XXX - Not complete yet!!!

Name

    SGIX_clipmap

Name Strings

    GL_SGIX_clipmap

Version

    $Date: 1996/06/12 17:19:43 $ $Revision: 1.9 $

Number

    33

Dependencies

    EXT_texture is required
    EXT_texture_object affects the definition of this extension
    SGIS_texture_lod affects the definition of this extension
    SGI_detail_texture affects the definition of this extension
    SGI_sharpen_texture affects the definition of this extension

Overview

    Mipmaps provide a general but expensive solution when the texture image
    is very large.  This extension defines clipmaps, which occupy a small
    subset of the memory required by equivalent mipmaps, but provide much
    of the mipmap rendering capabilities.  Clipmaps are especially useful
    for rendering terrain.

Issues

    *   Is lod clamping treated properly if we have no texel data?
    *   Should we always have to specify a virtual depth?

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:

	LINEAR_CLIPMAP_LINEAR_SGIX

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

	TEXTURE_CLIPMAP_FRAME_SGIX

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

	TEXTURE_CLIPMAP_CENTER_SGIX
	TEXTURE_CLIPMAP_OFFSET_SGIX
	TEXTURE_CLIPMAP_VIRTUAL_DEPTH_SGIX

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

	MAX_CLIPMAP_DEPTH_SGIX
	MAX_CLIPMAP_VIRTUAL_DEPTH_SGIX

Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the 1.0 Specification (Rasterization)

    GL Specification Table 3.7 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 integers

	Table 3.7: Texture parameters and their values.

    Notes:

    *   Clipmap filtering is supported only for 2D, borderless textures.

    *   Clipmap filtering is performed when the texture minification filter
	is LINEAR_CLIPMAP_LINEAR_SGIX.  We could define other clipmap
	filters with NEAREST sampling, but it probably isn't worth the
	trouble.
	
    *	Clipmap filtering is supported with all magnification filters
	except FILTER4_SGIS.  Detail and sharpen filtering are performed
	only when the texture data are supplied by the level 0 image.

    *   A clipmap virtualizes a mipmap by holding, at any given time, only a
	portion of a full mipmap pyramid.  A clipmap is complete only if
	levels 0 through B have the same dimension, levels B through N form
	the base of a valid mipmap, all image dimensions are a power of 2,
	and N is less than MAX_CLIPMAP_DEPTH_SGIX.

	Each image level P, where P ranges from B through N, is treated as
	though it contains the full image from the same level of the complete
	(N+1) level mipmap.

	Each image level P, where P ranges from 0 through B-1, is treated as
	though it contains a subimage from the same level of the complete
	(N+1) level mipmap.  This subimage is centered at texel coordinates

                S = (Scenter >> P)
                T = (Tcenter >> P)

	where Scenter and Tcenter are specified by the application.

	For instance, if the width of the clipmap is W=8, N=12, and Scenter=
	2048, the clipmap will contain the data given by 0's in the S-edge
	view below:

  0 .********************************00000000********************************.
  1 .********************************00000000********************************.
  2 .********************************00000000********************************.
  3 .********************************00000000********************************.
  4 .********************************00000000********************************.
  5 .********************************00000000********************************.
  6      ****************************00000000****************************
  7                      ************00000000************
  8                              ****00000000****
B=9                                  00000000
  10                                   0000
  11                                    00
N=12                                    0

	Likewise, if Scenter=2016 (32 less than above):

  0 .00000000****************************************************************.
  1 .****************00000000************************************************.
  2 .************************00000000****************************************.
  3 .****************************00000000************************************.
  4 .******************************00000000**********************************.
  5 .*******************************00000000*********************************.
  6      ****************************00000000*****************************
  7                      ************00000000************
  8                              ****00000000****
  9                                  00000000
  10                                   0000
  11                                    00
N=12                                    0

    *   Scenter and Tcenter are specified by the application by calling
	TexParameteriv or TexParameterfv with <target> set to TEXTURE_2D,
	<pname> set to <TEXTURE_CLIPMAP_CENTER_SGIX>, and <params> pointing
	to a vector whose first component is Scenter and whose second
	component is Tcenter.  The center can be changed at any time.

    *	The clipped levels (i.e. those levels P, where P<B) of the map can
	be framed with a region of texels guaranteed not to be displayed.
	The frames allow for paging of new texture data into a non-active
	portion of texture memory, enabling efficient roaming throughout the
	larger virtual mipmap.  The frames for all of the clipped levels are
	the same width and are measured as a fraction of the width of the
	clipped levels.  Specifically,

		width of frame in texels = Cframe * W / 2

	The floating point value Cframe can take on values from 0.0 to 1.0
	and is specified by the application.

