLOR_ATTACHMENT13_NV 0x8CED |
| COLOR_ATTACHMENT14_NV 0x8CEE |
| COLOR_ATTACHMENT15_NV 0x8CEF |
| |
| |
| 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 |
| 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 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. The color buffer does 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) |
| |
| Replace Section 4.2.1, "Selecting a Buffer for Writing" |
| |
| 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, DrawBuffersNV is used. |
| |
| The command |
| |
| void DrawBuffersNV(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 NONE, COLOR_ATTACHMENT0, or |
| COLOR_ATTACHMENTi_NV, where <i> is the index of the color attachment |
| point. Otherwise, an INVALID_ENUM error is generated. DrawBuffersNV |
| may only be called when the GL is bound to a framebuffer object. If |
| called when the GL is bound to the default framebuffer, an INVALID_- |
| OPERATION error is generated. |
| |
| 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 and |
| must be at least 1. The number of draw buffers supported can be |
| queried by calling GetIntegerv with the symbolic constant |
| MAX_DRAW_BUFFERS_NV. An INVALID_VALUE error is generated if <n> is |
| less than one or greater than MAX_DRAW_BUFFERS_NV. |
| |
| Except for NONE, a buffer may not appear more then once in the array |
| pointed to by <bufs>. Specifying a buffer more then once will result |
| in the error INVALID_OPERATION. |
| |
| If a fragment shader writes to "gl_FragColor", DrawBuffersNV |
| specifies a set of draw buffers into which the color written to |
| "gl_FragColor" is written. If a fragment shader writes to |
| gl_FragData, DrawBuffers 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. |
| |
| If DrawBuffersNV is supplied with a constant COLOR_ATTACHMENT<m> |
| where <m> is greater than or equal to the value of |
| MAX_COLOR_ATTACHMENTS_NV, then the error INVALID_OPERATION results. |
| |
| Indicating a buffer or buffers using DrawBuffersNV 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 to any buffer. |
| |
| The state required to handle color buffer selection is an integer |
| for each supported fragment color. For each framebuffer object, the |
| initial state of the draw buffer for fragment color zero is COLOR_- |
| ATTACHMENT0 and 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>_NV. |
| |
| Changes to Chapter 3 of the OpenGL Shading Language 1.0 Specification (Basics) |
| |
| Add a new section: |
| |
| 3.3.1 GL_NV_draw_buffers Extension |
| |
| To use the GL_NV_draw_buffers extension in a shader it must be |
| enabled using the #extension directive. |
| |
| The shading language preprocessor #define GL_NV_draw_buffers will be |
| defined to 1, if the GL_NV_draw_buffers extension is supported. |
| |
| Interactions with OpenGL ES 3.0 |
| |
| Section 4.2.1 of OpenGL ES 3.0, (Selecting a Buffer for Writing). Replace |
| the eighth and ninth paragraph: |
| |
| If the GL is bound to a draw framebuffer object, the ith buffer listed in |
| bufs must be COLOR_ATTACHMENTm or NONE, where m is less than the value of |
| MAX_COLOR_ATTACHMENTS. Specifying BACK or COLOR_ATTACHMENTm where m is |
| greater than or equal to MAX_COLOR_ATTACHMENTS will generate the error |
| INVALID_OPERATION. |
| |
| If an OpenGL ES Shading Language 1.00 or 3.00 fragment shader writes a |
| user-defined varying out variable, DrawBuffers 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 none of |
| gl_FragColor, gl_FragData, nor any user-defined output variables, the |
| values of the fragment colors following shader execution are undefined, and |
| may differ for each fragment color. If some, but not all user-defined |
| output variables are written, the values of fragment colors corresponding |
| to unwritten variables are similarly undefined. |
| |
| New State |
| |
| Add Table 6.X Framebuffer (State per framebuffer object): |
| |
| State Type Get Command Initial Value Description |
| --------------- ---- ------------ ------------- ----------- |
| DRAW_BUFFERi_NV Z10* GetIntegerv see 4.2.1 Draw buffer selected |
| for fragment color i |
| |
| Add the new Table 6.X "Framebuffer Dependent Values" : |
| |
| State Type Get Command Min Value Description |
| ------------------- ---- ----------- --------- ----------- |
| MAX_DRAW_BUFFERS_NV Z+ GetIntegerv 1 Maximum number of |
| active draw buffers |
| |
| Issues |
| |
| 1. What behavior should be expected if a draw buffer selects an attachment |
| for a draw framebuffer that has no image attached? |
| |
| Early drivers considered this an INVALID_OPERATION and the DrawBuffersNV |
| operation did not succeed. (Following precedent set in Issue 55 of |
| EXT_framebuffer_object) |
| |
| OpenGL ES 3.0 and Desktop GL 4.3 consider this legal, and the DrawBuffers |
| call succeeds. Missing attachments are simply not written to. |
| |
| RESOLVED: Behavior should match OpenGL ES 3.0. Application developers are |
| cautioned that early Tegra drivers may exhibit the previous behavior. |
| |
| See ARB_draw_buffers for additional relevant issues. |
| |
| Revision History |
| |
| Rev. Date Author Changes |
| ---- -------- --------- ------------------------------------ |
| 4 07/11/13 jhelferty Updated error behavior for missing |
| attachments to match ES 3.0. |
| Clarified ES 3.0 interactions. |
| 3 06/07/11 groth Clarified default behavior, state tables. |
| 2 04/26/11 groth Filled in many missing elements. |
| 1 03/03/08 kashida First revision based on |
| ARB_draw_buffers. |
