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Name Strings
Piers Daniell (pdaniell 'at'
Contributors to EXT_texture_filter_anisotropic
Jon Leech, Khronos
Daniel Koch, NVIDIA
Copyright (c) 2017-2019 The Khronos Group Inc. Copyright terms at
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
Last Modified Date: February 26, 2019
Revision: 6
ARB Extension #195
OpenGL 1.2 required.
This extension is written against the OpenGL 4.5 (Compatibility Profile)
Specification (October 24, 2016).
Texture mapping using OpenGL's existing mipmap texture filtering
modes assumes that the projection of the pixel filter footprint into
texture space is a square (ie, isotropic). In practice however, the
footprint may be long and narrow (ie, anisotropic). Consequently,
mipmap filtering severely blurs images on surfaces angled obliquely
away from the viewer.
Several approaches exist for improving texture sampling by accounting
for the anisotropic nature of the pixel filter footprint into texture
space. This extension provides a general mechanism for supporting
anisotropic texturing filtering schemes without specifying a
particular formulation of anisotropic filtering.
The extension permits the OpenGL application to specify on a per-texture
or -sampler object basis the maximum degree of anisotropy to account for
in texture filtering.
Increasing the maximum degree of anisotropy may
improve texture filtering, but may also significantly reduce the
implementation's texture filtering rate. Implementations are free
to clamp the specified degree of anisotropy to the implementation's
maximum supported degree of anisotropy.
A sampler or texture's maximum degree of anisotropy is specified
independently from its minification and magnification filter (as
opposed to being supported as an entirely new filtering mode).
Implementations are free to use the specified minification and
magnification filter to select a particular anisotropic texture
filtering scheme. For example, a NEAREST filter with a maximum
degree of anisotropy of two could be treated as a 2-tap filter that
accounts for the direction of anisotropy. Implementations are also
permitted to ignore the minification or magnification filter and
implement the highest quality of anisotropic filtering possible.
Applications seeking the highest quality anisotropic filtering
available are advised to request a LINEAR_MIPMAP_LINEAR minification
filter, a LINEAR magnification filter, and a large maximum degree
of anisotropy.
New Procedures and Functions
New Tokens
Accepted by the <pname> parameters of GetTexParameterfv,
GetTexParameteriv, TexParameterf, TexParameterfv, TexParameteri,
and TexParameteriv:
Accepted by the <pname> parameters of GetBooleanv, GetDoublev,
GetFloatv, and GetIntegerv:
Additions to Chapter 8 of the OpenGL 4.5 Specification (Textures and Samplers)
-- Sections 8.10 "Texture Parameters"
Add the following entry to the end of Table 8.25, "Texture parameters
and their values":
Name Type Legal Values
-------------------------- ------ --------------------------
TEXTURE_MAX_ANISOTROPY float greater or equal to 1.0
In the errors section for TexParameter*() on page 267 add the following:
"INVALID_VALUE is generated when TexParameter is called with <pname>
of TEXTURE_MAX_ANISOTROPY and any value specified by <param> or
<params> is less than 1.0."
-- Sections 8.14 "Texture Minification" and 8.15 "Texture Magnification"
After the first paragraph in Section 8.14:
"When the texture's value of TEXTURE_MAX_ANISOTROPY is equal to 1.0,
the GL uses an isotropic texture filtering approach as described in
this section and Section 8.15. However, when the texture's value
of TEXTURE_MAX_ANISOTROPY is greater than 1.0, the GL implementation
should use a texture filtering scheme that accounts for a degree
of anisotropy up to the smaller of the texture's value of
TEXTURE_MAX_ANISOTROPY or the implementation-defined value of
The particular scheme for anisotropic texture filtering is
implementation dependent. Additionally, implementations are free
to consider the current texture minification and magnification modes
to control the specifics of the anisotropic filtering scheme used.
The anisotropic texture filtering scheme may only access mipmap
levels if the minification filter is one that requires mipmaps.
Additionally, when a minification filter is specified, the
anisotropic texture filtering scheme may only access texture mipmap
levels between the texture's values for TEXTURE_BASE_LEVEL and
TEXTURE_MAX_LEVEL, inclusive. Implementations are also recommended
to respect the values of TEXTURE_MAX_LOD and TEXTURE_MIN_LOD to
whatever extent the particular anisotropic texture filtering
scheme permits this."
The following describes one particular approach to implementing
anisotropic texture filtering for the 2D texturing case:
"Anisotropic texture filtering substantially changes Section 8.14.
Previously a single scale factor P was determined based on the
pixel's projection into texture space. Now two scale factors,
Px and Py, are computed.
Px = sqrt(dudx^2 + dvdx^2)
Py = sqrt(dudy^2 + dvdy^2)
Pmax = max(Px,Py)
Pmin = min(Px,Py)
N = min(ceil(Pmax/Pmin),maxAniso)
Lambda' = log2(Pmax/N)
where maxAniso is the smaller of the texture's value of
TEXTURE_MAX_ANISOTROPY or the implementation-defined value of
It is acceptable for an implementation to round 'N' up to the nearest
supported sampling rate. For example an implementation may only
support power-of-two sampling rates.
