Name

    ARB_shader_texture_lod

Name Strings

    GL_ARB_shader_texture_lod

Contributors

    Bill Licea-Kane
    Evan Hart

    and contributors to the arb-gl2 working groups, the product
    of which provided the basis for this spec.

Contact

    Bill Licea-Kane, ATI Research (bill 'at' ati.com)

Notice

    Copyright (c) 2009-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

IP Status

    No known IP issues.

Status

    Complete. Approved by the ARB on May 13, 2009.

Version

    Last Modified Date: April 13, 2011
    Revision: 11

Number

    ARB Extension #60

Dependencies

    This extension is written against the OpenGL 2.0 Specification.

    This extension is written against The OpenGL Shading Language,
    Language Version 1.10, Document Revision 59.

    ARB_shader_objects is required.

    This extension interacts with ATI_shader_texture_lod.

    This extension interacts with ARB_texture_rectangle.

    This extension interacts with EXT_texture_filter_anisotropic.

Overview

    This extension adds additional texture functions to the
    OpenGL Shading Language which provide the shader writer
    with explicit control of LOD.

    Mipmap texture fetches and anisotropic texture fetches
    require an implicit derivatives to calculate rho, lambda
    and/or the line of anisotropy.  These implicit derivatives
    will be undefined for texture fetches occurring inside
    non-uniform control flow or for vertex shader texture
    fetches, resulting in undefined texels.

    The additional texture functions introduced with
    this extension provide explict control of LOD
    (isotropic texture functions) or provide explicit
    derivatives (anisotropic texture functions).

    Anisotropic texture functions return defined texels
    for mipmap texture fetches or anisotropic texture fetches,
    even inside non-uniform control flow.  Isotropic texture
    functions return defined texels for mipmap texture fetches,
    even inside non-uniform control flow.  However, isotropic
    texture functions return undefined texels for anisotropic
    texture fetches.

    The existing isotropic vertex texture functions:

        texture1DLod,   texture1DProjLod,
        texture2DLod,   texture2DProjLod,
        texture3DLod,   texture3DProjLod,
        textureCubeLod,
        shadow1DLod,    shadow1DProjLod,
        shadow2DLod,    shadow2DProjLod,

    are added to the built-in functions for fragment shaders.

    New anisotropic texture functions, providing explicit
    derivatives:

        texture1DGradARB(         sampler1D sampler,
                                  float P, float dPdx, float dPdy );
        texture1DProjGradARB(     sampler1D sampler,
                                  vec2  P, float dPdx, float dPdy );
        texture1DProjGradARB(     sampler1D sampler,
                                  vec4  P, float dPdx, float dPdy );

        texture2DGradARB(         sampler2D sampler,
                                  vec2  P, vec2  dPdx, vec2  dPdy );
        texture2DProjGradARB(     sampler2D sampler,
                                  vec3  P, vec2  dPdx, vec2  dPdy );
        texture2DProjGradARB(     sampler2D sampler,
                                  vec4  P, vec2  dPdx, vec2  dPdy );

        texture3DGradARB(         sampler3D sampler,
                                  vec3  P, vec3  dPdx, vec3  dPdy );
        texture3DProjGradARB(     sampler3D sampler,
                                  vec4  P, vec3  dPdx, vec3  dPdy );

        textureCubeGradARB(       samplerCube sampler,
                                  vec3  P, vec3  dPdx, vec3  dPdy );

        shadow1DGradARB(          sampler1DShadow sampler,
                                  vec3  P, float dPdx, float dPdy );
        shadow1DProjGradARB(      sampler1DShadow sampler,
                                  vec4  P, float dPdx, float dPdy );

        shadow2DGradARB(          sampler2DShadow sampler,
                                  vec3  P, vec2  dPdx, vec2  dPdy );
        shadow2DProjGradARB(      sampler2DShadow sampler,
                                  vec4  P, vec2  dPdx, vec2  dPdy );


        texture2DRectGradARB(     sampler2DRect sampler,
                                  vec2  P, vec2  dPdx, vec2  dPdy );
        texture2DRectProjGradARB( sampler2DRect sampler,
                                  vec3  P, vec2  dPdx, vec2  dPdy );
        texture2DRectProjGradARB( sampler2DRect sampler,
                                  vec4  P, vec2  dPdx, vec2  dPdy );

        shadow2DRectGradARB(      sampler2DRectShadow sampler,
                                  vec3  P, vec2  dPdx, vec2  dPdy );
        shadow2DRectProjGradARB(  sampler2DRectShadow sampler,
                                  vec4  P, vec2  dPdx, vec2  dPdy );

     are added to the built-in functions for vertex shaders
     and fragment shaders.

