Name ARB_shadow Name Strings GL_ARB_shadow Contact Brian Paul (brian_e_paul 'at' yahoo.com) Notice Copyright (c) 2002-2013 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html Specification Update Policy Khronos-approved extension specifications are updated in response to issues and bugs prioritized by the Khronos OpenGL Working Group. For extensions which have been promoted to a core Specification, fixes will first appear in the latest version of that core Specification, and will eventually be backported to the extension document. This policy is described in more detail at https://www.khronos.org/registry/OpenGL/docs/update_policy.php 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 accommodated 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 parameter of TexParameterf, TexParameteri, TexParameterfv, TexParameteriv, GetTexParameterfv, and GetTexParameteriv: TEXTURE_COMPARE_MODE_ARB 0x884C TEXTURE_COMPARE_FUNC_ARB 0x884D Accepted by the parameter of TexParameterf, TexParameteri, TexParameterfv, and TexParameteriv when the 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 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 is TEXTURE_COMPARE_MODE_ARB and parameter is not NONE or COMPARE_R_TO_TEXTURE. INVALID_ENUM is generated if TexParameter[if] parameter is TEXTURE_COMPARE_FUNC_ARB and parameter 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.