Name

    NV_explicit_multisample

Name Strings

    GL_NV_explicit_multisample

Contributors

    Eric Werness, NVIDIA Corporation
    Pat Brown, NVIDIA Corporation
    Jamie Gennis, NVIDIA Corporation
    Nacho Sanz-Pastor, Aechelon
    Mark Kilgard, NVIDIA Corporation

Contact

    Eric Werness, NVIDIA Corporation (ewerness 'at' nvidia.com)
    Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com)

Status

    Shipping in NVIDIA's Release 180 drivers (October 2008)

Version

    Last Modified Date:         10/27/08
    Revision:                   1

Number

    357

Dependencies

    OpenGL 2.0 is required.

    ARB_multisample is required.

    EXT_framebuffer_multisample affects the definition of this extension.

    NV_gpu_program4 affects the definition of this extension.

    EXT_gpu_shader4 affects the definition of this extension.

    EXT_timer_query affects the definition of this extension.

    This extension is written against the OpenGL 2.0 specification.

Overview

    In traditional multisample specs, the API only allows access to the samples
    indirectly through methods such as coverage values and downsampled
    readbacks. NV_explicit_multisample adds a set of new capabilities to allow
    more precise control over the use of multisamples. Specifically, it adds:

     * A query in the API to query the location of samples within the pixel

     * An explicit control for the multisample sample mask to augment the
       control provided by SampleCoverage

     * A new texture target to wrap a renderbuffer and allow a restricted class
       of accesses to the samples

     * The ability to fetch a specific sample from a multisampled texture from
       within a shader

     * A program option to enable the new behavior

New Procedures and Functions

    void GetBooleanIndexedvEXT(enum value, uint index, boolean *data);

    void GetIntegerIndexedvEXT(enum value, uint index, int *data);

    void GetMultisamplefvNV(enum pname, uint index, float *val);

    void SampleMaskIndexedNV(uint index, bitfield mask);

    void TexRenderbufferNV(enum target, uint renderbuffer);

New Tokens

    Accepted by the <pname> parameter of GetMultisamplefvNV:
    
        SAMPLE_POSITION_NV                          0x8E50

    Accepted by the <cap> parameter of Enable, Disable, and IsEnabled, and by
    the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv, and
    GetDoublev:

        SAMPLE_MASK_NV                              0x8E51

    Accepted by the <pname> parameter of GetBooleanIndexedvEXT and
    GetIntegerIndexedvEXT:

        SAMPLE_MASK_VALUE_NV                        0x8E52

    Accepted by the <pname> parameter of GetBooleanv, GetDoublev, GetIntegerv,
    and GetFloatv:

        TEXTURE_BINDING_RENDERBUFFER_NV             0x8E53
        TEXTURE_RENDERBUFFER_DATA_STORE_BINDING_NV  0x8E54
        MAX_SAMPLE_MASK_WORDS_NV                    0x8E59

    Accepted by the <target> parameter of BindTexture, and TexRenderbufferNV:

        TEXTURE_RENDERBUFFER_NV                     0x8E55

    Returned by the <type> parameter of GetActiveUniform:

        SAMPLER_RENDERBUFFER_NV                     0x8E56
        INT_SAMPLER_RENDERBUFFER_NV                 0x8E57
        UNSIGNED_INT_SAMPLER_RENDERBUFFER_NV        0x8E58

Additions to Chapter 2 of the OpenGL 2.0 Specification (OpenGL Operation)

    Modify Section 2.X.2, Program Grammar

    If a program specifies the NV_explicit_multisample program option, the
    <texTarget> rule is modified to add the texture target RENDERBUFFER and the
    <TEXop> rule is modified to add the instruction TXFMS.

    Add to section 2.15.4.1, Shader Only Texturing

    Multisample Texel Fetches

    Multisample texel fetches are very similar to standard texel fetches. Since
    multisample buffers can't have mipmaps, there is no LOD parameter. Instead,
    there is an integer parameter which selects the sample number to be fetched
    from the buffer. The number identifying the sample is the same as the value
    used to query the sample location using GetMultisamplefvNV.

    Additionally, this fetch may only be performed on a texture renderbuffer
    sampler. No other sample or fetch commands may be performed on a texture
    renderbuffer sampler.