New Procedures and Functions

    None

New Tokens

    None

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

    None

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

    In Section 3.8.8, replace the final paragraph on p. 171 with:

    "Let s(x, y) be the function that associates an s texture coordinate
    with each set of window coordinates (x, y) that lie within a
    primitive;  define t(x, y) and r(x, y) analogously.  For a
    one-dimensional texture, define t(x, y) = 0 and r(x, y) = 0; for a
    two-dimensional texture, define r(x, y) = 0.

    Let u(x, y) = wt * s(x, y), v(x, y) = ht * t(x, y), and
    w(x, y) = dt * r(x, y), where wt, ht, and dt are as defined by equations
    3.15, 3.16, and 3.17 with ws, hs, and ds equal to the width, height,
    and depth of the image array whose level is levelbase.  However, for
    rectangular textures let u(x, y) = s(x, y), v(x, y) = t(x, y), and
    w(x, y) = r(x, y)."

    Let
        dUdx = wt*dSdx; dUdy = wt*dSdy;
        dVdx = ht*dTdx; dVdy = ht*dTdy;
        dWdx = dt*dRdx; dWdy = dt*dRdx;                       (3.21a)

    where dSdx indicates the derivative of s with respect to window x,
    and similarly for the other derivatives, and

    For a rectangular texture, let
        dUdx = dSdx; dUdy = dSdy;
        dVdx = dTdx; dVdy = dTdy;
        dWdx = 0.0;                                           (3.21b)

    For a polygon, rho is given at a fragment with window coordinates
    (x, y) by

        rho = max (
              sqrt(dUdx*dUdx + dVdx*dVdx + dWdx*dWdx),
              sqrt(dUdy*dUdy + dVdy*dVdy + dWdy*dWdy)
              );                                              (3.21c)"


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

    None

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

    None

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

    None

Additions to Appendix A of the OpenGL 2.0 Specification (Invariance)

    None

Additions to the AGL/GLX/WGL Specifications

    None


Additions to version 1.10.59 of the OpenGL Shading Language Specification

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

        #define GL_ARB_shader_texture_lod 1

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

        #extension GL_ARB_shader_texture_lod : <behavior>

    Where <behavior> is as specified in section 3.3."


Additions to Chapter 8 of version 1.10.59 of the OpenGL Shading Language
Specification


    8.7  Texture Lookup Functions

    Delete the last paragraph, and replace with:

    "For the "Lod" functions, lod specifies lambda_base (see equation 3.18 in
    The OpenGL 2.0 Specification) and specifies dSdx, dTdx, dRdx = 0 and
    dSdy, dTdy, and dRdy = 0 (see equation 3.21a and 3.21b. in The OpenGL
    2.0 Specification).  The "Lod" functions are allowed in a vertex shader.
    If enabled by the preprocessor directive #extension, the "Lod" functions
    are also allowed in a fragment shader.

    For the "Grad" functions, dPdx is the explicit derivative of P with respect
    to window x, and similarly dPdy with respect to window y. For the “ProjGrad”
    functions, dPdx is the explicit derivative of the projected P with respect
    to window x, and similarly for dPdy with respect to window y.  For a
    one-dimensional texture, dPdx and dPdy are float.  For a two-dimensional
    texture, dPdx and dPdy are vec2.  Otherwise, dPdx and dPdy are vec3.


    Let

        dSdx = dPdx.s;
        dSdy = dPdy.s;

    and

                / 0.0;    for one-dimensional texture
        dTdx = (
                \ dPdx.t; otherwise

                / 0.0;    for one-dimensional texture
        dTdy = (
                \ dPdy.t; otherwise

    and
                / 0.0;    for one-dimenstional or two-dimensional texture
        dRdx = (          (including rectangular texture)
                \ dPdx.p; otherwise
                          (including cubemap texture)

                / 0.0;    for one dimensional or two-dimensional texture
        dRdy = (          (including rectangular texture)
                \ dPdy.p; otherwise
                          (including cubemap texture)

    (See equation 3.21a and 3.21b in  The OpenGL 2.0 Specification).

    If enabled by the preprocessor directive #extension, the "Grad" functions
    are allowed in vertex and fragment shaders.

    All other texture functions may require implicit derivatives.  Implicit
    derivatives are undefined within non-uniform control flow or for vertex
    shader texture fetches."

