Name ARB_texture_query_lod Name Strings GL_ARB_texture_query_lod Contact Eric Werness, NVIDIA Corporation (ewerness 'at' nvidia.com) Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com) Contributors Pat Brown, NVIDIA Greg Roth, NVIDIA Eric Werness, NVIDIA Alejandro PiƱeiro, Igalia, SL Notice Copyright (c) 2009-2013 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html 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 https://www.khronos.org/registry/OpenGL/docs/update_policy.php Status Complete. Approved by the ARB on July 3, 2009. Version Last Modified Date: 04/22/2020 Revision: 8 Number ARB Extension #73 Dependencies OpenGL 3.0 is required. OpenGL Shading Language 1.30 is required EXT_gpu_shader4 is required. EXT_texture_array is required. This extension interacts trivially with ARB_texture_cube_map_array This extension is written against the OpenGL 2.0 specification and version 1.30 of the OpenGL Shading Language Specification. Overview This extension provides a new set of fragment shader texture functions (textureLOD) that return the results of automatic level-of-detail computations that would be performed if a texture lookup were performed. 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) None. 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 the AGL/GLX/WGL Specifications None GLX Protocol TBD Errors None. New State None. New Implementation Dependent State None. 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 features described in this extension: #extension GL_ARB_texture_query_lod A new preprocessor #define is added to the OpenGL Shading Language: #define GL_ARB_texture_query_lod 1 Add to section 8.7 "Texture Lookup Functions" Syntax: vec2 textureQueryLOD(gsampler1D sampler, float coord) vec2 textureQueryLOD(gsampler2D sampler, vec2 coord) vec2 textureQueryLOD(gsampler3D sampler, vec3 coord) vec2 textureQueryLOD(gsamplerCube sampler, vec3 coord) vec2 textureQueryLOD(gsampler1DArray sampler, float coord) vec2 textureQueryLOD(gsampler2DArray sampler, vec2 coord) vec2 textureQueryLOD(gsamplerCubeArray sampler, vec3 coord) vec2 textureQueryLOD(sampler1DShadow sampler, float coord) vec2 textureQueryLOD(sampler2DShadow sampler, vec2 coord) vec2 textureQueryLOD(samplerCubeShadow sampler, vec3 coord) vec2 textureQueryLOD(sampler1DArrayShadow sampler, float coord) vec2 textureQueryLOD(sampler2DArrayShadow sampler, vec2 coord) vec2 textureQueryLOD(samplerCubeArrayShadow sampler, vec3 coord) Description: The textureQueryLOD function takes the components of and computes the LOD information that the texture pipe would use to make an access of that texture. The computed level of detail lambda_prime (equation 3.19), relative to the base level, is returned in the y component of the result vector. The level of detail is obtained after any LOD bias, but prior to clamping to [TEXTURE_MIN_LOD, TEXTURE_MAX_LOD]. The x component of the result vector contains information on the mipmap array(s) that would be accessed by a normal texture lookup using the same coordinates. If a single level of detail would be accessed, the level-of-detail number relative to the base level is returned. If multiple levels of detail are accessed, a floating-point number between the two levels is returned, with the fractional part equal to the fractional part of the computed and clamped level of detail. The algorithm used is given by the following pseudo-code: float ComputeAccessedLod(float computedLod) { // Clamp the computed LOD according to the texture LOD clamps. if (computedLod < TEXTURE_MIN_LOD) computedLod = TEXTURE_MIN_LOD; if (computedLod > TEXTURE_MAX_LOD) computedLod = TEXTURE_MAX_LOD; // Clamp the computed LOD to the range of accessible levels. if (computedLod < 0) computedLod = 0.0; if (computedLod > (float) maxAccessibleLevel) computedLod = (float) maxAccessibleLevel; // Return a value according to the min filter. if (TEXTURE_MIN_FILTER is LINEAR or NEAREST) { return 0.0; } else if (TEXTURE_MIN_FILTER is NEAREST_MIPMAP_NEAREST or LINEAR_MIPMAP_NEAREST) { return ceil(computedLod + 0.5) - 1.0; } else { return computedLod; } } The value is the level number of the smallest accessible level of the mipmap array (the value q in section 3.8.8) minus the base level. The returned value is then: vec2(ComputeAccessedLod(lambda_prime), lambda_prime); If textureQueryLOD is called on an incomplete texture, the results are undefined. textureQueryLOD is only available fragment shaders. Dependencies on ARB_texture_cube_map_array If ARB_texture_cube_map_array is not supported, remove the textureQueryLOD lookup functions taking cube map array samplers. Issues (1) Should we provide texture LOD functions for shadow sampler targets? RESOLVED: Yes. The level of detail computations for a texture used as a shadow map are completely identical to that for other textures. However, we provide separate data types for the two textures (e.g., sampler2D vs. sampler2DShadow), and there is no mechanism to cast from one to the other. If we didn't provide these functions, the only way to perform an LOD computation for a texture used as a shadow map would be to bind the same texture object to two different texture image units, one associated with a shadow sampler and the other associated with a normal sampler. Unlike regular shadow texture lookups, the reference depth value is not used for LOD calculations and is not accepted by the corresponding textureQueryLOD() built-infunctions. (2) Should LOD queries for array textures take a layer? RESOLVED: No. The layer number has no effect on the LOD calcuations, so can be safely ignored. As a result, LOD queries for sampler1DArray, sampler2DArray, and samplerCubeArray samplers take one, two, and three coordinates, respectively. (3) The core specification uses the "Lod" spelling, not "LOD". Should this extension be modified to use "Lod"? RESOLVED: The "Lod" spelling is the correct spelling for the core specification and the preferred spelling for use. However, use of "LOD" also exists, as the extension predated the core specification, so this extension won't remove use of "LOD". Revision History Rev. Date Author Changes ---- ---------- -------- ----------------------------------------- 8 04/22/2020 apinheiro Update OpenGL version required, to be consistent with GLSL version required (internal API issue 124) 7 04/10/2013 Jon Leech Add issue 3 regarding different spelling of "LOD" vs. "Lod" in extension & core. 6 08/02/2009 Jon Leech Reformat to 80 columns and assign ARB extension number. 5 07/14/2009 istewart Fixed preprocessor define and pseudocode for ComputeAccessedLod. 4 06/26/2009 pbrown Change prototype for textureQueryLOD for 1D and 2D arrays to not take a layer. Cube map arrays already didn't take a layer. 3 06/25/2009 groth Rename extension to ARB_texture_query_lod. 2 06/23/2009 groth correct filtering mode conditional in psuedocode 1 05/13/2009 groth Split off of gpu_shader4_1