xref: /third_party/openGLES/extensions/ARB/ARB_sampler_objects.txt (revision 5bd8dead)

Name

    ARB_sampler_objects

Name Strings

    GL_ARB_sampler_objects

Contact

    Graham Sellers, AMD (graham.sellers 'at' amd.com)

Contributors

    Graham Sellers, AMD
    Jaakko Konttinen, AMD
    Jeff Bolz, NVIDIA
    Daniel Koch, TransGaming
    Bruce Merry, ARM

Notice

    Copyright (c) 2010-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Specification Update Policy

    Khronos-approved extension specifications are updated in response to
    issues and bugs prioritized by the Khronos OpenGL Working Group. For
    extensions which have been promoted to a core Specification, fixes will
    first appear in the latest version of that core Specification, and will
    eventually be backported to the extension document. This policy is
    described in more detail at
        https://www.khronos.org/registry/OpenGL/docs/update_policy.php

Status

    Complete. Approved by the ARB at the 2010/01/22 F2F meeting.
    Approved by the Khronos Board of Promoters on March 10, 2010.

Version

    Version 13, November 12, 2014

Number

    ARB Extension #81

Dependencies

    This extension is written against the OpenGL 3.2 (Compatibility
    Profile) specification.

    This extension interacts with EXT_texture_filter_anisotropic.

    OpenGL 1.0 is required.

Overview

    In unextended OpenGL textures are considered to be sets of image
    data (mip-chains, arrays, cube-map face sets, etc.) and sampling
    state (sampling mode, mip-mapping state, coordinate wrapping and
    clamping rules, etc.) combined into a single object. It is typical
    for an application to use many textures with a limited set of
    sampling states that are the same between them. In order to use
    textures in this way, an application must generate and configure
    many texture names, adding overhead both to applications and to
    implementations. Furthermore, should an application wish to sample
    from a texture in more than one way (with and without mip-mapping,
    for example) it must either modify the state of the texture or
    create two textures, each with a copy of the same image data. This
    can introduce runtime and memory costs to the application.

    This extension separates sampler state from texture image data. A
    new object type is introduced, the sampler (representing generic
    sampling parameters). The new sampler objects are represented by a
    new named type encapsulating the sampling parameters of a
    traditional texture object. Sampler objects may be bound to texture
    units to supplant the bound texture's sampling state. A single
    sampler may be bound to more than one texture unit simultaneously,
    allowing different textures to be accessed with a single set of
    shared sampling parameters. Also, by binding different sampler
    objects to texture units to which the same texture has been bound,
    the same texture image data may be sampled with different sampling
    parameters.

IP Status

    No known IP claims.

New Procedures and Functions

    void GenSamplers(sizei count, uint *samplers);
    void DeleteSamplers(sizei count, const uint * samplers);
    boolean IsSampler(uint sampler);
    void BindSampler(uint unit, uint sampler);
    void SamplerParameteri(uint sampler, enum pname, int param);
    void SamplerParameterf(uint sampler, enum pname, float param);
    void SamplerParameteriv(uint sampler, enum pname, const int *params);
    void SamplerParameterfv(uint sampler, enum pname, const float *params);
    void SamplerParameterIiv(uint sampler, enum pname, const int *params);
    void SamplerParameterIuiv(uint sampler, enum pname, const uint *params);
    void GetSamplerParameteriv(uint sampler, enum pname, int *params);
    void GetSamplerParameterfv(uint sampler, enum pname, float *params);
    void GetSamplerParameterIiv(uint sampler, enum pname, int *params);
    void GetSamplerParameterIuiv(uint sampler, enum pname, uint *params);

New Tokens

    Accepted by the <value> parameter of the GetBooleanv, GetIntegerv,
    GetInteger64v, GetFloatv and GetDoublev functions:

        SAMPLER_BINDING                                 0x8919

Additions to Chapter 2 of the OpenGL 3.2 Specification (Compatibility Profile) (OpenGL Operation)

    None.