    Add to section 2.X.4.4, Program Texture Access

    Add row to table X.17

                                                     coordinates used
      texTarget          Texture Type               s t r  layer  shadow
      ----------------   ---------------------      -----  -----  ------
      RENDERBUFFER       TEXTURE_RENDERBUFFER_NV      <not supported>


    Add row to table X.18

                                      coordinates used
      texTarget          supported      i j k  layer  lod
      ----------------   ---------      -----  -----  ---
      RENDERBUFFER          no          - - -    -     -

    Add after the discussion of TexelFetch

    The TXFMS instruction provides the ability to extract a single sample from a
    specified RENDERBUFFER texture image using the function

      result_t_vec TexelFetchMultisample(int_vec coord, int_vec offset);

    The extracted texel is converted to an (R,G,B,A) vector according to Table
    3.21.  The result vector is interpreted as floating-point, signed integer,
    or unsigned integer, according to the data type modifier of the
    instruction.  If the internal format of the texture is not compatible with
    the instruction's data type modifier, the extracted texel value is
    undefined.

    <coord> is a four-component signed integer vector used to identify the
    single sample accessed. Since a renderbuffer must be 2D with no mipmaps, the
    x and y components are always used to select the pixel and the fourth
    component is used to select the sample. All of the restrictions of
    TexelFetch apply to TexelFetchMultisample, with the additional requirement
    that it must be called on a RENDERBUFFER texture target. If the sample number
    is greater than or equal to the value of SAMPLES for the render buffer
    associated with the renderbuffer texture, the results are undefined. 

Additions to Chapter 3 of the OpenGL 2.0 Specification (Rasterization)

    (Insert into section 3.2.1, Multisampling after the discussion of the query
    for SAMPLES)

    To query the location of a given sample, the function

        void GetMultisamplefvNV(enum pname, uint index, float *val);

    is used with a pname of SAMPLE_POSITION_NV and an index corresponding to the
    sample for which the location should be returned. The sample location is
    returned as two floating point values each between 0 and 1 corresponding to
    the x and y locations respectively in GL pixel space of that sample. The
    error INVALID_OPERATION is generated if <index> is greater than or equal to
    the value of SAMPLES.

    (Insert new Section 3.8.4, Renderbuffer Textures.  Renumber subsequent
    sections.)

    In addition to one-, two-, and three-dimensional and cube map textures
    described in previous sections, one additional type of texture is supported.
    A renderbuffer texture is similar to a two-dimensional texture.  However,
    unlike other texture types, the texel array is not stored as part of the
    texture.  Instead, a renderbuffer object is attached to a renderbuffer
    texture and the texel array is taken from the data store of the attached
    renderbuffer object.  When the contents of a renderbuffer are modified,
    those changes are reflected in the contents of any renderbuffer texture to
    which the buffer object is attached.  Also unlike other textures,
    renderbuffer textures do not have multiple image levels; only a single data
    store is available.

    The command

      void TexRenderbufferNV(enum target, uint renderbuffer);

    attaches the renderbuffer object named <renderbuffer> to the active
    renderbuffer texture. If <renderbuffer> is zero, any renderbuffer object
    attached to the buffer texture is detached, and no new renderbuffer object
    is attached.  If <renderbuffer> is non-zero, but is not the name of an
    existing renderbuffer object, the error INVALID_OPERATION is generated.
    <target> must be TEXTURE_RENDERBUFFER_NV.  The name of the renderbuffer
    object that is bound as the data store of the active renderbuffer texture
    can be queried by calling GetIntegerv with <value> set to
    TEXTURE_RENDERBUFFER_DATA_STORE_BINDING_NV.

    When a renderbuffer texture is accessed in a shader, the access takes one
    vector of integers describing which pixel to fetch and an integer
    corresponding to the sample numbers described in section 3.2.1 describing
    which sample within the pixel to fetch. No standard sampling instructions
    are allowed to the renderbuffer texture target.

    Modify Section 3.8.10, Texture Completeness (p. 177)

    (insert after second paragraph of section, p.177)

    For renderbuffer textures, a texture is <complete> if a renderbuffer object
    is attached to it, and each dimension of that renderbuffer is positive.

    Modify Section 3.8.11, Texture State and Proxy State (p. 178)

    (insert into the first paragraph of the section, p. 178) ... a zero
    compressed size, and zero-sized components).  The renderbuffer texture
    target contains an integer identifying the renderbuffer object that
    provides the data store for the texture, initially zero.  Next, there are
    the two sets of texture properties; ...

    Modify Section 3.8.12, Texture Objects (p. 180)

    (modify first paragraphs of section, p. 180, simply adding references to
     renderbuffer textures, which are treated as texture objects)

    In addition to the default textures TEXTURE_1D, TEXTURE_2D, TEXTURE_3D,
    TEXTURE_CUBE_MAP, and TEXTURE_RENDERBUFFER_NV, named one-, two-, and
    three-dimensional, cube map, and renderbuffer texture objects can be created
    and operated upon. The name space for texture objects is the unsigned
    integers, with zero reserved by the GL.