It is also acceptable for an implementation to approximate the ideal
functions Px and Py with functions Fx and Fy subject to the following
1. Fx is continuous and monotonically increasing in |du/dx| and |dv/dx|.
Fy is continuous and monotonically increasing in |du/dy| and |dv/dy|.
2. max(|du/dx|,|dv/dx|} <= Fx <= |du/dx| + |dv/dx|.
max(|du/dy|,|dv/dy|} <= Fy <= |du/dy| + |dv/dy|.
Instead of a single sample, Tau, at (u,v,Lambda), 'N' locations in
the mipmap at LOD Lambda are sampled within the texture footprint of
the pixel. This sum Tau_Aniso is defined using the single sample Tau.
When the texture's value of TEXTURE_MAX_ANISOTROPHY is greater
than 1.0, use Tau_Aniso instead of Tau to determine the fragment's
texture value.
Tau_Aniso = 1/N \ Tau(u(x - 1/2 + i/(N+1), y), v(x - 1/2 + i/(N+1), y)), Px > Py
Tau_Aniso = 1/N \ Tau(u(x, y - 1/2 + i/(N+1)), v(x, y - 1/2 + i/(N+1))), Py >= Px
It is acceptable to approximate the u and v functions with equally spaced
samples in texture space at LOD Lambda:
Tau_Aniso = 1/N \ Tau(u(x,y)+dudx(i/(N+1)-1/2), v(x,y)+dvdx(i/(N+1)-1/2)), Px > Py
Tau_Aniso = 1/N \ Tau(u(x,y)+dudy(i/(N+1)-1/2), v(x,y)+dvdy(i/(N+1)-1/2)), Py >= Px
Additions to the GLX Specification
New State
(table 23.19, p721) add the entry:
Get Value Type Get Command Initial Value Description Sec Attribute
-------------------------- ---- ----------------- -------------- --------------- ----- ---------
TEXTURE_MAX_ANISOTROPY R GetTexParameterfv 1.0 Maximum degree 8.14 texture
of anisotropy
(table 23.23, p725) add the entry:
Get Value Type Get Command Initial Value Description Sec Attribute
-------------------------- ---- --------------------- -------------- --------------- ----- ---------
TEXTURE_MAX_ANISOTROPY R GetSamplerParameterfv 1.0 Maximum degree 8.14 -
of anisotropy
New Implementation State
(table 23.64, p766) add the entry:
Get Value Type Get Command Minimum Value Description Sec Attribute
------------------------------ ---- ------------ -------------- --------------- ----- ---------
MAX_TEXTURE_MAX_ANISOTROPY R GetFloatv 16.0 Limit of 8.14 -
maximum degree
of anisotropy
1) Should there be a particular anisotropic texture filtering minification
and magnification mode?
RESOLUTION: NO. The maximum degree of anisotropy should control
when anisotropic texturing is used. Making this orthogonal to
the minification and magnification filtering modes allows these
settings to influence the anisotropic scheme used. Yes, such
an anisotropic filtering scheme exists in hardware.
2) What should the minimum value for MAX_TEXTURE_MAX_ANISOTROPY be?
RESOLUTION: 16.0. To support this extension, at least 16 to 1
anisotropy should be supported. 16.0 was chosen because this is what
all modern GPUs support at the time the ARB version of this extension
was forked from the EXT version. The EXT version of this extension
specified 2, but it was written in 1999.
3) Should an implementation-defined limit for the maximum maximum degree of
anisotropy be "get-able"?
RESOLUTION: YES. But you should not assume that a high maximum
maximum degree of anisotropy implies anything about texture
filtering performance or quality.
4) Should anything particular be said about anisotropic 3D texture
and cube texture filtering?
RESOLUTION: NO. It was decided by the OpenGL working group to leave
anisotropic filtering for 3D or cube textures as undefined. Anisotropic
filtering is only well-defined for 2D and 2D array textures. For
defined results with 3D and cube textures isotropic filtering should
be used by setting the anisotropy level to 1.0.
5) Should the maximum degree of anisotropy be a sampler parameter?
RESOLVED: Yes. The original EXT extension was created before sampler
objects were defined.
Revision History
Rev. Date Author Changes
---- ---------- --------- -----------------------------------------------
6 2019-02-26 pdaniell Update issue 4 question and resolution.
5 2017-06-02 Jon Leech Include anisotropy state in sampler objects
4 2017-05-29 dgkoch Minor typography/gramatical fixes from OpenGL 4.6.
3 2017-05-24 pdaniell Fix issue 2 to reflect the new minimum max
anisotrpic version.
2 2017-04-25 pdaniell Increase minimum max anisotropic value to 16.
1 2017-04-03 pdaniell Initial draft for OpenGL 4.6 forked from