Additions to Chapter 3 of the OpenGL 3.2 Specification (Compatibility Profile) (Rasterization)

    Additions to Section 3.8.12 Texture Completeness

    Add subheading "Effects of Sampler Objects on Texture Completeness"

    If a sampler object and a texture object are simultaneously bound to
    the same texture unit, then the sampling state for that unit is
    taken from the sampler object (see section 3.9.2, "Sampler
    Objects"). This can have an effect on the effective completeness of
    the texture. In particular, if the texture is not mipmap complete
    and the sampler object specifies a MIN_FILTER requiring mipmaps, the
    texture will be considered incomplete for the purposes of that
    texture unit. However, if the sampler object does not require
    mipmaps, the texture object will be considered complete. This means
    that a texture can be considered both complete and incomplete
    simultaneously if it is bound to two or more texture units along
    with sampler objects with different states.

    Additions to Section 3.9 Texturing

    Modify the prologue to Section 3.9 as follows:

    Renumber Section 3.9.14 "Texture Objects" to Section 3.9.1. renumber
    subsequent sections

    Replace Section 3.9.1 "Texture Objects"

    3.9.1 Texture Objects
    ---------------------

    Textures in GL are represented by named objects. The name space for
    texture objects is the unsigned integers, with zero reserved by the
    GL to represent the default texture object. The default texture
    object is bound to each of the TEXTURE_1D, TEXTURE_2D, TEXTURE_3D,
    TEXTURE_1D_ARRAY, TEXTURE_2D_ARRAY, TEXTURE_RECTANGLE,
    TEXTURE_BUFFER, TEXTURE_CUBE_MAP, TEXTURE_2D_MULTISAMPLE, and
    TEXTURE_2D_MULTISAMPLE_ARRAY targets during context initialization.

    A new texture object is created by binding an unused name to one of
    these texture targets. The command

        void GenTextures( sizei n, uint *textures );

    returns <n> previously unused texture names in <textures>. These
    names are marked as used, for the purposes of GenTextures only, but
    they acquire texture state and a dimensionality only when they are
    first bound, just as if they were unused. The binding is effected by
    calling

        void BindTexture( enum target, uint texture );

    with <target> set to the desired texture target and <texture> set to
    the unused name. The resulting texture object is a new state vector,
    comprising all the state and with the same initial values listed in
    section 3.9.13. The new texture object bound to <target> is, and
    remains a texture of the dimensionality and type specified by
    <target> until it is deleted.

    BindTexture may also be used to bind an existing texture object to
    any of these targets. The error INVALID_OPERATION is generated if an
    attempt is made to bind a texture object of different dimensionality
    than the specified <target>. If the bind is successful no change is
    made to the state of the bound texture object, and any previous
    binding to <target> is broken.

** This error applies only to 3.2 core profile / 3.1 w/o ARB_compatibility /
** 3.0 deprecated contexts.
    BindTexture fails and an INVALID_OPERATION error is generated if
    <texture> is not zero or a name returned from a previous call to
    GenTextures, or if such a name has since been deleted. While a
    texture object is bound, GL operations on the target to which it is
    bound affect the bound object, and queries of the target to which it
    is bound return state from the bound object. If texture mapping of
    the dimensionality of the target to which a texture object is bound
    is enabled, the state of the bound texture object directs the
    texturing operation.

    Texture objects are deleted by calling

        void DeleteTextures( sizei n, uint *textures );

    <textures> contains <n> names of texture objects to be deleted.
    After a texture object is deleted, it has no contents or
    dimensionality, and its name is again unused. If a texture that is
    currently bound to any of the target bindings of BindTexture is
    deleted, it is as though BindTexture had been executed with the same
    target and texture zero. Additionally, special care must be taken
    when deleting a texture if any of the images of the texture are
    attached to a framebuffer object. See section 4.4.2 for details.

    Unused names in <textures> are silently ignored, as is the name zero.

    The texture object name space, including the initial one-, two-, and
    three- dimensional, one- and two-dimensional array, rectangular,
    buffer, cube map, two-dimensional multisample, and two-dimensional
    multisample array texture objects, is shared among all texture
    units. A texture object may be bound to more than one texture unit
    simultaneously. After a texture object is bound, any GL operations
    on that target object affect any other texture units to which the
    same texture object is bound.

    Texture binding is affected by the setting of the state
    ACTIVE_TEXTURE. If a texture object is deleted, it as if all texture
    units which are bound to that texture object are rebound to texture
    object zero.

    Insert new Section 3.9.2 "Sampler Objects", renumber subsequent
    sections.

