Name

    NV_texture_expand_normal

Name Strings

    GL_NV_texture_expand_normal

Contact

    Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com)

Notice

    Copyright NVIDIA Corporation, 2002.

IP Status

    NVIDIA Proprietary.

Status

    Implemented, November 2002
    Shipping in Release 40 NVIDIA driver for CineFX hardware, January 2003.

Version

    Last Modified:      2005/06/16
    NVIDIA Revision:    4

Number

    286

Support

    NVIDIA plans to discontinue this extension for future GPU
    architectures.  Support for NV3x (GeForce FX), NV4x (GeForce 6
    Series), and G7x (GeForce 7x00) architectures will continue.

    As an alternative to the EXT_texture_expand_normal functionality,
    developers can either use the signed fixed-point texture formats
    provided by NV_texture_shader (such as GL_SIGNED_RGBA8_NV) or perform
    the "expand normal" operation with shader instructions (typically
    just a MAD)..

Dependencies

    OpenGL 1.1 is required.

Overview

    This extension provides a remapping mode where unsigned texture
    components (in the range [0,1]) can be treated as though they
    contained signed data (in the range [-1,+1]).  This allows
    applications to easily encode signed data into unsigned texture
    formats.

    The functionality of this extension is nearly identical to the
    EXPAND_NORMAL_NV remapping mode provided in the NV_register_combiners
    extension, although it applies even if register combiners are used.

Issues

    (1) When is the remapping applied?

      RESOLVED:  It would be possible to remap after loading each texel,
      remap after all filtering is done, or something in between.
      Ignoring implementation-dependent rounding errors, it really
      doesn't matter.

      The spec language says that the remapping is applied after filtering
      texel values within each level.  For LINEAR_MIPMAP_LINEAR, this
      means that the remapping is "done" twice.  This approach was chosen
      solely to simplify the spec language, and does not necessarily
      reflect NVIDIA's implementation.

    (2) Should the remapping mode apply to textures with signed
    components?

      RESOLVED:  No -- the EXPAND_NORMAL_NV mapping is ignored for
      such textures.

    (3) NV_texture_shader provides several internal formats with a mix
    of signed and unsigned components.  For example, the base formats
    DSDT_MAG_NV, and DSDT_MAG_INTENSITY_NV have this property, and
    there is a variant of RGBA where the RGB components are signed,
    but the A component is unsigned.  What should happen in this case?

      RESOLVED:  The unsigned components are remapped; the signed
      components are unmodified.

    (4) What should be said about signed fixed-point precision and range
    of actual implementations?

      RESOLVED:  The fundamental problem is that it is not possible
      to derive a linear mapping taking unsigned values that exactly
      represents -1.0, 0.0, and +1.0.

      The mapping chosen for current NVIDIA implementations does not
      exactly represent +1.0.  For an n-bit fixed-point component,
      0 maps to -1.0, 2^(n-1) maps to 0.0, and 2^n-1 (maximum value)
      maps to 1.0 - 1/(2^(n-1)).  This same conversion is applied to
      stored textures using the signed texture types in NV_texture_shader.

      This specification is written using the conventional OpenGL mapping
      where -1.0 and +1.0 can be represented exactly, but 0.0 can not.
      The specification is simpler and avoids precision-dependent language
      describing the mapping.  We expect some leeway in how the remapping
      is applied.

      This issue is discussed in more detail in the issues section
      of the NV_texture_shader specification (the question is phrased
      identically).

    (5) Are texture border color components remapped?

      RESOLVED:  Yes -- if the border values are used for filtering,
      border color components are remapped identically to normal texel
      components.

New Procedures and Functions

    None.

New Tokens

    Accepted by the <pname> parameters of TexParameteri,
    TexParameteriv, TexParameterf, TexParameterfv, GetTexParameteri,
    and GetTexParameteriv:

        TEXTURE_UNSIGNED_REMAP_MODE_NV               0x888F


Additions to Chapter 2 of the OpenGL 1.4 Specification (OpenGL Operation)

    None.


Additions to Chapter 3 of the OpenGL 1.4 Specification (Rasterization)

    Modify Section 3.8.4, Texture Parameters, p.135

    (modify Table 3.19, p. 137)

        Name                Type   Legal Values
        -----------------   ----   ----------------------
        TEXTURE_UNSIGNED_   enum   EXPAND_NORMAL_NV, NONE
          REMAP_MODE_NV


    Modify Section 3.8.8,  Texture Minification, p.140

    (add after the last paragraph before the "Mipmapping" subsection,
    p. 144)

    After the texture filter is applied, the filtered texture values are
    optionally rescaled, converting unsigned texture components encoded
    in the range [0,1] to signed values in the range [-1,+1].  If the
    texture parameter TEXTURE_UNSIGNED_REMAP_MODE_NV is EXPAND_NORMAL_NV,
    the filtered values for each unsigned component of the texture is
    transformed by

        tau = 2 * tau - 1.

    For components

Additions to Chapter 4 of the OpenGL 1.4 Specification (Per-Fragment
Operations and the Frame Buffer)

    None.


Additions to Chapter 5 of the OpenGL 1.4 Specification (Special Functions)

    None.


Additions to Chapter 6 of the OpenGL 1.4 Specification (State and
State Requests)

    None.


Additions to Appendix A of the OpenGL 1.4 Specification (Invariance)

    None.


Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    None.


Errors

    None.


New State

(add to table 6.15, p. 230)
                                                         Initial
Get Value                       Type  Get Command        Value   Description         Sec.   Attribute
------------------------------  ----  -----------------  ------- ------------------  -----  ---------
TEXTURE_UNSIGNED_REMAP_MODE_NV  nxZ2  GetTexParameteriv  NONE    unsigned component  3.8.8  texture
                                                                 remapping


Revision History

Rev.    Date    Author   Changes
----  -------- --------  --------------------------------------------
 4    06/14/05  mjk      Add to-be-discontinued "Status" message and
                         discussion of alternatives.

 3    10/07/02  pbrown   Minor wording changes on precision issues -- this
                         remapping should produce roughly the same results
                         as storing the texture in signed form.

 2    10/07/02  pbrown   Added issue about where the remapping is applied,
                         and what happens to border colors.

 1    10/07/02  pbrown   Initial revision.