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Name

    SGIX_texture_phase

Name Strings

    GL_SGIX_texture_phase

Version

    $Date: 1997/12/08 19:59:59 $ $Revision: 1.3 $

Number

    ??

Dependencies

    None

Overview

    This extension allows textures to filtered using a modified LINEAR
    filter that treats the texture data as phase angle.  (In addition
    to the NEAREST and LINEAR filters defined in the original GL
    Specification.) Such filtering is useful for dopler data where
    texel values correspond to positions on a unit circle or phase
    angles.

Issues

    *	

Reasoning

    *	

New Procedures and Functions

    None

New Tokens

    Accepted by the <param> parameter of TexParameteri and
    TexParameterf, and by the <params> parameter of TexParameteriv and
    TexParameterfv, when their <pname> parameter is
    TEXTURE_MIN_FILTER or TEXTURE_MAG_FILTER:

	PHASE_SGIX		0x832A

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

    None

Additions to Chapter 3 of the GL Specification (Rasterization)

    The additional token value PHASE_SGIS is accepted as an enumerated
    value for the texture minification and magnification filters, causing 
    Table 3.7 to be replaced with the table below:

	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,
							PHASE_SGIS
	TEXTURE_MAG_FILTER		integer		NEAREST, LINEAR,
							FILTER4_SGIS,
							PHASE_SGIX
	TEXTURE_BORDER_COLOR		4 floats	any 4 values in [0,1]

	Table 3.7: Texture parameters and their values.

    Phase filtering is specified by calling TexParameteri,
    TexParameterf, TexParameteriv, or TexParameterfv with <pname> set
    to TEXTURE_MIN_FILTER or TEXTURE_MAG_FILTER, and <param> or <params> 
    set to PHASE_SGIS. Because phase filtering is defined only for
    non-mipmapped textures, there is no difference between its
    definition for minification and magnification.

    First consider the 1-dimensional case.  Let T be a computed texture
    value (one of R_t, G_t, B_t, or A_t).  Let T[i] be the component
    value of the texel at location i in a 1-dimensional texture image.
    Then, if the appropriate texture filter mode is PHASE_SGIX, a
    2-texel group is selected:


	      /	floor(u - 1/2) mod 2**n,	TEXTURE_WRAP_S is REPEAT
	i0 = (
	      \	floor(u - 1/2),			TEXTURE_WRAP_S is CLAMP


	      /	(i0 + 1) mod 2**n,		TEXTURE_WRAP_S is REPEAT
	i1 = (
	      \	i0 + 1,				TEXTURE_WRAP_S is CLAMP


   Let
   
	A = frac(u - 1/2)

    where frac(x) denotes the fractional part of x, and u is the
    texture image coordinate in the s direction, as illustrated in
    Figure 3.10 of the GL Specification.  Then the texture value T is
    found as:

	t0 = T[i0]
	t1 = T[i1]
	del = t1 - t0
	if |del| > 0.5 then
	    t0 = t0 + sign(del)
	T = frac((1-A) * t0 + A * t1)

    If any of the selected T[i] in the above equation refer to a border
    texel with unspecified value, then the border color given by the
    current setting of TEXTURE_BORDER_COLOR is used instead of the
    unspecified value.

    For 2-dimensional textures the calculations for i0, i1, and
    A are identical to the 1-dimensional case.  A 4-texel group is
    selected, requiring two j values computed as


	      /	floor(v - 1/2) mod 2**m,	TEXTURE_WRAP_T is REPEAT
	j0 = (
	      \	floor(v - 1/2),			TEXTURE_WRAP_T is CLAMP


	      /	(j0 + 1) mod 2**m,		TEXTURE_WRAP_T is REPEAT
	j1 = (
	      \	j0 + 1,				TEXTURE_WRAP_T is CLAMP

    Let

	B = frac(v - 1/2)

    where v is the texture image coordinate in the t direction, as
    illustrated in Figure 3.10 of the GL Specification.  Then the
    texture value T is found as:

	t00 = T[i0,j0]
	t01 = T[i0,j1]
	t10 = T[i1,j0]
	t11 = T[i1,j1]
	del = t10 - t00
	if |del| > 0.5 then
	    t00 = t00 + sign(del)
	t0 = frac((1-A) * t00 + A * t10)
	del = t11 - t01
	if |del| > 0.5 then
	    t01 = t01 + sign(del)
	t1 = frac((1-A) * t01 + A * t11)
	del = t1 - t0
	if |del| > 0.5 then
	    t0 = t0 + sign(del)
	T = frac((1-A) * t0 + A * t1)

    If any of the selected T[i,j] in the above equation refer to a
    border texel with unspecified value, then the border color given by
    the current setting of TEXTURE_BORDER_COLOR is used instead of the
    unspecified value.

    Phase texture filtering is similarly defined for 3-dimensional
    textures.


Additions to Chapter 4 of the GL Specification (Per-Fragment Operations
and the Framebuffer)

    None

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

    None

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

    None

Additions to the GLX Specification

    None

GLX Protocol

    
Errors


New State


New Implementation Dependent State