    3.9.2 Sampler Objects
    ---------------------

    The state necessary for texturing can be divided into two categories
    as described in Section 3.9.13. An OpenGL texture object represents
    both sets. The first set represents dimensionality and other image
    parameters, and the second set represents sampling state.
    Additionally, a sampler object may be created to encapsulate only
    the second set, or the sampling state, of a texture object.

    A new sampler object is created by binding an unused name to a
    texture unit. The command

        void GenSamplers( sizei count, uint *samplers );

    returns <count> previously unused sampler object names in
    <samplers>. The name zero is reserved by the GL to represent no
    sampler being bound to a sampler unit. The names are marked as used,
    for the purposes of GenSamplers only, but they acquire state only
    when they are first used as a parameter to BindSampler,
    SamplerParameter*, GetSamplerParameter*, or IsSampler. When a
    sampler object is first used in one of these functions, the
    resulting sampler object is initialized with a new state vector,
    comprising all the state and with the same initial values listed in
    table 6.23.

    When a sampler object is bound to a texture unit, its state
    supersedes that of the texture object bound to that texture unit. If
    the sampler name zero is bound to a texture unit, the currently
    bound texture's sampler state becomes active. A single sampler
    object may be bound to multiple texture units simultaneously.

    A sampler binding is effected by calling

        void BindSampler( uint unit, uint sampler );

    with <unit> set to the texture unit to which to bind the sampler and
    <sampler> set to the name of a sampler object returned from a
    previous call to GenSampler.

    <unit> must be between zero and the value of
    MAX_TEXTURE_IMAGE_UNITS-1. <sampler> is the name of a sampler object
    that has previously been reserved by a call to GenSamplers.

    If the bind is successful no change is made to the state of the
    bound sampler object, and any previous binding to <unit> is broken.

    BindSampler fails and an INVALID_OPERATION error is generated if
    <sampler> is not zero or a name returned from a previous call to
    GenSamplers, or if such a name has since been deleted with
    DeleteSamplers. An INVALID_VALUE error is generated if <unit> is
    greater than or equal to the value of
    MAX_COMBINED_TEXTURE_IMAGE_UNITS.

    If state is present in a sampler object bound to a texture unit that
    would have been rejected by a call to TexParameter for the texture
    bound to that unit, the behavior of the implementation is as if the
    texture were incomplete. For example, if TEXTURE_WRAP_S,
    TEXTURE_WRAP_T or TEXTURE_WRAP_R is set to REPEAT or MIRRORED_REPEAT
    on the sampler object bound to a texture unit and the texture bound
    to that unit is a TEXTURE_RECTANGLE, the texture will be considered
    incomplete.

    The currently bound sampler may be queried by calling GetIntegerv
    with <pname> set to SAMPLER_BINDING. When a sampler object is
    unbound from the texture unit (by binding another sampler object, or
    the sampler object named zero, to that texture unit) the modified
    state is again replaced with the sampler state associated with the
    texture object bound to that texture unit.

    The parameters represented by a sampler object are a subset of those
    described in Section 3.9.6 "Texture Parameters". Each parameter of a
    sampler object is set by calling

        void SamplerParameter{if}{v}( uint sampler, enum pname, T param );
        void SamplerParameterI{u ui}v( uint sampler, enum pname, T *params );

    <sampler> is the name of a sampler object previously reserved by a
    call to GenSamplers. <pname> is the name of a parameter to modify
    and <param> is the new value of that parameter. An INVALID_OPERATION
    error is generated if <sampler> is not the name of a sampler object
    previously returned from a call to GenSamplers. The values accepted
    in the <pname> parameter are TEXTURE_WRAP_S, TEXTURE_WRAP_T,
    TEXTURE_WRAP_R, TEXTURE_MIN_FILTER, TEXTURE_MAG_FILTER,
    TEXTURE_BORDER_COLOR, TEXTURE_MIN_LOD, TEXTURE_MAX_LOD,
    TEXTURE_LOD_BIAS, TEXTURE_COMPARE_MODE, TEXTURE_COMPARE_FUNC and
    TEXTURE_MAX_ANISOTROPY_EXT. Texture state listed in table 6.22 but
    not listed here and in the sampler state in table 6.23 is not part
    of the sampler state, and remains in the texture object.