    A texture object is created by binding an unused name to TEXTURE_1D,
    TEXTURE_2D, TEXTURE_3D, TEXTURE_CUBE_MAP, or TEXTURE_RENDERBUFFER_NV. 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 values listed in section 3.8.11, set to the same initial
    values. If the new texture object is bound to TEXTURE_1D, TEXTURE_2D,
    TEXTURE_3D, TEXTURE_CUBE_MAP, or TEXTURE_RENDERBUFFER_NV, it is and remains
    a one-, two-, three-dimensional, cube map, or renderbuffer texture
    respectively until it is deleted.

    BindTexture may also be used to bind an existing texture object to either
    TEXTURE_1D, TEXTURE_2D, TEXTURE_3D, TEXTURE_CUBE_MAP, or
    TEXTURE_RENDERBUFFER_NV. 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.

    ...

    In the initial state, TEXTURE_1D, TEXTURE_2D, TEXTURE_3D, TEXTURE_CUBE_MAP,
    and TEXTURE_RENDERBUFFER_NV have one-, two-, three-dimensional, cube map,
    and renderbuffer texture state vectors respectively associated with them. In
    order that access to these initial textures not be lost, they are treated as
    texture objects all of whose names are 0. The initial one-, two-,
    three-dimensional, cube map, and renderbuffer texture is therefore operated
    upon, queried, and applied as TEXTURE_1D, TEXTURE_2D, TEXTURE_3D,
    TEXTURE_CUBE_MAP, or TEXTURE_RENDERBUFFER_NV respectively while 0 is bound
    to the corresponding targets.

    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 one of the targets
    TEXTURE_1D, TEXTURE_2D, TEXTURE_3D, TEXTURE_CUBE_MAP, or
    TEXTURE_RENDERBUFFER_NV is deleted, it is as though BindTexture had been
    executed with the same target and texture zero. Unused names in textures are
    silently ignored, as is the value zero.

    (modify second paragraph, p. 182, adding buffer textures, plus cube map
    textures, which is an oversight in the core specification)

    The texture object name space, including the initial one-, two-, and
    three-dimensional, cube map, and renderbuffer 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.

Additions to Chapter 4 of the OpenGL 2.0 Specification (Per-Fragment Operations
and the Frame Buffer)

    Modify Section 4.1.3, Multisample Fragment Operations

    Modify the first paragraph to include SAMPLE_MASK and SAMPLE_MASK_VALUE on
    the list of values the coverage value is modified based on.

    Modify the discussion of SAMPLE_COVERAGE to start with "Next" instead of
    "Finally"

    Add after the discussion of SAMPLE_COVERAGE:

    Finally, if SAMPLE_MASK is enabled, the fragment coverage is ANDed with the
    coverage value SAMPLE_MASK_VALUE. The value of SAMPLE_MASK_VALUE is
    specified using

        void SampleMaskIndexedNV(GLuint index, GLbitfield mask);
    
    with <mask> set to the desired mask for <maskNumber>. SAMPLE_MASK_VALUE is
    queried by calling GetIntegerIndexedv with <pname> set to SAMPLE_MASK_VALUE
    and the index set to <maskNumber>. Bit B of mask M corresponds to sample
    32*M+B as described in Section 3.2.1. The error INVALID_OPERATION is
    generated if the mask word indexed is greater than or equal to
    MAX_SAMPLE_MASK_WORDS.


Additions to Chapter 5 of the OpenGL 2.0 Specification (Special Functions)

    On p. 241, add the following to the list of commands not compiled into a
    display list: GetMultisamplefvNV, TexRenderbufferNV.

Additions to Chapter 6 of the OpenGL 2.0 Specification (State and
State Requests)

    Modify the second paragraph of section 6.1.1 (Simple Queries)
    p244 to read as follows:

    ...<data> is a pointer to a scalar or array of the indicated
    type in which to place the returned data.

        void GetBooleanIndexedvEXT(enum target, uint index, boolean *data);
        void GetIntegerIndexedvEXT(enum target, uint index, int *data);

    are used to query indexed state.  <target> is the name of 
    the indexed state and <index> is the index of the particular
    element being queried.  <data> is a pointer to a scalar or array
    of the indicated type in which to place the returned data. In addition ...