    Add the following entries to the texture function table:

        texture1DGradARB(         sampler1D sampler,
                                  float P, float dPdx, float dPdy );
        texture1DProjGradARB(     sampler1D sampler,
                                  vec2  P, float dPdx, float dPdy );
        texture1DProjGradARB(     sampler1D sampler,
                                  vec4  P, float dPdx, float dPdy );

        texture2DGradARB(         sampler2D sampler,
                                  vec2  P, vec2  dPdx, vec2  dPdy );
        texture2DProjGradARB(     sampler2D sampler,
                                  vec3  P, vec2  dPdx, vec2  dPdy );
        texture2DProjGradARB(     sampler2D sampler,
                                  vec4  P, vec2  dPdx, vec2  dPdy );

        texture3DGradARB(         sampler3D sampler,
                                  vec3  P, vec3  dPdx, vec3  dPdy );
        texture3DProjGradARB(     sampler3D sampler,
                                  vec4  P, vec3  dPdx, vec3  dPdy );

        textureCubeGradARB(       samplerCube sampler,
                                  vec3  P, vec3  dPdx, vec3  dPdy );

        shadow1DGradARB(          sampler1DShadow sampler,
                                  vec3  P, float dPdx, float dPdy );
        shadow1DProjGradARB(      sampler1DShadow sampler,
                                  vec4  P, float dPdx, float dPdy );

        shadow2DGradARB(          sampler2DShadow sampler,
                                  vec3  P, vec2  dPdx, vec2  dPdy );
        shadow2DProjGradARB(      sampler2DShadow sampler,
                                  vec4  P, vec2  dPdx, vec2  dPdy );


        texture2DRectGradARB(     sampler2DRect sampler,
                                  vec2  P, vec2  dPdx, vec2  dPdy );
        texture2DRectProjGradARB( sampler2DRect sampler,
                                  vec3  P, vec2  dPdx, vec2  dPdy );
        texture2DRectProjGradARB( sampler2DRect sampler,
                                  vec4  P, vec2  dPdx, vec2  dPdy );

        shadow2DRectGradARB(      sampler2DRectShadow sampler,
                                  vec3  P, vec2  dPdx, vec2  dPdy );
        shadow2DRectProjGradARB(  sampler2DRectShadow sampler,
                                  vec4  P, vec2  dPdx, vec2  dPdy );


Interactions with ATI_texture_shader_lod

    Each texture function added to the texture function table
    by ATI_texture_shader_lod is suffixed with "_ATI".

    The equivalent texture functions added to the texture
    function table by this extension are suffixed with "GradARB".

    It is anticipated that this extension might get promoted
    to core.  The promoted texture functions added to the
    texture function table would then be suffixed with "Grad".

    Note that ATI_texture_shader_lod does not contain the
    interactions with ARB_texture_rectangle and EXT_texture_anisotropic.
    Nor does it overload the texture2DRect and shadow2DRect functions.
    This extension is a slight superset of ATI_texture_shader_lod.

Interactions with ARB_texture_rectangle

    If ARB_texture_rectangle is not supported, delete references
    to rectangular textures and references to equation 3.21b.

Interactions with EXT_texture_anisotropic

    The Lod functions set the derivatives ds/dx, dt/dx, dr/dx,
    dx/dy, dt/dy, and dr/dy = 0.  Therefore Rhox and Rhoy = 0
    0, Rhomax and Rhomin = 0.

Issues

    (1) What should this extension be called?

    RESOLVED:  ARB_shader_texture_lod

    (2) There are several existing built-in functions we
    could use.  Should we use them, and do we need to
    name decorate them with suffix _ARB?

    It makes sense to use the existing built-ins suffixed
    with "Lod."  We will simply drop the restriction that
    they are allowed only in the vertex shader.

    Since this extension simply extends the scope of
    the functions, I don't think we need to suffix with _ARB.

    RESOLVED:  Yes, use existing "Lod" built-in names without
    an _ARB suffix.


    (3) Should we add additional texture functions with derivative
    built-ins?

    There are two approaches.

    Existing texture functions could be overloaded to
    accept optional derivatives.  (Similar to the Lod bias
    texture functions.)  These names *should* probably
    be name decorated with the suffix ARB."

    New texture functions suffixed with GradARB
    could be added.

    The advantage of overloading the existing functions
    is it reduces the plethora of texture function names.

    The disadvantage of overloading the existing functions
    is that not all of the existing texture functions
    make sense with explicit derivatives.  (Example,
    projective texture fetch with explict derivatives.)

    We have two sets of precedent here.  The existing absolute
    LOD texture functions use explicit name decoration.
    The existing LOD bias texture functions use overloading.

    RESOLVED:  We will use existing built-in texture functions
    suffixed with "Lod" *WITHOUT* name decoration for absolute
    LOD functions.

    We will use existing built-in texture functions suffixed with
    GradARB for derivatives.