    If the values for TEXTURE_BORDER_COLOR are specified with a call to
    SamplerParameterIiv or SamplerParameterIuiv, the values are
    unmodified and stored with an internal data type of integer. If
    specified with SamplerParameteriv, they are converted to
    floating-point using equation 2.2. Otherwise, the values are
    unmodified and stored as floating-point.

    An INVALID_ENUM error is generated if <pname> is not the name of a
    parameter accepted by SamplerParameter*. If the value of <param> is
    not an acceptable value for the parameter specified in <pname>, an
    error is generated as specified in the description of TexParameter*.

    Modifying a parameter of a sampler object affects all texture units
    to which that sampler object is bound. Calling TexParameter has no
    effect on the sampler object bound to the active texture unit. It
    will modify the parameters of the texture object bound to that unit.

    Sampler objects are deleted by calling

        void DeleteSamplers( sizei count, const uint *samplers );

    <samplers> contains <count> names of sampler objects to be deleted.
    After a sampler object is deleted, its name is again unused. If a
    sampler object that is currently bound to a sampler unit is deleted,
    it is as though BindSampler is called with <unit> set to the unit
    the sampler is bound to and <sampler> zero. Unused names in
    <samplers> are silently ignored, as is the reserved name zero.

Additions to Chapter 4 of the OpenGL 3.2 Specification (Compatibility Profile) (Per-Fragment Operations and the Framebuffer)

    None.

Additions to Chapter 5 of the OpenGL 3.2 Specification (Compatibility Profile) (Special Functions)

    Add a new category of commands not usable in display lists
    to section 5.4.1:

   "Sampler objects: GenSamplers and DeleteSamplers."

Additions to Chapter 6 of the OpenGL 3.2 Specification (Compatibility Profile) (State and State Requests)

    Insert new section 6.1.5 "Sampler Queries" following existing
    section 6.1.4 and renumber following sections:

    6.1.5 Sampler Queries

    The command

        boolean IsSampler( uint sampler );

    may be called to determine whether <sampler> is the name of a
    sampler object IsSampler will return TRUE if <sampler> is the name
    of a sampler object previously returned from a call to GenSamplers
    and FALSE otherwise. Zero is not the name of a sampler object. The
    current values of the parameters of a sampler object may be queried
    by calling

        void GetSamplerParameter{if}v( uint sampler, enum pname, T *params );
        void GetSamplerParameterI{i ui}v( uint sampler, enum pname, T *params );

    <sampler> is the name of the sampler object from which to retrieve
    parameters. <pname> is the name of the parameter whose value to
    retrieve and <params> is the address of an array into which the
    current value of the desired parameter will be placed.
    GetSamplerParameter* accepts the same values for <pname> as
    SamplerParameter*. An INVALID_OPERATION error is generated if
    <sampler> is not the name of a sampler object previously returned
    from a call to GenSamplers. An INVALID_ENUM error is generated if
    <pname> is not the name of a parameter accepted by
    GetSamplerParameter*.

    Querying value TEXTURE_BORDER_COLOR with GetSamplerParameterIiv or
    GetSamplerParameterIuiv returns the border color values as signed
    integers or unsigned integers, respectively; otherwise the values
    are returned as described in section 6.1.2. If the border color is
    queried with a type that does not match the original type with which
    it was specified, the result is undefined.


    In section 6.1.19, append to the paragraph starting "When PushAttrib
    is called with TEXTURE_BIT set...":

   "... are restored to their pushed values. The bindings and state for
    sampler objects are not pushed or popped."

Additions to Appendix A of the OpenGL 3.2 Specification (Compatibility Profile) (Invariance)

    None.

Interactions with EXT_texture_filter_anisotropic

    If EXT_texture_filter_anisotropic is not supported, remove all
    references to TEXTURE_MAX_ANISOTROPY_EXT.

Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    None.

Errors

    INVALID_OPERATION is generated by BindSampler if <sampler> is not
    zero or a name returned from a previous call to GenSamplers.

    INVALID_VALUE is generated by BindSampler if <unit> is greater than
    MAX_TEXTURE_IMAGE_UNITS-1.

    INVALID_OPERATION is generated by SamplerParameter* and
    GetSamplerParameter* if <sampler> is not a name returned from a
    previous call to GenSamplers.

