GL_ARB_shadow

Name

    ARB_shadow

Name Strings

    GL_ARB_shadow

Contact

    Brian Paul (brian_e_paul 'at' yahoo.com)

Status

    Complete. Approved by ARB on February 14, 2002.

Version

    Last Modified Date: 21 January 2002

Number

    ARB Extension #23

Dependencies

    OpenGL 1.1 is required.
    ARB_depth_texture is required.
    This extension is written against the OpenGL 1.3 Specification.

Overview

    This extension clarifies the GL_SGIX_shadow extension.

    This extension supports comparing the texture R coordinate to a depth
    texture value in order to produce a boolean texture value.	This can
    be used to implement shadow maps.

    The extension is written in generic terms such that other texture
    comparison modes can be accomodated in the future.

IP Status

    XXX None?

Issues

    (1) How is this extension different from GL_SGIX_shadow?

      - It defines GL behaviour when the currently bound texture is not
	a depth texture.
      - It specifies that R is clamped to [0,1].
      - We use the standard GL_LEQUAL and GL_GEQUAL tokens instead of
	defining new ones.
      - The result may be ALPHA, LUMINANCE or INTENSITY.
      - A bit more is said about how depth textures are sampled.
      - The extension is generalized for comparison modes.

    (2) Should we use GL_LEQUAL and GL_EQUAL instead of
      GL_TEXTURE_LEQUAL_R_SGIX and GL_TEXTURE_GEQUAL_R_SGIX?

      RESOLUTION: Yes.	The old tokens are misleading.	For example,
      the GL_TEXTURE_LEQUAL_R_SGIX token should really have been named
      GL_R_LEQUAL_TEXTURE_SGIX since we're comparing R <= TEXTURE.
      This extension uses the standard GL_LEQUAL and GL_GEQUAL tokens.
      Also, the original shadow spec seems to be inconsistant with
      what was really implemented in hardware.

    (3) Use TEXTURE_COMPARE_OPERATOR_ARB or TEXTURE_COMPARE_FUNC_ARB?

      RESOLVED: Use TEXTURE_COMPARE_FUNC_ARB to be more consistant with
      the conventions of glDepthFunc(), glStencilFunc(), etc which use
      the GL_LEQUAL, GL_GEQUAL, etc tokens.

    (4) Should the result of the texture comparison be a LUMINANCE,
      INTENSITY or ALPHA texel?

      RESOLVED: Allow any of them. This is controlled by
      DEPTH_TEXTURE_MODE_ARB defined in ARB_depth_texture extension.

    (5) What if TEXTURE_COMPARE_MODE_ARB is set to COMPARE_R_TO_TEXTURE
      but the the currently bound texture is not a depth texture?

      RESOLVED: If the currently bound texture is a color (or paletted
      or color index) texture then the texture unit treats it in the
      usual manner and all texture comparison logic is bypassed.

    (6) Should the R value be clamped to [0,1] before the comparison?

      RESOLUTION: Yes, that makes sense since the depth texels are in
      the range [0,1].	Note that clamping R to [0,1] really only matters
      at the values 0 and 1.

    (7) How is bilinear or trilinear filtering implemented?

      RESOLUTION: We suggest an implementation behaviour but leave the
      details up to the implementation.  Differences here amount to the
      quality and softness of shadow edges.  Specific filtering
      algorithms could be expressed via layered extensions.  We're
      intentionally vague here to avoid IP and patent issues.

    (8) Is GL_ARB_shadow the right name for this extension?

      RESOLVED: Probably.  While this extension is expressed in rather
      generic terms which may be used by future extensions, it implements
      a rather specific operation at this time.

    (9) What about GL_SGIX_shadow_ambient?

      RESOLUTION: Omit that functionality.  It can be accomplished with
      advanced texture extensions such as GL_NV_register_combiners.
      GL_SGIX_shadow_ambient usually can't be implemented with existing
      hardware so it'll be offered as GL_ARB_shadow_ambient, rather than
      burdon this extension with it.

New Procedures and Functions

    None

New Tokens

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

    TEXTURE_COMPARE_MODE_ARB	0x884C
    TEXTURE_COMPARE_FUNC_ARB	0x884D

    Accepted by the <param> parameter of TexParameterf, TexParameteri,
    TexParameterfv, and TexParameteriv when the <pname> parameter is
    TEXTURE_COMPARE_MODE_ARB:

    COMPARE_R_TO_TEXTURE_ARB	0x884E

Additions to Chapter 2 of the 1.3 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the 1.3 Specification (Rasterization)

    Section 3.8.4, Texture Parameters, p. 133, append table 3.19 with the
    following:

	Name			    Type  Legal Values
	--------------------------  ----  -------------------------------
	TEXTURE_COMPARE_MODE_ARB    enum  NONE, COMPARE_R_TO_TEXTURE
	TEXTURE_COMPARE_FUNC_ARB    enum  LEQUAL, GEQUAL

    After section 3.8.12, Texture Environments and Texture Functions,
    p. 149, insert the following new sections (and renumber subsequent
    sections):

	"3.8.13 Texture Comparison Modes

	TEXTURE_COMPARE_MODE_ARB can be used to compute the texture value
	according to a comparison function.  TEXTURE_COMPARE_MODE_ARB
	specifies the comparison operands, and TEXTURE_COMPARE_FUNC_ARB
	specifies the comparison function.  The format of the resulting
	texture sample is specified by the DEPTH_TEXTURE_MODE_ARB.