Additions to the AGL/GLX/WGL Specifications

    None

GLX Protocol

    The following rendering command is sent to the server as part of
    a glXRender request:

    SampleMaskIndexedNV

        2      12              rendering command length
        2      XXXX            rendering command opcode
        4      CARD32          index
        4      BITFIELD        mask

    TexRenderbufferNV

        2      12              rendering command length
        2      XXXX            rendering command opcode
        4      ENUM            target
        4      CARD32          renderbuffer
   
        
    The following new non-rendering commands are added:

    GetMultisamplefvNV

        1      CARD8           opcode(X assigned)                                    
        1      XXX             GLX opcode
        2      4               request length
        4      GLX_CONTEXT_TAG context tag      
        4      ENUM            pname
        4      CARD32          index
    =>  
        1      1               reply
        1                      unused
        2      CARD16          sequence number
        4      0               reply length
        8                      unused
        4      FLOAT32         val[0]
        4      FLOAT32         val[1]
        8                      unused
  
  
Dependencies on EXT_framebuffer_multisample

    If EXT_framebuffer_multisample is not available, all discussion involving
    textures and renderbuffers is deleted. This only leaves the ability to query
    the sample parameters of the current framebuffer.

Dependencies on EXT_timer_query

    If EXT_timer_query is not available, include its discussion of int64EXT and
    uint64EXT.

Dependencies on NV_gpu_program4 and EXT_gpu_shader4

    If NV_gpu_program4 and EXT_gpu_shader4 are not supported, and no other
    mechanism is provided to perform texture lookups into renderbuffer textures,
    this extension is pointless, given that it provides no fixed-function
    mechanism to access renderbuffer textures.

    If EXT_gpu_shader4 is supported, the language below describes the
    modifications to the shading language to support renderbuffer textures. If
    not, the language below and in Chapter 2 should be removed.

Errors

    The error INVALID_VALUE is generated by GetBooleanIndexedvEXT and
    GetIntegerIndexedvEXT if target is SAMPLE_MASK_VALUE_NV and index is greater
    than or equal to MAX_SAMPLE_MASK_WORDS_NV.

    The error INVALID_VALUE is generated by SampleMaskIndexedNV if index is greater
    than or equal to MAX_SAMPLE_MASK_WORDS_NV.


New State

    Get Value                       Get Command    Type    Initial Value    Attribute
    ---------                       -----------    ----    -------------    ---------
    SAMPLE_MASK_NV                  IsEnabled      B       FALSE            multisample/enable
    SAMPLE_MASK_VALUE_NV            GetInteger-    nxZ+    ~0               multisample
                                    IndexedvEXT

    Where n is the number of sample mask words (the value of
    MAX_SAMPLE_MASK_WORDS_NV) the implementation supports.

    (add to table 6.15, Texture State Per Texture Unit/Binding Point p. 276)

                                                            Initial
    Get Value                           Type    Get Command  Value  Description                 Sec.    Attribute
    ---------------------------------   ----    ----------- ------- --------------------------- ------  ---------
    TEXTURE_BINDING_RENDERBUFFER_NV     2*xZ+   GetIntegerv    0    Texture object bound to     3.8.12  texture
                                                                    TEXTURE_RENDERBUFFER_EXT

    (add to table 6.16, Texture State Per Texture Object, p. 276)

                                                            Initial
    Get Value                           Type    Get Command  Value  Description                 Sec.    Attribute
    ---------------------------------   ----    ----------- ------- --------------------------- ------  ---------
    TEXTURE_RENDERBUFFER_DATA_STORE_    nxZ+    GetIntegerv    0    Renderbuffer object bound   3.8.4   texture
      BINDING_NV                                                    as the data store for the
                                                                    active image unit's
                                                                    renderbuffer texture

    (add to table 9.nnn "Framebuffer dependent Values" introduced by
    the EXT_framebuffer_object specification)

                                                        Initial
    Get Value           Type        Get Command         Value           Description      Sec.    Attribute
    ------------------  ----------  ------------------  --------------  ---------------  ------  ---------
    SAMPLE_POSITION_NV  N*2*R[0,1]  GetMultisamplefvNV  implementation  Explicit sample  3.2.1   -
                                                        dependent       positions

    Where N is the number of samples (the value of SAMPLES) the framebuffer supports.

New Implementation Dependent State

                                                      Minimum
    Get Value                    Type    Get Command  Value   Description       Sec.    Attribute
    ---------                    ------- -----------  ------- ----------------  ------  ---------
    MAX_SAMPLE_MASK_WORDS_NV     Z       GetIntegerv  1       maximum number    x.x.x   -
                                                              of sample mask
                                                              words

Modifications to the OpenGL Shading Language Specification, Version 1.10.59

    Including the following line in a shader can be used to control the language
    featured described in this extension:

      #extension GL_NV_explicit_multisample : <behavior>

    where <behavior> is as specified in section 3.3.

