Name

    ARB_sample_locations

Name Strings

    GL_ARB_sample_locations

Contact

    Piers Daniell, NVIDIA Corporation (pdaniell 'at' nvidia.com)

Contributors

    Jeff Bolz, NVIDIA
    Pat Brown, NVIDIA
    Mathias Heyer, NVIDIA

Notice

    Copyright (c) 2015 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete. Approved by the ARB on June 26, 2015.
    Ratified by the Khronos Board of Promoters on August 7, 2015.

Version

    Last Modified Date:         June 11, 2015
    Revision:                   4

Number

    ARB Extension #181

Dependencies

    This extension is written against the OpenGL 4.5 specification
    (Compatibility Profile).

    This extension interacts with OpenGL ES 3.1.

Overview

    This extension allows an application to modify the locations of samples
    within a pixel used in multisample rasterization.  Additionally, it allows
    applications to specify different sample locations for each pixel in a
    group of adjacent pixels, which may increase antialiasing quality
    (particularly if a custom resolve shader is used that takes advantage of
    these different locations).

    It is common for implementations to optimize the storage of depth values
    by storing values that can be used to reconstruct depth at each sample
    location, rather than storing separate depth values for each sample. For
    example, the depth values from a single triangle can be represented using
    plane equations.  When the depth value for a sample is needed, it is
    automatically evaluated at the sample location. Modifying the sample
    locations causes the reconstruction to no longer evaluate the same depth
    values as when the samples were originally generated.  This extension
    provides a command to "evaluate" and store per-sample depth values using
    the currently programmed sample locations, which allows the application to
    manage this issue if/when necessary.

    The programmable sample locations are used during rasterization and for
    evaluation of depth functions during normal geometric rendering. The
    programmable locations are associated with a framebuffer object rather
    than an individual depth buffer, so if the depth buffer is used as a
    texture the texture sampling may be done at the standard sample
    locations. Additionally, commands that do not render geometric primitives
    (e.g. ReadPixels, BlitFramebuffer, CopyTexSubImage2D, etc.) may use the
    standard sample locations to evaluate depth functions rather than the
    programmable locations. If a single depth buffer is used at different
    times with different sample locations, the depth functions may be
    interpreted using the current sample locations.

New Procedures and Functions

    void FramebufferSampleLocationsfvARB(enum target, uint start,
                                         sizei count, const float *v);
    void NamedFramebufferSampleLocationsfvARB(uint framebuffer, uint start,
                                              sizei count, const float *v);

    void EvaluateDepthValuesARB();

New Tokens

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

        SAMPLE_LOCATION_SUBPIXEL_BITS_ARB                0x933D
        SAMPLE_LOCATION_PIXEL_GRID_WIDTH_ARB             0x933E
        SAMPLE_LOCATION_PIXEL_GRID_HEIGHT_ARB            0x933F
        PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB      0x9340

    Accepted by the <pname> parameter of GetMultisamplefv:

        // Alias of SAMPLE_POSITION. Before NV_expms, the spec used "location".
        SAMPLE_LOCATION_ARB                              0x8E50
        PROGRAMMABLE_SAMPLE_LOCATION_ARB                 0x9341

    Accepted by the <pname> parameter of FramebufferParameteri,
    GetFramebufferParameteriv:

        FRAMEBUFFER_PROGRAMMABLE_SAMPLE_LOCATIONS_ARB    0x9342
        FRAMEBUFFER_SAMPLE_LOCATION_PIXEL_GRID_ARB       0x9343

Additions to Chapter 9 of the OpenGL 4.5 (Compatibility Profile) Specification
(Framebuffers and Framebuffer Objects)

    Modify Section 9.2.1 (Framebuffer Object Parameters), p. 330

    Change the error:

    "An INVALID_OPERATION error is generated if the default framebuffer is
    bound to <target>." into "An INVALID_OPERATION error is generated if the
    default framebuffer is bound to <target> and <pname> is neither
    FRAMEBUFFER_PROGRAMMABLE_SAMPLE_LOCATIONS_ARB nor
    FRAMEBUFFER_SAMPLE_LOCATION_PIXEL_GRID_ARB."

    Modify Section 9.2.3 (Framebuffer Object Queries), p. 332

    Change the error:

    "An INVALID_OPERATION error is generated if the default framebuffer is
    bound to <target>." into "An INVALID_OPERATION error is generated if the
    default framebuffer is bound to <target> and <pname> is neither
    FRAMEBUFFER_PROGRAMMABLE_SAMPLE_LOCATIONS_ARB nor
    FRAMEBUFFER_SAMPLE_LOCATION_PIXEL_GRID_ARB."

