Name EXT_provoking_vertex Name Strings GL_EXT_provoking_vertex Contributors Cynthia Allison, NVIDIA Gregory Roth, NVIDIA Daniel Koch, TransGaming Gavriel State, TransGaming Jason Green, TransGaming Ian Romanick, Intel Marcus Steyer, NVIDIA Pat Brown, NVIDIA Stefan Dosinger, CodeWeavers Henri Verbeet, CodeWeavers Contact Mark Kilgard, NVIDIA (mjk 'at' nvidia.com) Status Implemented by NVIDIA, March 2009 Version Last Modified Date: May 11, 2009 Version: 12 Number 364 Dependencies This extension is written against the OpenGL 2.1 Specification but can apply to any prior specification. ARB_geometry_shader4, EXT_geometry_shader4, NV_geometry_shader4, and NV_gpu_program4 interact with this extension EXT_transform_feedback, NV_transform_feedback, and the transform feedback functionality made core by OpenGL 3.0 are clarified by this extension. Overview This extension provides an alternative provoking vertex convention for rendering lines, triangles, and (optionally depending on the implementation) quads. The provoking vertex of a primitive is the vertex that determines the constant primary and secondary colors when flat shading is enabled. In OpenGL, the provoking vertex for triangle, quad, line, and (trivially) point primitives is the last vertex used to assemble the primitive. The polygon primitive is an exception in OpenGL where the first vertex of a polygon primitive determines the color of the polygon, even if actually broken into triangles and/or quads. See section 2.14.7 (Flatshading) of the OpenGL 2.1 specification, particularly Table 2.12 for more details. Alternatively the provoking vertex could be the first vertex of the primitive. Other APIs with flat-shading functionality such as Reality Lab and Direct3D have adopted the "first vertex of the primitive" convention to determine the provoking vertex. However, these APIs lack quads so do not have a defined provoking vertex convention for quads. The motivation for this extension is to allow applications developed for APIs with a "first vertex of the primitive" provoking vertex to be easily converted to OpenGL. New Procedures and Functions void ProvokingVertexEXT(enum mode); New Tokens Accepted by the parameter of ProvokingVertexEXT: FIRST_VERTEX_CONVENTION_EXT 0x8E4D LAST_VERTEX_CONVENTION_EXT 0x8E4E Accepted by the parameter of GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev: PROVOKING_VERTEX_EXT 0x8E4F QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION_EXT 0x8E4C Additions to Chapter 2 of the OpenGL 2.1 Specification (OpenGL Operation) -- Section 2.14.7 "Flatshading" (page 69) Replace the entire section with: "A primitive may be flatshaded, meaning that all vertices of the primitive are assigned the same color index or the same primary or secondary colors. These colors are the colors of the vertex that spawned the primitive. Table 2.12 summaries the possibilities. Flatshading is controlled by void ShadeModel(enum shadeMode); void ProvokingVertexEXT(enum provokeMode); shadeMode value must be either the symbolic constants SMOOTH or FLAT. If shadeMode is SMOOTH (the initial state), the vertex colors are treated individually. If shadeMode is FLAT, flatshading is turned on. provokeMode value must be either FIRST_VERTEX_CONVENTION_EXT or LAST_VERTEX_CONVENTION_EXT. If provokeMode is LAST_VERTEX_CONVENTION_EXT (the initial state), the "last vertex convention" column of table 2.12 applies when flat shading; otherwise, the "first vertex convention" column applies when flat shading. The provoking vertex behavior of quad primitives is implementation dependent. Implementations can choose to either respect (follow) the state set by ProvokingVertexEXT for quad primitives (and, in this case, return true for the QUADS_FOLLOW_PROVOKING_VERTEX_EXT implementation-dependent state) or unconditionally implement the last vertex convention for quad primitives (and, in this case, return false for QUADS_FOLLOW_PROVOKING_VERTEX_EXT). ShadeModel and ProvokingVertexEXT each require one bit of state. First vertex Last vertex Primitive type of polygon i convention convention =========================== ============ ================================================== point i i <- same independent line 2i-1 2i line loop i i+1, if i that is not either FIRST_VERTEX_CONVENTION_EXT or LAST_VERTEX_CONVENTION_EXT. New State (table 6.11, page 276) add the following entry: Get Value Type Get Command Initial Value Description Sec Attribute -------------------- ---- ----------- -------------------------- ---------------- ------ --------- PROVOKING_VERTEX_EXT Z2 GetIntegerv LAST_VERTEX_CONVENTION_EXT Provoking vertex 2.14.7 lighting convention (table 6.36, page 301) add the following entry: Get Value Type Get Command Initial Value Description Sec Attribute --------------------------------- ---- ----------- ------------- ----------------- ------ --------- QUADS_FOLLOW_PROVOKING_VERTEX_EXT B GetBooleanv - True if quad 2.14.7 - primitives follow provoking vertex convention New Implementation Dependent State None NVIDIA Implementation Details GeForce 8 (G8x, G9x, GT1xx) and up GPUs report true for QUADS_FOLLOW_PROVOKING_VERTEX_EXT. GeForce 6 and 7 (NV4x, G7x) GPUs report false for QUADS_FOLLOW_PROVOKING_VERTEX_EXT. Issues 1. What should this extension be called? RESOLVED: EXT_provoking_vertex The phrase "provoking vertex" is not used in the core OpenGL specification but it is the accepted jargon for the functionality in question. 