    A new preprocessor #define is added to the OpenGL Shading Language:

      #define GL_NV_explicit_multisample 1

    Add to section 3.6 "Keywords"

    The following new sampler types are added:

      samplerRenderbuffer, isamplerRenderbuffer, usamplerRenderbuffer

    Add to section 4.1 "Basic Types"

    Add the following sampler type to the "default sampler types" table:

      samplerRenderbuffer        handle for accessing a renderbuffer texture

    Add the following sampler type to the "unsigned integer sampler types"
    table:

      isamplerRenderbuffer       handle for accessing an integer renderbuffer texture

    Add the following sampler type to the "integer sampler types" table:

      usamplerRenderbuffer       handle for accessing an unsigned integer renderbuffer texture

    Add to section 8.7 "Texture Lookup Functions"

    Add new functions to the set of allowed texture lookup functions:

    Syntax:

      vec4 texelFetchRenderbuffer(samplerRenderbuffer sampler, ivec2 coord, int sample)

    Description:

      Use integer texture coordinate <coord> to lookup a single sample <sample>
      on the texture bound to <sampler> as described in section 2.15.4.1 of the
      OpenGL specification "Multisample Texel Fetches".

    Syntax:

      ivec2 textureSizeRenderbuffer(samplerRenderbuffer sampler)
      ivec2 textureSizeRenderbuffer(isamplerRenderbuffer sampler)
      ivec2 textureSizeRenderbuffer(usamplerRenderbuffer sampler)

    Description:

      Returns the dimensions, width and height of level 0 for the texture bound
      to <sampler>, as described in section 2.15.4.1 of the OpenGL specification
      section "Texture Size Query".

Examples

Issues

    (1) Should the NUMBER_OF_SAMPLES parameter reuse an existing enum?

      RESOLVED. Just use SAMPLES_ARB.

    (2) How should we deal with sample patterns of more than
        sizeof(GLbitfield)*8 bits for the sample mask?

      RESOLVED. The API uses indexed calls to allow indefinite extension of the
      number of samples exposed.

    (3) What's the interaction between sample mask, sample coverage, and alpha
        to coverage?

      UNRESOLVED. In the hardware there's just one method for doing overall
      coverage, so we can combine the two API states into the methods as we see
      fit. The more interesting question is the interaction between the shader
      sample mask from GPU_program4_1 and the API-level masks, which we should
      try to keep similar to the API-level interface.

    (4) Should we expose the CSAA weights in this API?

      RESOLVED. Not in this extension. For now, only maskable samples can be
      queried. A future extension can allow this functionality.

    (5) What should the default value of SAMPLE_MASK be? 0? ~0?

      RESOLVED. ~0 to match with other state like stencil mask. 

    (6) Should the sample position query work for non-multisample?
      
      RESOLVED. The value of SAMPLES for non-multisample is zero, so the sample
      position query will return an error for all index values in this case.

    (7) Integer/unsigned integer renderbuffer sampler types?
     
      RESOLVED. There are integer/unsigned integer renderbuffer sampler types.

    (8) Do we need a program option to enable the new behavior?

      RESOLVED. Yes.

    (9) What should TEXTURE_RENDERBUFFER_DATA_STORE_BINDING_NV be if the
        renderbuffer object it refers to has been deleted?

      RESOLVED.  TEXTURE_RENDERBUFFER_DATA_STORE_BINDING_NV remains unchanged
      when the renderbuffer is deleted.

    (10) Do renderbuffer textures support texture parameters
         (TexParameter) or queries (GetTexParameter,
         GetTexLevelParameter, GetTexImage)?

      RESOLVED: No.  None of the existing parameters apply to renderbuffer
      textures, and this extension doesn't introduce the need for any new ones.
      Renderbuffer textures have no levels, and the size in texels is implicit
      (based on the data store).  Given that the texels themselves are obtained
      from a renderbuffer object, it seems more appropriate to retrieve such
      data with renderbuffer queries.

      Note that the spec edits above don't add explicit error language for any
      of these cases.  That is because each of the functions enumerate the set
      of valid <target> parameters.  Not editing the spec to allow
      TEXTURE_RENDERBUFFER_NV in these cases means that target is not legal, and
      an INVALID_ENUM error should be generated.

    (11) What is the behavior of TexelFetchMultisample when given an
         out-of-bounds value?

      RESOLVED: Undefined. This spec explicitly lists the behavior as undefined
      when given a sample number greater than SAMPLES-1 and inherits the
      undefined behavior specified for TexelFetch when the normal
      two-dimensional coordinate is outside the range of the texture.


Revision History

    Rev.    Date    Author    Changes
    ----  --------  --------  -----------------------------------------

    1     7/22/08   ewerness  First revision.