Name

    SGIX_texture_mipmap_anisotropic

Name Strings

    GL_SGIX_texture_mipmap_anisotropic

Version

    $Date: 1998/10/21 00:42:24 $ $Revision: 1.4 $

Number

    XXX

Dependencies

    EXT_texture3D affects the definition of this extension
    SGIX_texture_lod_bias affects the definition of this extension

Overview

    This extension improves the filtering quality over the standard mipmap
    algorithm.  The limitation of the standard mipmap algorithm is that a 
    pixel's footprint in texture space is approximated by a symmetrically
    filtered sample.  The approximation becomes worse as the pixel footprint
    becomes more anisotropic.  The poor approximation usually results in
    the mapped image appearing excessively blurry.  This extension replaces
    a single symmetric sample with the average of distributed smaller samples, 
    which better approximates the pixel footprint shape.  This anisotropic
    filtering method uses the existing mipmap and clipmap structures.

Issues

    *	Should this extension handle all anisotropic filtering methods?
        -- No.  Since all other known anisotropic filter methods (summed-area, 
           ripmap, non-mip supersampling) use different data structures,
  	   the control mechanisms will be different and therefore should
  	   be separate extensions.

New Procedures and Functions

    None

New Tokens

    Accepted by the <pname> parameter of TexParameteri, TexParameterf,
    TexParameteriv, TexParameterfv, GetTexParameteriv, and GetTexParameterfv:

	TEXTURE_MIPMAP_ANISOTROPY_SGIX	0x832E

    Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,
    GetFloatv, and GetDoublev:

        MAX_MIPMAP_ANISOTROPY_SGIX	0x832F


Additions to Chapter 2 of the 1.1 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the 1.1 Specification (Rasterization)

    GL Specification Table 3.9 is updated as follows:

	Name				Type	    Legal Values
	----				----	    ------------
	TEXTURE_WRAP_S			integer	    CLAMP, REPEAT
	TEXTURE_WRAP_T			integer	    CLAMP, REPEAT
	TEXTURE_WRAP_R_EXT		integer	    CLAMP, REPEAT
	TEXTURE_MIN_FILTER		integer	    NEAREST, LINEAR,
						    NEAREST_MIPMAP_NEAREST,
						    NEAREST_MIPMAP_LINEAR,
						    LINEAR_MIPMAP_NEAREST,
						    LINEAR_MIPMAP_LINEAR,
						    FILTER4_SGIS,
						    LINEAR_CLIPMAP_LINEAR_SGIX,
	TEXTURE_MAG_FILTER		integer	    NEAREST, LINEAR,
						    FILTER4_SGIS,
						    LINEAR_DETAIL_SGIS,
						    LINEAR_DETAIL_ALPHA_SGIS,
						    LINEAR_DETAIL_COLOR_SGIS,
						    LINEAR_SHARPEN_SGIS,
						    LINEAR_SHARPEN_ALPHA_SGIS,
						    LINEAR_SHARPEN_COLOR_SGIS
	TEXTURE_BORDER_COLOR		4 floats    any 4 values in [0,1]
	DETAIL_TEXTURE_LEVEL_SGIS	integer	    any non-negative integer
	DETAIL_TEXTURE_MODE_SGIS	integer	    ADD, MODULATE
	TEXTURE_MIN_LOD_SGIS		float	    any value
	TEXTURE_MAX_LOD_SGIS		float	    any value
	TEXTURE_BASE_LEVEL_SGIS		integer	    any non-negative integer
	TEXTURE_MAX_LEVEL_SGIS		integer	    any non-negative integer
        GENERATE_MIPMAP_SGIS            boolean     TRUE or FALSE
        TEXTURE_CLIPMAP_FRAME_SGIX      float       any non-negative value
        TEXTURE_CLIPMAP_CENTER_SGIX     2 integers  any 2 non-negative integers
        TEXTURE_CLIPMAP_OFFSET_SGIX     2 integers  any 2 non-negative integers
        TEXTURE_CLIPMAP_VIRTUAL_DEPTH_SGIX 3 integers  any 3 non-negative integer
	TEXTURE_LOD_BIAS_S_SGIX		float	    any value
	TEXTURE_LOD_BIAS_T_SGIX		float	    any value
	TEXTURE_LOD_BIAS_R_SGIX		float	    any value
        TEXTURE_MIPMAP_ANISOTROPY	int	    any value >= 1

	Table 3.9: Texture parameters and their values.

   Texture Minification
   ------------------------

   This extensions substantially changes Section 3.8.1.  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),TEXTURE_MIPMAP_ANISOTROPY_SGIX);
   Lamda = log2(Pmax/N)
   

   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.

             i=N
             ---
   Tau = 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
             ---
   Tau = 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
             ---
   Tau = 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
             ---
   Tau = 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 1.1 Specification (Per-Fragment Operations
and the Frame Buffer)

    None

Additions to Chapter 5 of the 1.1 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.1 Specification (State and State Requests)

    None

Additions to the GLX Specification

    None

Dependencies on EXT_texture3D

    If EXT_texture3D is supported, the functions Px, Py, and Tau are modified
    to include dwdx, dwdy, and w.


Dependencies on SGIX_texture_lod_bias

    If SGIX_texture_lod_bias is supported, dudx, dvdx, dwdx, dudy, dvdy, dwdy 
    are replaced with dlodudx, dlodvdx, dlodwdx, dlodudy, dlodvdy, dlodwdy in 
    the Px and Py functions.

Errors

    None

New State

								Initial
    Get Value			   Get Command	    Type	Value	  Attrib
    ---------			   -----------	    ----	-------	  ------
    TEXTURE_MIPMAP_ANISOTROPY_SGIX GetTexParameteriv Z1*        1         texture

New Implementation Dependent State

                                                                        Minimum
    Get Value                           Get Command     Type            Value
    ---------                           -----------     ----            -------
    MAX_MIPMAP_ANISOTROPY_SGIX          GetIntegerv     Z1*              2