2. How should quads be handled? RESOLVED: Ideally, quadrilateral primitives (GL_QUADS and GL_QUAD_STRIP) would follow the provoking vertex mode. Other existing APIs with different flatshading conventions do not support quads. Rather than force support for both the first and last convention for quads (which no other API supports), instead this extension provides implementations the flexibility to advertise whether or not quads respect the provoking vertex or not. This resolution ensures more hardware vendors can support this extension. Hardware vendors which support both OpenGL and Direct3D's provoking vertex conventions must have support for "first vertex" for triangles and lines because Direct3D demands these conventions. Direct3D does not demand a convention for quads. However every vendor supporting OpenGL can support the "last vertex" mode for quads. Leaving the quad behavior up to the OpenGL implementation means hardware can simply always switch to the OpenGL quad behavior when emitting quads. See issue #12 for more details about how the implementation-dependent handling of quads is advertised. 3. How should the specification language be written for this extension? RESOLVED: Update table 2.12 for all supported primitives. The current language describes how points and lines are handled in prose but the behavior for triangles, quads, and polygons is specified in table 2.12. Put all the Begin/End batch types in a single table with two columns specifying the "first vertex convention" and "last vertex convention" provoking vertex modes respectively. A unified table is less ambiguous than a combination of a table and prose. 4. What token names for the provoking vertex conventions should be used? RESOLVED: GL_FIRST_VERTEX_CONVENTION_EXT and GL_LAST_VERTEX_CONVENTION_EXT (the initial state, consistent with OpenGL's unextended operation). The word "convention" is used because (see issue #2), the "first vertex" or "last vertex" rules are not iron-clad as they may or may do not apply to quads. The provoking vertex behavior for polygons and triangle fans also isn't strictly first or last vertex: Polygons always use the first vertex (no matter the provoking vertex convention). Triangle fans don't really use the first vertex (the spoke vertex) when using the "first vertex" provoking vertex rule; see issue #7. 5. IRIS GL had a provoking vertex convention for polygons where the last vertex of a polygon primitive determined the flat shaded color of the polygon. Should we support this convention? RESOLVED: No. Interesting IRIS GL applications relying on this convention are assuredly non-existent at this point. This convention also requires waiting until all the vertices for a polygon (which OpenGL does not bound) are specified before the polygon can begin being rasterized. The IRIS GL convention was dubious for this reason and OpenGL's designers were correct to abandon IRIS GL's polygon provoking vertex convention. 6. How should line loops behave? RESOLVED: Line loops in GL_FIRST_VERTEX_CONVENTION_EXT mode should behave as though it were a line strip with the first vertex repeated at the end. In other words, the first vertex of the line loop provokes the color for the line primitive that closes the line loop. Direct3D does not support line loops. 7. How are triangle fans handled? RESOLVED: The first vertex of a triangle fan is the spoke vertex. Triangle fans in GL_FIRST_VERTEX_CONVENTION_EXT mode should use the first non-spoke vertex of the primitive as the provoking vertex. In other words, the spoke vertex is never the provoking vertex (in either convention). The rationale for this is to match DirectX 9's triangle fan behavior. The rationale for the DirectX 9 behavior is (presumably) that if the spoke vertex was considered the "first vertex" of every primitive in a triangle fan, every flat shaded primitive in a triangle fan would necessarily have the spoke vertex's color, which isn't very interesting. (DirectX 10 does not support triangle fans.) 8. How does the provoking vertex convention affect primitives generated by a geometry shader? RESOLVED: The provoking vertex convention affects primitives whether they are generated by geometry shaders or conventional (non-geometry shader) primitive assembly. Geometry shaders only generate point, line strips, and triangle strips (not line loops, triangle fans, polygons, or quads). (So the GL_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION_EXT is irrelevant when a geometry program or shader is active.) This makes the supporting the first and last vertex conventions for primitives generated by geometry shaders "simple" because in the points, line strip, and triangle strip cases, the convention really is to use either first or last vertex to define the provoking vertex (no caveats). There's no special specification language to support the fact that the provoking vertex convention applies to primitives generated by geometry shaders because flat shading behavior is described in Chapter 3's rasterization discussion which is all subsequent to the geometry shader