skia / external / github.com / KhronosGroup / OpenGL-Registry / d15191e446614fcec3369577495dfb45b131a27a / . / extensions / EXT / EXT_provoking_vertex.txt

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 <mode> parameter of ProvokingVertexEXT: | |

FIRST_VERTEX_CONVENTION_EXT 0x8E4D | |

LAST_VERTEX_CONVENTION_EXT 0x8E4E | |

Accepted by the <pname> 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<n | |

1, if i==n | |

line strip i i+1 | |

independent triangle 3i-2 3i | |

triangle strip i i+2 | |

triangle fan i+1 i+2 | |

independent quad 4i-3 4i, if QUADS_FOLLOW_PROVOKING_VERTEX_EXT = true | |

4i 4i, if QUADS_FOLLOW_PROVOKING_VERTEX_EXT = false <- same | |

quad strip 2i-1 2i+2, if QUADS_FOLLOW_PROVOKING_VERTEX_EXT = true | |

2i+2 2i+2, if QUADS_FOLLOW_PROVOKING_VERTEX_EXT = false <- same | |

single polygon (i=1) 1 1 <- same | |

line adjacency 4i-2 4i-1 | |

line strip adjacency i+1 i+2 | |

triangle adjacency 6i-5 6i-1 | |

triangle strip adjacency 2i-1 2i+3 | |

Table 2.12: Polygon flatshading color selection. The color used for | |

flat shading the ith polygon generated by the indicated Begin/End | |

type are derived from the current color (if lighting is disabled) | |

in effect when the indicated vertex is specified. If lighting | |

is enabled or a vertex shader is used, the colors are produced | |

by lighting or vertex shading (respectively) the indicated vertex. | |

Vertices are numbered 1 through n, where n is the number of vertices | |

between the Begin/End pair." | |

Additions to Chapter 3 of the OpenGL 2.1 Specification (Rasterization) | |

None | |

Additions to Chapter 4 of the OpenGL 2.1 Specification (Per-Fragment | |

Operations and the Frame Buffer) | |

None | |

Additions to Chapter 5 of the OpenGL 2.1 Specification (Special | |

Functions) | |

None | |

Additions to Chapter 6 of the OpenGL 2.1 Specification (State and | |

State Requests) | |

None | |

Additions to the AGL/GLX/WGL Specifications | |

None | |

Additions to the OpenGL Shading Language | |

None | |

Dependencies on ARB_geometry_shader4, EXT_geometry_shader4, | |

NV_geometry_shader4, and/or NV_gpu_program4: | |

If none of ARB_geometry_shader4, EXT_geometry_shader4, | |

NV_geometry_shader4, or NV_gpu_program4 are supported, ignore | |

the rows of table 2.12 for line adjacency, line strip adjacency, | |

triangle adjacency, and triangle strip adjacency. | |

Dependencies on EXT_transform_feedback, NV_transform_feedback, and/or the | |

transform feedback functionality integrated into the core by OpenGL 3.0: | |

Clarify the statement describing when transform feedback occurs | |

(in section 2.18 of the OpenGL 3.1 specification) to read: | |

"The vertices are fed back after vertex color clamping, but before | |

flatshading and clipping." | |

(The "flatshading and" phrase is newly clarifying.) | |

GLX Protocol | |

A new GL rendering command is added. The following command is sent to the | |

server as part of a glXRender request: | |

ProvokingVertexEXT | |

2 8 rendering command length | |

2 4227 rendering command opcode | |

4 ENUM provokeMode | |

Errors | |

INVALID_ENUM is generated when ProvokingVertexEXT is called with | |

a <provokeMode> 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 | |