New State

    Append to Table 6.20, "Textures (state per texture unit and binding point)"

    +-----------------------+-----------+-------------------+-----------------+-------------------------+--------+------------+
    | Get Value             | Type      | Get Command       | Initial Value   | Description             | Sec    | Attribute  |
    +-----------------------+-----------+-------------------+-----------------+-------------------------+--------+------------+
    | SAMPLER_BINDING       | 48* x Z+  | GetIntegerv       | 0               | Sampler object bound    | 3.9.2  | -          |
    |                       |           |                   |                 | to active texture unit  |        |            |
    +-----------------------+-----------+-------------------+-----------------+-------------------------+--------+------------+

    Add new Table 6.23, "Textures (state per sampler object)", renumber subsequent tables.

    +---------------------------+-----------+----------------------+------------------------+------------------------------+--------+------------+
    | Get Value                 | Type      | Get Command          | Initial Value          | Description                  | Sec    | Attribute  |
    +---------------------------+-----------+----------------------+------------------------+------------------------------+--------+------------+
    | TEXTURE_BORDER_COLOR      | n x C     | GetSamplerParameter  | 0,0,0,0                | Border color                 | 3.9    | -          |
    | TEXTURE_MIN_FILTER        | n x Z6    | GetSamplerParameter  | NEAREST_MIPMAP_LINEAR  | Minification function        | 3.9.9  | -          |
    | TEXTURE_MAG_FILTER        | n x Z2    | GetSamplerParameter  | LINEAR                 | Magnification function       | 3.9.10 | -          |
    | TEXTURE_WRAP_S            | n x Z5    | GetSamplerParameter  | REPEAT                 | Texcoord s wrap mode         | 3.9.9  | -          |
    | TEXTURE_WRAP_T            | n x Z5    | GetSamplerParameter  | REPEAT                 | Texcoord t wrap mode         | 3.9.9  | -          |
    | TEXTURE_WRAP_R            | n x Z5    | GetSamplerParameter  | REPEAT                 | Texcoord r wrap mode         | 3.9.9  | -          |
    | TEXTURE_MIN_LOD           | n x R     | GetSamplerParameter  | -1000                  | Minimum level of detail      | 3.9    | -          |
    | TEXTURE_MAX_LOD           | n x R     | GetSamplerParameter  | 1000                   | Maximum level of detail      | 3.9    | -          |
    | TEXTURE_LOD_BIAS          | n x R     | GetSamplerParameter  | 0.0                    | Texture level of detail      | 3.9.9  | -          |
    |                           |           |                      |                        | bias (biastexobj)            |        |            |
    | TEXTURE_COMPARE_MODE      | n x Z2    | GetSamplerParameter  | NONE                   | Comparison mode              | 3.9.16 | -          |
    | TEXTURE_COMPARE_FUNC      | n x Z8    | GetSamplerParameter  | LEQUAL                 | Comparison function          | 3.9.16 | -          |
    | TEXTURE_MAX_ANISOTROPY_EXT| n x R     | GetSamplerParameter  | 1.0                    | Maximum degree of anisotropy | 3.9    | -          |
    +---------------------------+-----------+----------------------+------------------------+------------------------------+--------+------------+

New Implementation Dependent State

    None.

Issues

    1) Should BindSampler take a <target> parameter?

    DISCUSSION: Currently it does not. Binding a sampler to a texture
    unit binds it to all targets. Binding a texture to a target of a
    texture unit continues to determine the target precedence.

    2) What is the set of state associated with a sampler object?
       Specifically, should TEXTURE_BASE_LEVEL and TEXTURE_MAX_LEVEL be
       part of the sampler or the texture?

    DISCUSSION: TEXTURE_BASE_LEVEL and TEXTURE_MAX_LEVEL are presently
    part of the image state (texture) and are thus not included in the
    sampler object. TEXTURE_WRAP_S, TEXTURE_WRAP_T, TEXTURE_WRAP_R,
    TEXTURE_MIN_FILTER, TEXTURE_MAG_FILTER, TEXTURE_BORDER_COLOR,
    TEXTURE_MIN_LOD, TEXTURE_MAX_LOD, TEXTURE_LOD_BIAS,
    TEXTURE_COMPARE_MODE, TEXTURE_COMPARE_FUNC are the states included
    in the sampler object.

    3) How would one use the same sampler state to access multiple
       textures?