processing inserted into Chapter 2. DirectX 10 geometry shaders can output flat attributes according to Direct3D's "first vertex provokes" convention for line and triangle output primitives from a geometry shader. So matching the DirectX 10 geometry shader behavior for flat shading requires setting the provoking vertex to GL_FIRST_VERTEX_CONVENTION_EXT. This said, the OpenGL default convention of "last vertex" for the provoking vertex tends to be more useful for geometry shaders. By deferring the computation of the flat shaded color to the last vertex of every primitive, that tends to give the geometry shader compiler the maximum allowance for scheduling computation and texturing operations required to compute the flat shaded color as long as possible (that is, until the last vertex of the primitive). 9. Should there be an OPTION or #pragma for geometry shader assembly and GLSL respectively to request the specific provoking vertex convention for the shader? RESOLVED: No. The provoking vertex is context state that doesn't belong within a shader as a pragma anymore than the stencil state belongs within the shader. Overriding context state based on a pragma in a shader introduces unfortunate validation interactions that will slow down shader binds. Geometry shaders written for DirectX 10 and using flat attributes expect the "first vertex" provoking vertex convention but the application is better off specifying the proper provoking vertex convention for shaders just as is done with other context state. TransGaming supports this resolution to not support a pragma. 10. How do geometry shader input primitives interact with this extension? RESOLVED: Table 2.12 includes the new primitives types introduced by geometry shaders (GL_LINES_ADJACENCY_ARB, GL_LINE_STRIP_ADJACENCY_ARB, GL_TRIANGLES_ADJACENCY_ARB, and GL_TRIANGLE_STRIP_ADJACENCY_ARB). However the entries for these primitive types are only relevant when these new primitive types are used with NO geometry shader enabled. When a geometry shader is enabled, the only primitive output types are points, line strips, and triangle strips. 11. To what attribute set should the provoking vertex belong? RESOLVED: Lighting (GL_LIGHTING_BIT). This is because the provoking vertex bit is described in the same context as the shade model (GL_SHADE_MODEL) setting, and the shade model state is part of the lighting attribute set. 12. How should the allowance for handling quadrilateral primitives be advertised? RESOLVED: Because this extension is intended to facilitate supporting Direct3D content that depends on the Direct3D's provoking vertex convention yet Direct3D does not support quad primitives (as OpenGL provides with GL_QUAD_STRIP and GL_QUADS), the particular provoking vertex behavior of quads is not crucial to this extension's intended application. In the interest of making this extension's functionality for triangle and line primitives broadly available (the primitives Direct3D does support with a first vertex provoking vertex convention), this extension does not mandate a single uniform behavior for quad primitives. Mandating a particular behavior for quad primitives would, for some implementations, encumber the performance of this extension in the non-quad case or make this implementation of this extension needlessly complex to implement. Instead the GL_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION_EXT implementation-dependent boolean indicates whether or not quads (generated by GL_QUADS or GL_QUAD_STRIP) should abide by the provoking vertex convention or not. Whether or not the GL_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION_EXT state is true or false, the provoking vertex behavior of quads is well-defined in either case. The recommended, though implementation-dependent, value for GL_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION_EXT is true because this means quads, will like lines and triangles, follow the GL_PROVOKING_VERTEX_EXT state as indicated. 13. How does the provoking vertex state interact with primitive restart? RESOLVED: Orthogonally so no specific specification language describing the interaction is required. Specifically a primitive restart acts as a glEnd/glBegin sequence so it restarts the primitive numbering to 1 for the vertex immediately following the restart index. 14. Should the provoking vertex behavior apply to both the primary and secondary color? RESOLVED: Yes, the provoking vertex decides both the primary and secondary color of a flat-shaded primitive. That's consistent with Direct3D's provoking vertex convention as well as OpenGL's current behavior. 15. Should the provoking vertex behavior be specified with a glEnable/glDisable token instead of glProvokingVertexEXT? RESOLVED: The provoking vertex API is closely related to glShadeModel which uses an enumerated mode rather than glEnable/glDisable to specify flat or smooth shading so the API mimics the glShadeModel approach. This results in a fairly readable API usage that is more easily groaked by unfamiliar programmers: glProvokingVertexEXT(GL_FIRST_VERTEX_CONVENTION_EXT); instead of: glEnable(GL_FIRST_VERTEX_CONVENTION_EXT); It is also not clear that the provoking vertex convention is really a single enable. The convention behaves differently depending on the primitive type. For example, GL_POLYGON always uses the first vertex as the provoking vertex regardless of the provoking vertex state. 