	3.8.13.1 Depth Texture Comparison Mode

	If the currently bound texture's format is DEPTH_COMPONENT then
	TEXTURE_COMPARE_MODE_ARB, TEXTURE_COMPARE_FUNC_ARB and
	DEPTH_TEXTURE_MODE_ARB control the output of the texture unit
	as described below.  However, if the currently bound texture is
	not DEPTH_COMPONENT then the texture unit operates in the normal
	manner and texture comparison is bypassed.

	Let Dt (D subscript t) be the depth texture value, in the range
	[0, 1].  Let R be the interpolated texture coordinate clamped to
	the range [0, 1].  Then the effective texture value Lt, It, or At
	is computed by

	if TEXTURE_COMPARE_MODE_ARB = NONE

	    r = Dt

	else if TEXTURE_COMPARE_MODE_ARB = COMPARE_R_TO_TEXTURE_ARB

	    if TEXTURE_COMPARE_FUNC_ARB = LEQUAL

		 { 1.0,  if R <= Dt
	     r = {
		 { 0.0,  if R > Dt

	    else if TEXTURE_COMPARE_FUNC_ARB = GEQUAL

		 { 1.0,  if R >= Dt
	     r = {
		 { 0.0,  if R < Dt

	    endif

	    if DEPTH_TEXTURE_MODE_ARB = LUMINANCE

		Lt = r

	    else if DEPTH_TEXTURE_MODE_ARB = INTENSITY

		It = r

	    else if DEPTH_TEXTURE_MODE_ARB = ALPHA

		At = r

	    endif

	endif

	If TEXTURE_MAG_FILTER is not NEAREST or TEXTURE_MIN_FILTER is
	not NEAREST or NEAREST_MIPMAP_NEAREST then r may be computed by
	comparing more than one depth texture value to the texture R
	coordinate.  The details of this are implementation-dependent
	but r should be a value in the range [0, 1] which is proportional
	to the number of comparison passes or failures.

Additions to Chapter 4 of the 1.3 Specification (Per-Fragment Operations and the Frame Buffer)

    None

Additions to Chapter 5 of the 1.3 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.3 Specification (State and State Requests)

    In section 6.1.3, p. 200, insert the following after the fourth
    paragraph:

    "The texture compare mode and texture compare function may be queried
    by calling GetTexParameteriv or GetTexParameterfv with <pname> set to
    TEXTURE_COMPARE_MODE_ARB, or TEXTURE_COMPARE_FUNC_ARB, respectively."

Additions to the GLX Specification

    None

Errors

    INVALID_ENUM is generated if TexParameter[if] parameter <pname>
    is TEXTURE_COMPARE_MODE_ARB and parameter <param> is not NONE or
    COMPARE_R_TO_TEXTURE.

    INVALID_ENUM is generated if TexParameter[if] parameter <pname>
    is TEXTURE_COMPARE_FUNC_ARB and parameter <param> is not LEQUAL or
    GEQUAL.

New State

    In table 6.16, Texture Objects, p. 224, add the following:

    Get Value			Type  Get Command	    Initial Value  Description	   Sec.   Attribute
    --------------------------	----  --------------------  -------------  --------------  -----  ---------
    TEXTURE_COMPARE_MODE_ARB	Z_2   GetTexParameter[if]v  NONE	   compare mode    3.8.13 texture
    TEXTURE_COMPARE_FUNC_ARB	Z_2   GetTexParameter[if]v  LEQUAL	   compare func    3.8.13 texture

New Implementation Dependent State

    None

Revision History

    19 March 2001
	- initial revision
    20 March 2001
	- use GL_LEQUAL, GL_GEQUAL tokens
	- removed TEXTURE_COMPARE_FAIL_VALUE_ARB
    16 April 2001
	- renamed TEXTURE_COMPARE_OPERATOR_ARB to TEXTURE_COMPARE_FUNC_ARB
	- replace TEXTURE_COMPARE_ARB with TEXTURE_COMPARE_MODE_ARB
    22 April 2001
	- added TEXTURE_COMPARE_RESULT
    23 April 2001
	- minor tweaks
    22 June 2001
	- fixed grammatical errors
    16 November 2001
	- Change default value of TEXTURE_COMPARE_MODE_ARB to LUMINANCE.
    17 November 2001
	- Resolved issue 5
	- cleaned up new section 3.8.7.1 yet again
    12 December 2001
	- rewritten against the OpenGL 1.3 spec
    3 January 2002
	- fixed a typo found by Bimal
    18 January 2002
	- Since depth textures can now allow ALPHA, INTENSITY, LUMINANCE mode,
	  there was no need for TEXTURE_COMPARE_RESULT_ARB.
    21 January 2002
	- Fixed error to be INVALID_ENUM instead of INVALID_OPERATION.

Implementation Support

   List of OpenGL implementations supporting the GL_ARB_shadow extension

Original File

   Original text file for the GL_ARB_shadow extension