	Cframe is a non-negative fraction of half the level width specified
	by calling TexParameterf or TexParameterfv with <target> set to
	TEXTURE_2D, <pname> set to <TEXTURE_CLIPMAP_FRAME_SGIX>, and <param>
	set to Cframe or <params> pointing to Cframe.  Cframe is clamped to
	the range [0,1].  The frame width can be changed at any time.

    *	During filtering a level of detail is determined just as it would be
	for the equivalent (N+1) level mipmap.  If the level of detail falls
	into the clipped portion of the clipmap and the required texels are
	not available within the framed region of the two closest image
	levels, the level of detail is increased to the nearest image level
	that does include the required texels.  The values of TEXTURE_WRAP_S
	and TEXTURE_WRAP_T have no effect during clipmap minification.

	If SGIS_texture_lod is supported, the level of detail is clamped to
	the range defined by TEXTURE_MIN_LOD_SGIS, TEXTURE_MAX_LOD_SGIS,
	TEXTURE_BASE_LEVEL_SGIS, and TEXTURE_MAX_LEVEL_SGIS.  (Issue:  what
	if texels do not exist on the level given by max_lod?)

    *	As the center moves, only texels along the edges of the clipmap levels
	change.  To allow for incremental loading only of these texels via
	TexSubImage2DEXT, torroidal offset values are added to the texture
	addresses of each level.  For the Pth level where P ranges from 0 to
	B-1, these offsets are

		Soffsetp = (Soffset >> P)
		Toffsetp = (Toffset >> P)

	where Soffset and Toffset are specified by the application.  Note that
	Soffset and Toffset for the top level defines the offsets for
	subsequent levels by a simple shift just as with the center. 

	Soffset and Toffset are specified by calling TexParameteriv or
	TexParameterfv with <target> set to TEXTURE_2D, <pname> set to
	<TEXTURE_CLIPMAP_OFFSET_SGIX>, and <params> pointing to a vector
	whose first component is Soffset and whose second component is
	Toffset.  The offset can be changed at any time.

    *   To index into a clipmap of greater than MAX_CLIPMAP_DEPTH_SGIX levels
	of detail, additional parameters are provided to restrictively index
	a smaller clipmap of (N+1) levels located wholly within a complete,
	larger clipmap.  The larger clipmap is complete only if levels 0
	through B have the same dimension, levels B through V form the base
	of a valid mipmap, all image dimensions are a power of 2, and V is
	less than MAX_CLIPMAP_VIRTUAL_DEPTH_SGIX.  These constraints are
	precisely those given above with the exception of the depth, V, being
	allowed to range up to MAX_CLIPMAP_VIRTUAL_DEPTH_SGIX.

        The second, smaller clipmap has a base level located at level D of
	the larger clipmap and has a depth of N.  Both parameters D and N are
	specified by the application.  The smaller clipmap is valid only if D
	is between 0 and V, (D+N) is less than or equal to V, and D is less
	than MAX_CLIPMAP_DEPTH_SGIX.

	Scenter, Tcenter, Soffset, and Toffset are still relative the level
	0, the base level of the larger, virtual mipmap, as are the minimum
	and maximum levels of detail specified in the SGIS_texture_lod
	extension.

	For D=2, N=5, V=12, and Scenter=2048, the valid texels are given by
	the x's in the diagram below.

  0 ...******************************00000000******************************...
  1 ...******************************00000000******************************...
D=2 ...******************************xxxxxxxx******************************...
  3 ...******************************xxxxxxxx******************************...
  4 ...******************************xxxxxxxx******************************...
  5 ...******************************00xxxx00******************************...
  6      ****************************000xx000****************************
D+N=7                    ************000x0000************
  8                              ****00000000****
  9                                  00000000
  10                                   0000
  11                                    00
V=12                                    0

	Likewise, if Scenter=2016 (32 less than above):

  0 .00000000**************************************************************...
  1 .****************00000000**********************************************...
D=2 ...**********************xxxxxxxx**************************************...
  3 ...**************************xxxxxxxx**********************************...
  4 ...****************************xxxxxxxx********************************...
  5 ...*****************************00xxxx00*******************************...
  6      ****************************000xx000******************************
D+N=7                    ************000x0000************
  8                              ****00000000****
  9                                  00000000
  10                                   0000
  11                                    00
V=12                                    0

	Texel address and level of detail generation proceed with the smaller
	clipmap precisely as described above for the original clipmap.  If
	the level of detail is such that a level below D would be indexed,
	texture magnification is performed even though the finer texture
	information may be present in texture memory.  Likewise, levels of
	detail coarser than D+N may demonstrate aliasing.  The onus is on the
	application to choose parameters D and N to prevent unwanted
	filtering operations.