16. Does the OpenGL Shading Language (GLSL) 1.30 "flat" varying qualifier respect the provoking vertex state? RESOLVED: Yes. The GLSL 1.30 specification says "This variable [qualified as flat] will come from a single provoking vertex, as described by the OpenGL Graphics System Specification." This extension amends how the provoking vertex is described so no GLSL specification update is required. This does imply that user-declared varyings in a GLSL shader declared with "flat" will have the provoking vertex convention applied to determine their value. 17. How does the provoking vertex apply to Direct3D 10? RESOLVED: Direct3D 10 has deprecated the D3DSHADEMODE state for controlling flat or smooth (Gouraud) shading. However there is still the concept of a provoking vertex (called the "leading vertex" by Direct3D 10) which still corresponds to this extension's "first vertex" convention. Use of the leading (provoking) vertex for constant (flat) interpolation is indicated by Direct3D 10's "nointerpolation" variable storage class (sometimes called an interpolation modifier). 18. Does the NV_gpu_program4 "FLAT" attribute modifier respect the provoking vertex state? RESOLVED: Yes. NVIDIA's NV_gpu_program4 extension, describing an OpenGL assembly for Shader Model 4.0, allows a FLAT modifier to be specified for fragment program inputs. The NV_gpu_program4 specification says "If an attribute is flat-shaded, it will be taken from the output attribute of the provoking vertex of the primitive using the same data type." This extension amends how the provoking vertex is described so no NV_gpu_program4 specification update is required. 19. How does this extension interact with transform feedback? RESOLVED: Attribute components written out by transform feedback are NOT affected by the flatshading or provoking vertex state. While this specification is written against OpenGL 2.1, transform feedback was made core functionality with OpenGL 3.0 and then the order of the transform feedback was moved in the OpenGL 3.1 specification. Therefore the subsequent discussion uses the more recent 3.1 sectioning. Specifically the OpenGL 3.1 specification (section 2.18: Transform Feedback) says "The vertices are fed back after vertex color clamping, but before clipping." This statement is unclear because flatshading (section 2.13.7: Flatshading) happens inbetween vertex color clamping (section 2.13.6: Clamping or Masking) and primitive clipping (section 2.20: Primitive Clipping). Base on this issue the sentence is clarified to read: "The vertices are fed back after vertex color clamping, but before [flatshading and] clipping." For reference, the original EXT_transform_feedback extension has this same language ("The vertices are fed back after vertex color clamping, but before clipping.") but follows that sentence with: "If a geometry shader is active, the vertices recorded are those emitted from the geometry shader." Technically geometry shading takes place prior to even vertex color clamping. Clearly flat shading needs to happen prior to clipping so that clipped vertices can share the flat shaded attributes of the primitive prior to any potential clipping. This resolution is consistent with DirectX 10's behavior. Technically, DirectX 10 says that vertices output through transform feedback (which DirectX 10 calls "stream output") only have to be defined for constant attributes of the primitive's leading vertex (constant attributes are those that OpenGL would call flatshaded). Other constant attributes for non-leading vertices may be undefined. Leaving such constant attributes undefined is undesirable, particularly given how OpenGL operates. It is well-defined and more useful to simply output the value of the vertex's attribute prior to any flatshading. This is particularly desirable for OpenGL because with this extension (and even prior to supporting this extension), the provoking vertex is not always the leading vertex. To clarify further, while this resolution is consistent with DirectX 10, an OpenGL implementation that supports transform feedback has no undefined behavior specified. The simplest way to describe what happens is that attribute components written out by transform feedback are the attribute component values of vertices AFTER (optional) geometry shading and vertex color clamping but PRIOR to flatshading and primitive clipping. Revision History Rev. Date Author Changes ---- -------- --------- --------------------------------------------- 12 5/11/09 mjk Resolve issue 12 with feedback from Daniel Koch 11 4/17/09 mjk grammar and typo fixes 10 3/26/09 mjk typo fixes, more contributors listed, clarify transform feedback interaction 9 3/12/09 mjk dependencies on geometry shading extensions 8 3/11/09 mjk Fix issues 2, 4 & 7, add issues 16, 17, & 18 7 3/7/09 mjk Fix line adj typo, re-order table 6 3/3/09 mjk Assign enums, add issues 11-15 5 1/16/08 mjk Geometry shader interactions 4 1/6/08 mjk Add line loop behavior, change triangle fan 3 1/31/07 mjk Jamie review feedback 2 1/24/07 mjk Fix quad entries in table 1 1/11/07 mjk Initial version