Additions to Chapter 14 of the OpenGL 4.5 (Compatibility Profile) Specification
(Fixed-Function Primitive Assembly and Rasterization)

    Modify Section 14.3.1 (Multisampling), p. 527

    Remove the paragraph describing "GetMultisamplefv", and append the
    following to the section.

    Each framebuffer has a set of default sample locations, depending on the
    number of samples in the attachments or the value of FRAMEBUFFER_DEFAULT_-
    SAMPLES (for framebuffer objects) or the number of samples in the pixel
    format (for the default framebuffer). Each framebuffer also has a set of
    programmable sample locations, which may be used instead of the default
    sample locations. The programmable sample locations are controlled by the
    commands:

        void FramebufferSampleLocationsfvARB(enum target, uint start,
                                             sizei count, const float *v);
        void NamedFramebufferSampleLocationsfvARB(uint framebuffer, uint start,
                                             sizei count, const float *v);

    <target> or <framebuffer> selects the framebuffer whose programmable
    sample locations are modified. There are <N> pairs of programmable sample
    locations values in a framebuffer, where <N> is the value of
    PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB. Each programmable sample
    location is specified as a pair of floating point values in the range
    [0,1], corresponding to the x and y locations respectively in GL pixel
    space. (0.5, 0.5) thus corresponds to the pixel center. Sample locations
    outside of [0,1] result in undefined behavior.  These commands accept
    <count> pairs of values in <v> and update locations for samples <start> to
    <start>+<count>-1.  The location for sample <i> is taken from
    v[2*(i-start)] and v[2*(i-start)+1].

    Errors:

      An INVALID_VALUE error is generated if the sum of <start> and <count> is
      greater than PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB.

    Default and programmable sample locations may be queried with the command:

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

    <index> corresponds to which sample location should be returned, and the
    sample location is returned as two floating-point values in val[0] and
    val[1]. If <pname> is SAMPLE_LOCATION_ARB (aliasing SAMPLE_POSITION),
    a default sample location is returned and <index> must be less than
    the value of SAMPLES. If <pname> is PROGRAMMABLE_SAMPLE_LOCATION_ARB,
    a programmable sample location is returned and <index> must be less
    than the value of PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB.

    Sample locations are rounded on use to the precision indicated by the value
    of SAMPLE_LOCATION_SUBPIXEL_BITS_ARB (i.e. rounded to the nearest
    2^{-subpixelbits}). This precision may depend on the number of samples in
    the framebuffer. The initial programmable sample locations are all
    (0.5,0.5).

    Errors:

        An INVALID_ENUM error is generated if <target> is not
        DRAW_FRAMEBUFFER, READ_FRAMEBUFFER, or FRAMEBUFFER.

        An INVALID_OPERATION error is generated by
        NamedFramebufferSampleLocationsfvARB if <framebuffer> is not
        the name of an existing framebuffer object.

        An INVALID_VALUE error is generated if <pname> is SAMPLE_LOCATION_ARB
        and <index> is greater than or equal to the value of SAMPLES.

        An INVALID_VALUE error is generated if <pname> is
        PROGRAMMABLE_SAMPLE_LOCATION_ARB and <index> is greater than or equal
        to the value of PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB.

        An INVALID_ENUM error is generated if <pname> is not
        SAMPLE_LOCATION_ARB or PROGRAMMABLE_SAMPLE_LOCATION_ARB.

    Programmable sample locations are enabled by calling FramebufferParameteri
    with a <pname> of FRAMEBUFFER_PROGRAMMABLE_SAMPLE_LOCATIONS_ARB and a
    non-zero value of <param>. The initial value of FRAMEBUFFER_PROGRAMMABLE_-
    SAMPLE_LOCATIONS_ARB is zero.

    Programmable sample locations can vary across pixels, based on the pixel x
    and y coordinate. A framebuffer has a sample location pixel grid which may
    depend on the number of samples. This grid size can be queried by calling
    GetIntegerv with a <pname> of SAMPLE_LOCATION_PIXEL_GRID_WIDTH_ARB and
    SAMPLE_LOCATION_PIXEL_GRID_HEIGHT_ARB, which returns the grid dimensions for
    the draw framebuffer.

