EXT_texture_query_lod

GL_EXT_texture_query_lod

Gert Wollny (gert wollny 'at' collabora.com)

Pat Brown, NVIDIA
Greg Roth, NVIDIA
Eric Werness, NVIDIA

Copyright (c) 2019 Collabora LTD 
Copyright (c) 2009-2013 The Khronos Group Inc. Copyright terms at
    http://www.khronos.org/registry/speccopyright.html

Complete

Last Modified Date:         04/02/2019
Revision:                   1
Based on ARB_texture_query_lod version 4, modified 2013/10/04.

OpenGL ES extension #310

OpenGL ES 3.0 is required.

OpenGL Shading Language 3.00 ES is required

This extension interacts trivially with EXT_texture_cube_map_array

This extension is written against the OpenGL ES 3.2 specification and
version 3.20 ES of the OpenGL Shading Language Specification.

This extension adds 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.

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

  #extension GL_EXT_texture_query_lod

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

  #define GL_EXT_texture_query_lod 1

Change section 8.9.1 "Texture Query Functions"

Remove the first paragraph and add to the table:

Syntax:

  vec2 textureQueryLOD(gsampler2D sampler, vec2 coord)
  vec2 textureQueryLOD(gsampler3D sampler, vec3 coord)
  vec2 textureQueryLOD(gsamplerCube sampler, vec3 coord)
  vec2 textureQueryLOD(gsampler2DArray sampler, vec2 coord)
  vec2 textureQueryLOD(gsamplerCubeArray sampler, vec3 coord)
  vec2 textureQueryLOD(sampler2DShadow sampler, vec2 coord)
  vec2 textureQueryLOD(samplerCubeShadow sampler, vec3 coord)
  vec2 textureQueryLOD(sampler2DArrayShadow sampler, vec2 coord)
  vec2 textureQueryLOD(samplerCubeArrayShadow sampler, vec3 coord)

Description:

  The textureQueryLOD function takes the components of <coord> 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 8.7), 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 <maxAccessibleLevel> is the level number of the smallest
  accessible level of the mipmap array (the value q in section
  8.14.3) 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.

  If EXT_texture_cube_map_array is not supported, remove the
  textureQueryLOD lookup functions taking cube map array samplers.

See the issue list in GL_ARB_texture_query_lod.

Rev.    Date      Author      Changes
----  ----------  --------    -----------------------------------------
2     20/02/2019  Gert Wollny remove references to 1D textures and non-GLES
                              extensions

1     19/02/2019  Gert Wollny Initial EXT version based on ARB.
                              No functional changes.