    We establish a precedent that new texture functions
    will have explicit names.  The existing LOD bias texture
    functions are exceptions to the precedent.

    (4) What is done with the vec3 dPdx and vec3 dPdy for
    cube map texture fetches?

    For cube maps, it is incorrect to simply say "dPdx
    specificies ds/dx, dt/dx, and dr/dx, and dPdy specifies
    ds/dy, dt/dy and dr/dy (see equation 3.21a in The
    OpenGL 2.0 Specification)."

    This is currently tough to specify given the core specification.

    3.8.6 covers cube map face selection and transformation of
    the incoming rx, ry, rz to s and t.

    3.8.8 covers derivatives, and states:

    "Let s(x, y) be the function that associates an s texture
    coordinate with each set of window coordinates (x, y) that
    lie within a primitive; define t(x, y) and r(x, y) analogously."

    Because of the cube mape face selection of 3.8.6, s(x, y)
    and t(x, y) may be discontinuous within a primitive.  A discontinuous
    function has undefined derivatives at the discontinuity.  (This
    discontinuity occurs at the cube map edges.)

    The core spec is silent on how this discontinuity may be resolved.
    Nor does the core spec provide an alternative method for calculating
    rho given the continuous original rx, ry, rz, and their well
    specified derivatives (dPdx and dPdy).

    The changes required to the core spec are substantial and may
    require much discussion on the tradeoffs between tightly
    specifying a method and providing sufficient freedom for implementers.

    RESOLVED:  For now, we will leave this as unspecified.
    Implementors are cautioned to take sufficient steps so
    that the calculation of rho based on explicit dPdx
    and dPdy provides comparable quality to the existing
    implicit derivative methods.

    (5) Should the projective versions of the anisotropic texture
    functions be added?

    ATI_shader_texture_lod resolved issue 3 to use overloaded
    functions (opposite of the way issue 3 is resolved here),
    including overloading the projective texture functions.
    We have insufficient feedback to permit removing them in this
    extension.  We err on the side of caution and keep them.
    This extension is therefore a functional equivalent to
    ATI_shader_texture_lod.  (Slight superset, see issue 6.)

    RESOLVED:  Add them since ATI_shader_texture_lod introduced
    overloaded versions.

    (6) Should anisotropic rectangular texture functions be added?

    Even though derivatives are not required for Lod selection
    for rectangular textures (there is only the baselevel for
    rectangular textures) the derivatives are required for
    anisotropic texture fetches.  (Perhaps
    ARB_texture_rectangle should have an interaction with
    EXT_texture_anisotropic clarifying this.)

    ATI_shader_texture_lod is silent on these interactions.
    Adding the interactions and functions makes this extension
    a slight superset of ATI_shader_texture_lod.

    RESOLVED:  Add them.

    (7) How are ProjGrad functions handled?

    The dPdx and dPdy derivatives are of the projected P.  The shader
    writer is responsible for this projection.  That is, for a 2D
    texture, dPdx = dFdx( P.st/P.q );  dPdy = dFdy( P.st/P.q );

Revision History:

10 - 2011-04-13 (Jon Leech)
    * Remove nonexistent textureCubProjLod from list of existing
      isotropic vertex texture functions (Khronos Bug 7205).

9 - 2010-12-09 (Jon Leech)
    * Corrected typos where 1D texture functions were specified
      as taking sampler2D parameters (Khronos Bug 7008).

8 - 2009-05-24 (Jon Leech)
    * Added to the Registry
    * Corrected minor typos

7 - 2007-08-22
    * Shipping status.

6 - 2006-07-06
    * Clarified ProjGrad.
    * Added issue 7.
    * Clarified cubemap.

5 - 2006-06-08
    * u(x,y),v(x,y) and w(x,y) are used for wrap calculations.
      Their definitions (with ARB_texture_rectangle interaction)
      are restored.
    * Clean up defined/undefined language of functions in
      overview.

4 - 2006-05-25
    * Follow ASCII conventions.  See examples in core
      spec equations 3.23, 3.24, & 3.25 and EXT_texture_3D.

3 - 2006-05-25
    * Add #extension and #define
    * Explicitly reference page/paragraph in core edits.
    * Rework texture function descriptions.  Add language
      to shading language spec that makes it clear that the
      extension must be enabled for the functions to be
      available.

2 - 2006-05-25
    * Name changes.
    * Added interaction with ARB_texture_rectangle.
    * Added interaction with EXT_texture_anisotropic.
    * Clarified that implicit derivatives are undefined
      within non-uniform control flow and the vertex shader.
    * Relocated issues to the tail of the spec.

1 - 2006-05-17
    Initial ARB version