    If FRAMEBUFFER_PROGRAMMABLE_SAMPLE_LOCATIONS_ARB is enabled and the
    framebuffer parameter FRAMEBUFFER_SAMPLE_LOCATION_PIXEL_GRID_ARB is enabled
    (non-zero), sample locations are selected as follows:

        grid_x = value of SAMPLE_LOCATION_PIXEL_GRID_WIDTH_ARB;
        grid_y = value of SAMPLE_LOCATION_PIXEL_GRID_HEIGHT_ARB;

        pixel_x = <window x coordinate> mod grid_x;
        pixel_y = <window y coordinate> mod grid_y;

        num_samples = value of SAMPLES;
        sample_i = (pixel_y*grid_x + pixel_x)*num_samples + sample_index;

        float *table = FRAMEBUFFER_PROGRAMMABLE_SAMPLE_LOCATIONS_ARB;
        sample_location.xy = (table[2*sample_i], table[2*sample_i+1]);

    If FRAMEBUFFER_PROGRAMMABLE_SAMPLE_LOCATIONS_ARB is enabled and
    FRAMEBUFFER_SAMPLE_LOCATION_PIXEL_GRID_ARB is disabled, sample locations are
    selected as follows:

        sample_i = sample_index;

        float *table = FRAMEBUFFER_PROGRAMMABLE_SAMPLE_LOCATIONS_ARB;
        sample_location.xy = (table[2*sample_i], table[2*sample_i+1]);

    If a framebuffer is incomplete, querying the value of
    SAMPLE_LOCATION_SUBPIXEL_BITS_ARB, SAMPLE_LOCATION_PIXEL_GRID_WIDTH_ARB, and
    SAMPLE_LOCATION_PIXEL_GRID_HEIGHT_ARB will return zero.

    Add new Subsection 14.3.1.X (Resolving Depth Values)

    It is common for implementations to optimize the storage of depth values
    by storing values that can be used to reconstruct depth at each sample
    location, rather than storing separate depth values for each sample. For
    example, the depth values from a single triangle can be represented using
    plane equations.  When the depth value for a sample is needed, it is
    automatically evaluated at the sample location. Modifying the sample
    locations causes the reconstruction to no longer evaluate the same depth
    values as when the samples were originally generated.  The choice of using
    separate depth values for each sample or some other reconstruction method
    is implementation-dependent and may not be queried.

    If per-sample depth values need to be reconstructed, some commands may
    evaluate depth values using default sample locations even if programmable
    sample locations are enabled.  This evaluation can occur either when
    reading sample values or when updating one sample requires the
    implementation to reconstruct depth values for a group of neighboring
    samples. These commands include ReadPixels, DrawPixels, CopyPixels,
    GetTexImage, CopyTexImage, CopyTexSubImage, TexImage, TexSubImage, and
    BlitFramebuffer. Texturing from a depth texture whose values need to be
    reconstructed may also evaluate depth functions as the default sample
    locations.

    The command

        void EvaluateDepthValuesARB();

    evaluates depth values for all samples in the current depth buffer
    (subject to the pixel ownership and scissor tests) and stores each value
    in the depth buffer.  This can be used to ensure that later accesses will
    use depth values consistent with the sample locations used when the
    samples were generated.  If the current framebuffer has no depth buffer,
    EvaluateDepthValuesARB will have no effect.


New Implementation Dependent State

                                                      Minimum
    Get Value                    Type    Get Command  Value   Description                   Sec.
    ---------                    ------- -----------  ------- ------------------------      ------
    PROGRAMMABLE_SAMPLE_-          Z+    GetIntegerv     2    Size of programmable sample   14.3.1
        LOCATION_TABLE_SIZE_ARB                               location table

New State

    Table 23.84 (Framebuffer Dependent Values):

    Get Value                               Get Command       Type        Minimum Value    Description             Sec.    Attribute
    ---------                               -----------       ----        -------------    -----------             ----    ---------
    SAMPLE_LOCATION_SUBPIXEL_BITS_ARB       GetIntegerv        Z+             4            Precision of sample     14.3.1  -
                                                                                           locations
    SAMPLE_LOCATION_PIXEL_GRID_WIDTH_ARB    GetIntegerv        Z+             1            Size of programmable    14.3.1  -
                                                                                           location grid
    SAMPLE_LOCATION_PIXEL_GRID_HEIGHT_ARB   GetIntegerv        Z+             1            Size of programmable    14.3.1  -
                                                                                           location grid
    SAMPLE_LOCATION_ARB                     GetMultisamplefv   n*2*R[0,1] impl-dependent   Default sample          14.3.1 -
                                                                                           positions
    Table 23.30. (Framebuffer):

