Name EXT_texture_filter_anisotropic Name Strings GL_EXT_texture_filter_anisotropic Notice Copyright NVIDIA Corporation, 1999. Version Last updated May 23, 2018 Number OpenGL Extension #187 OpenGL ES Extension #41 Dependencies Written based on the wording of the OpenGL 1.2 specification. Overview 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 object basis the maximum degree of anisotropy to account for in texture filtering. Increasing a texture object's 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 texture's maximum degree of anisotropy is specified independent from the texture's 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. Issues 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. What should the minimum value for MAX_TEXTURE_MAX_ANISOTROPY_EXT be? RESOLUTION: 2.0. To support this extension, at least 2 to 1 anisotropy should be supported. 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. Should anything particular be said about anisotropic 3D texture filtering? Not sure. Does the implementation example shown in the spec for 2D anisotropic texture filtering readily extend to 3D anisotropic texture filtering? New Procedures and Functions None New Tokens Accepted by the parameters of GetTexParameterfv, GetTexParameteriv, TexParameterf, TexParameterfv, TexParameteri, and TexParameteriv: TEXTURE_MAX_ANISOTROPY_EXT 0x84FE Accepted by the parameters of GetBooleanv, GetDoublev, GetFloatv, and GetIntegerv: MAX_TEXTURE_MAX_ANISOTROPY_EXT 0x84FF Additions to Chapter 2 of the 1.2 Specification (OpenGL Operation) None Additions to Chapter 3 of the 1.2 Specification (Rasterization) -- Sections 3.8.3 "Texture Parameters" Add the following entry to the end of Table 3.17: Name Type Legal Values -------------------------- ------ -------------------------- TEXTURE_MAX_ANISOTROPY_EXT float greater or equal to 1.0 -- Sections 3.8.5 "Texture Minification" and 3.8.6 "Texture Magnification" After the first paragraph in Section 3.8.5: "When the texture's value of TEXTURE_MAX_ANISOTROPY_EXT is equal to 1.0, the GL uses an isotropic texture filtering approach as described in this section and Section 3.8.6. However, when the texture's value of TEXTURE_MAX_ANISOTROPY_EXT 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_EXT or the implementation-defined value of MAX_TEXTURE_MAX_ANISOTROPY_EXT. 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 3.8.5. 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) Lamda' = log2(Pmax/N) where maxAniso is the smaller of the texture's value of TEXTURE_MAX_ANISOTROPY_EXT or the implementation-defined value of MAX_TEXTURE_MAX_ANISOTROPY_EXT. It is acceptable for 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 conditions: 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,Lamda), 'N' locations in the mipmap at LOD Lamda, are sampled within the texture footprint of the pixel. Instead of a single sample, Tau, at (u,v,lambda), 'N' locations in the mipmap at LOD Lamda are sampled within the texture footprint of the pixel. This sum TauAniso is defined using the single sample Tau. When the texture's value of TEXTURE_MAX_ANISOTROPHY_EXT is greater than 1.0, use TauAniso instead of Tau to determine the fragment's texture value. i=N --- TauAniso = 1/N \ Tau(u(x - 1/2 + i/(N+1), y), v(x - 1/2 + i/(N+1), y)), Px > Py / --- i=1 i=N --- TauAniso = 1/N \ Tau(u(x, y - 1/2 + i/(N+1)), v(x, y - 1/2 + i/(N+1))), Py >= Px / --- i=1 It is acceptable to approximate the u and v functions with equally spaced samples in texture space at LOD Lamda: i=N --- TauAniso = 1/N \ Tau(u(x,y)+dudx(i/(N+1)-1/2), v(x,y)+dvdx(i/(N+1)-1/2)), Px > Py / --- i=1 i=N --- TauAniso = 1/N \ Tau(u(x,y)+dudy(i/(N+1)-1/2), v(x,y)+dvdy(i/(N+1)-1/2)), Py >= Px / --- i=1 " Additions to Chapter 4 of the 1.2 Specification (Per-Fragment Operations and the Frame Buffer) None Additions to Chapter 5 of the 1.2 Specification (Special Functions) None Additions to Chapter 6 of the 1.2 Specification (State and State Requests) None Additions to the GLX Specification None Errors INVALID_VALUE is generated when TexParameter is called with of TEXTURE_MAX_ANISOTROPY_EXT and a value or value of what points to less than 1.0. New State (table 6.13, p203) add the entry: Get Value Type Get Command Initial Value Description Sec Attribute -------------------------- ---- ----------------- -------------- --------------- ----- --------- TEXTURE_MAX_ANISOTROPY_EXT R GetTexParameterfv 1.0 Maximum degree 3.8.5 texture of anisotropy New Implementation State (table 6.25, p215) add the entry: Get Value Type Get Command Minimum Value Description Sec Attribute ------------------------------ ---- ------------ -------------- --------------- ----- --------- MAX_TEXTURE_MAX_ANISOTROPY_EXT R GetFloatv 2.0 Limit of 3.8.5 - maximum degree of anisotropy Issues 1) Should TEXTURE_MAX_ANISOTROPY_EXT be accepted by SamplerParameter*? Yes, for implementations supporting sampler objects. The per-texture sampling state is overridden by the sampler object state, if present. The anisotropy parameter should not be an exception, as this would reduce the usefulness of sampler objects when anisotropic filtering is supported. This also matches the interaction described in ARB_sampler_objects, and the same behavior is still expected for API versions with core support for sampler objects. Revision History 2018-05-23 (Nicolas Capens) - clarify interaction with sampler objects. 11/12/14 (Jon Leech) - Fix spelling of TEXTURE_MAX_ANISOTROPY (public Bug 1263). 9/26/07 (Jon Leech) - assigned OpenGL ES extension number so the extension can live in both API registries. 4/25/00 - clarify that TexParameterf and TexParameteri accept TEXTURE_MAX_ANISOTROPY_EXT as a pname.