    DISCUSSION: Generate a sampler object, set its state. Then, bind the
    same sampler object to multiple texture units (perhaps with
    different textures bound to them) and use those units as usual.

    4) How would one access the same texture with different samplers?

    DISCUSSION: Generate a sampler object for each intended set of
    sampling parameters and bind them to different texture units. Bind
    the same texture to each of those units. That same texture will be
    sampled according to the parameters contained in the sampler object
    bound to the respective unit.

    5) Does it matter what order samplers and textures are bound to
       texture units?

    No. As long as a sampler is bound to a texture unit, it acts as the
    source of the sampler state for that unit. Textures may be switched
    in and out without affecting the sampler state. This includes the
    default texture object.

    6) If a sampler object is bound to multiple texture units and its
    state is modified, is that state visible to all of those units?

    Yes.

    7) Should sampler objects be made visible to the shading language.

    This is left for a future extension. Separating samplers and texture
    images in the shader allows for a run-time combinatorial explosion
    of sampler- texture pairs that would be difficult to count. It is
    also very clumsy to specify in a way that interacts cleanly with
    prior versions of OpenGL. This may become more feasible to apply
    against a future version of OpenGL, but this extension is applicable
    to any version of OpenGL.

    8) Can sampler objects be shared between contexts?

    Yes. The semantics of sharing a sampler object between contexts is
    the same as that of sharing texture objects.

    9) What happens when a sampler object bound to a texture unit has a
       parameter that is incompatible with the texture that is bound to
       that unit?

    The behavior is as if the texture is incomplete.

    10) When are sampler objects created? When do they start to exist?

    Sampler objects are created when they are first used by any function
    taking the name of a sampler object as a parameter. Their names must
    be reserved by a call to GenSamplers. For all intents and purposes,
    it is as if they start existing when GenSamplers creates their
    names.

Revision History

    Rev.    Date      Author    Changes
    ----  ----------  --------- -----------------------------------------
    13    11/12/2014  Jon Leech Fix spelling of TEXTURE_MAX_ANISOTROPY
                                (public Bug 1263).
    12    07/22/2011  Jon Leech Fix valid <unit> values to be based
                                on MAX_COMBINED_TEXTURE_IMAGE_UNITS
                                instead of MAX_TEXTURE_IMAGE_UNITS (bug
                                6406).
    11    07/07/2010  Jon Leech Do not allow sampler bindings and sampler
                                object state to be pushed and popped, by
                                modifying all new state table entries to
                                have no attribute group and adding a
                                clarifying statement in section 6.1.20.
                                Do not allow GenSamplers and
                                DeleteSamplers in display lists (bug
                                6056).
    10    03/29/2010  pbrown    Further fixes in the listed prototypes;
                                use separate lines for external parsing.
     9    03/27/2010  pbrown    Fix an incorrect prototype for BindSampler
                                in the new functions section (bug 6146).
     8    03/22/2010  Jon Leech Change error for invalid <sampler> names
                                passed to *SamplerParameter* from
                                INVALID_VALUE to INVALID_OPERATION for
                                consistency (bug 6026). Expand Errors
                                section. Reformat to 80 columns.
     7    02/10/2010  Jon Leech Add const to SamplerParameter*v 'params'.
     6    01/26/2010  pbrown    Assign enum for SAMPLER_BINDING.
     5    01/05/2010  gsellers  Add SamplerParameterI*, GetSamplerParameterI*
                                (bug 5806).
                                Fix ambiguity about when sampler objects
                                are created. Samplers names are reserved
                                by GenSamplers but actually created
                                (default state initialized) on first
                                use. There is no bind-to-create for
                                sampler objects. (bug 5813).
                                Add language about effect of sampler
                                objects on effective texture
                                completeness (bug 5838). Specify that
                                illegal sampler states for particular
                                texture types result in incomplete
                                textures, rather than undefined
                                behavior.
     4    12/11/2009  Jon Leech Reorganization and consistency edits
                                resulting from 3.3 spec integration.
                                Remove ARB suffixes.
     3    11/19/2009  gsellers  Feedback from bmerry.
     2    11/11/2009  gsellers  Incorporate feedback.
     1    11/09/2009  gsellers  Initial version based on draft
                                EXT_separate_sampler_objects.