    Get Value                               Get Command     Type    Initial Value    Description             Sec.    Attribute
    ---------                               -----------     ----    -------------    -----------             ----    ---------
    FRAMEBUFFER_PROGRAMMABLE_-              GetFramebuffer-  B          FALSE        Enable programmable     14.3.1  -
        SAMPLE_LOCATIONS_ARB                Parameteriv                              sample locations
    FRAMEBUFFER_SAMPLE_LOCATION_-           GetFramebuffer-  B          FALSE        Enable varying          14.3.1  -
        PIXEL_GRID_ARB                      Parameteriv                              locations per pixel
    PROGRAMMABLE_SAMPLE_LOCATION_ARB        GetMultisamplefv n*2*R[0,1] (0.5,0.5)    Programmable sample     14.3.1  -
                                                                                     locations


Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    None.

Modifications to the OpenGL Shading Language Specification, Version 4.50

    None.

Errors

    INVALID_VALUE is generated by FramebufferSampleLocationsfvARB or
    NamedFramebufferSampleLocationsfvARB if the sum of <start> and <count> is
    greater than PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB.

    INVALID_VALUE is generated by GetMultisamplefv if <pname> is
    PROGRAMMABLE_SAMPLE_LOCATION_ARB and <index> is greater than or equal to
    the value of PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB.

    Some errors in the original specification are relaxed to allow for new
    functionality provided by this extension (e.g., allowing programmable
    sample location framebuffer parameters to be set on the default
    framebuffer).

Interactions with OpenGL ES 3.1

    If implemented in OpenGL ES 3.1, remove references to GetDoublev,
    DrawPixels, CopyPixels and GetTexImage.

Issues

    (1) Why is SAMPLE_LOCATION_ARB added?

    RESOLVED: Prior to the NV_explicit_multisample spec (and its incorporation
    into ARB_texture_multisample), the spec used the word "location" to
    describe the placement of the sample within a pixel, rather than
    "position". This alias of SAMPLE_POSITION is added to return to that
    convention.

    (2) How should values that depend on the number of samples be handled for
    incomplete framebuffers?

    RESOLVED: Return zero. Other such queries (e.g. RED_BITS) are undefined
    but do not generate an error. Let's be more well-defined, but still not
    generate an error.

    (3) Should programmable sample locations affect non-multisample rendering?

    RESOLVED: No, the programmable sample locations only apply to multisample
    rasterization rules. Non-multisample rasterization is generally defined by
    rules involving "fragment centers" or (for lines) a diamond around the
    fragment center. Multisample rasterization is defined by rules involving
    sample points being inside or outside of a region defined by the primitive.
    Thus, only multisample rasterization rules are affected by modifying the
    sample locations.

    (4) How does this extension differ from AMD_sample_positions?

    RESOLVED:  There are a few differences between this extension and
    AMD_sample_positions:

    - This extension allows the sample locations to vary per-pixel within a
      grid of adjacent pixels.

    - This extension specifies some limitations on texturing on various copy
      operations when a source image was generated with programmable
      locations, or to existing samples the framebuffer when the sample
      locations change.  It also provides an explicit "resolve" operation that
      allows one to ensure that correct values are stored in all samples when
      non-framebuffer operations not supporting the programmable locations are
      used.  This issue is not explicitly handled in the AMD extension.

    - This extension has explicit framebuffer parameters enabling programmable
      sample locations and multi-pixel grid support.  The AMD extension lets
      you effectively delete all programmable sample locations and revert to
      default locations via programming a new location using a NULL pointer.

    - The sample locations in this extension are explicitly framebuffer object
      state, with separate state available for the default framebuffer (zero).
      It's unclear from the AMD extension whether the state is global or per
      framebuffer object, though a note in comment (1) ("Any other fbos will
      not be affected by this change") suggests it might also be framebuffer
      object state.

    (5) Can PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB be less than SAMPLES
    for a specific framebuffer?

    RESOLVED: In order to allow the application to specify the sample
    locations for all samples, the implementation must make the size of
    PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB at least as big as the
    framebuffer SAMPLES size. It would be invalid for an implementation
    to report a PROGRAMMABLE_SAMPLE_LOCATION_TABLE_SIZE_ARB less than
    any framebuffer SAMPLES size.

Revision History

    Revision 1
    - Branch from NV_sample_locations

    Revision 2  pdaniell 5/26
    - Rename ResolveDepthValuesARB() to EvaluateDepthValuesARB().

    Revision 3  ewerness 6/5
    - Complete rename of ResolveDepthValuesARB() to EvaluateDepthValuesARB().

    Revision 4  pdaniell 6/11
    - Add issue (5) to cover question in bug 14100.