	The finest level of the clipmap, D, the depth of the clipmap, N+1, and
	the depth of the virtual clipmap, V+1, are specified by calling
	TexParameteriv, or TexParameterfv with <target> set to TEXTURE_2D,
	<pname> set to <TEXTURE_CLIPMAP_VIRTUAL_DEPTH_SGIX>, and <params> set
	to (D,N+1,V+1).  These parameters can be changed at any time.

	If the depth of the virtual clipmap is zero, clipmap virtualization
	is ignored, and texturing proceeds as with a non-virtual clipmap.

    *	Special Case:
	TEXTURE_CLIPMAP_LOD_OFFSET_SGIX is ignored
	TEXTURE_CLIPMAP_VIRTUAL_DEPTH_SGIX === TEXTURE_CLIPMAP_DEPTH_SGIX

Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations
and the Frame Buffer)

    None

Additions to Chapter 5 of the 1.0 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.0 Specification (State and State Requests)

    None

Additions to the GLX Specification

    None

Dependencies on EXT_texture

    EXT_texture is required

Dependencies on EXT_texture_object

    If EXT_texture_object is implemented, the state values named

	TEXTURE_CLIPMAP_FRAME_SGIX
	TEXTURE_CLIPMAP_CENTER_SGIX
	TEXTURE_CLIPMAP_OFFSET_SGIX
	TEXTURE_CLIPMAP_VIRTUAL_DEPTH_SGIX

    are added to the state vector of each texture object. When an attribute
    set that includes texture information is popped, the bindings and
    enables are first restored to their pushed values, then the bound
    textures have their TEXTURE_CLIPMAP_FRAME_SGIX,
    TEXTURE_CLIPMAP_CENTER_SGIX, TEXTURE_CLIPMAP_OFFSET_SGIX, and
    TEXTURE_CLIPMAP_VIRTUAL_DEPTH_SGIX parameters restored to their pushed
    values.

Dependencies on SGIS_texture_lod

    If SGIS_texture_lod is not supported, references to user-defined lod
    clamping and ranges in this document are invalid and should be ignored.

Dependencies on SGIS_detail_texture

    If SGIS_detail_texture is not supported, references to detail texture
    mapping in this document are invalid and should be ignored.

Dependencies on SGIS_sharpen_texture

    If SGIS_sharpen_texture is not supported, references to sharpen texture
    mapping in this document are invalid and should be ignored.

Errors

    INVALID_VALUE is generated if TexParameteriv, TexParameterfv,
    parameter <pname> is TEXTURE_CLIPMAP_CENTER_SGIX, and either of the two
    parameters <params> points to are negative.

    INVALID_VALUE is generated if TexParameteriv, TexParameterfv,
    parameter <pname> is TEXTURE_CLIPMAP_OFFSET_SGIX, and either of the two
    parameters <params> points to are negative.

    INVALID_VALUE is generated if TexParameteriv, TexParameterfv,
    parameter <pname> is TEXTURE_CLIPMAP_VIRTUAL_DEPTH_SGIX, and any of the
    three parameters <params> points to are negative.

    INVALID_VALUE is generated if TexParameteriv, TexParameterfv,
    parameter <pname> is TEXTURE_CLIPMAP_VIRTUAL_DEPTH_SGIX, and the sum of
    the clipmap level of detail offset and the clipmap depth is greater than
    the virtual clipmap depth.

New State

							     Initial
    Get Value			 Get Command	    Type     Value	Attrib
    ---------			 -----------	    ----     -------	------
    TEXTURE_CLIPMAP_FRAME_SGIX   GetTexParameterf  Z+   0		texture
    TEXTURE_CLIPMAP_CENTER_SGIX	 GetTexParameterfv  2 x Z+   0,0	texture
    TEXTURE_CLIPMAP_OFFSET_SGIX	 GetTexParameterfv  2 x Z+   0,0	texture
    TEXTURE_CLIPMAP_VIRTUAL_DEPTH_SGIX GetTexParameterfv 3 x Z+   0,0,0	texture


New Implementation Dependent State

									Minimum
    Get Value				Get Command	Type		Value
    ---------				-----------	----		-------
    MAX_CLIPMAP_DEPTH_SGIX		GetIntegerv	Z+		10
    MAX_CLIPMAP_VIRTUAL_DEPTH_SGIX	GetIntegerv	Z+		10