| Name |
| |
| ARB_shadow_ambient |
| |
| Name Strings |
| |
| GL_ARB_shadow_ambient |
| |
| 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: 12 December 2001 |
| |
| Number |
| |
| ARB Extension #24 |
| |
| Dependencies |
| |
| OpenGL 1.1 is required. |
| ARB_shadow is required. |
| ARB_depth_texture is required. |
| This extension is written against the OpenGL 1.3 Specification. |
| |
| Overview |
| |
| This is based on the GL_SGIX_shadow_ambient extension and is layered |
| upon the GL_ARB_shadow extension. |
| |
| Basically, this extension allows the user to specify the texture |
| value to use when the texture compare function fails. Normally |
| this value is zero. By allowing an arbitrary value we can get |
| functionality which otherwise requires an advanced texture |
| combine extension (such as GL_NV_register_combiners) and multiple |
| texture units. |
| |
| IP Status |
| |
| None. |
| |
| Issues |
| |
| (1) How is this extension different from GL_SGIX_shadow_ambient? |
| |
| It's functionally identical. Since the original spec isn't in |
| the extension registry this one was written from scratch. |
| |
| (2) Is this extension really needed? |
| |
| RESOLUTION: Yes, if an advanced texture combine extension is not |
| available. From Mark Kilgard's GDC presentation on shadow mapping: |
| |
| The desired color for a lit, textured, shadow fragment is: |
| |
| ( ambient + diffuseShade * diffuse) * decal + specular * shade |
| |
| where diffuseShade = dimming + ( 1.0 - dimming ) * shade, and |
| shade is the result of the shadow test in [0,1]. dimming |
| expresses the scattered light in a scene. |
| |
| If we limit shade to the range [<dimming>, 1] (where <dimming> is |
| TEXTURE_FAIL_VALUE_ARB) then we can express the diffuse term with |
| the standard GL_MODULATE texture env mode. |
| |
| Furthermore, this extension allows implementing shadow arithmetic |
| with a single texture stage. Otherwise, two texture stages are |
| needed in order to compute diffuseShade * diffuse * decal. |
| |
| This extension is also useful for implementing shadowed lighting |
| expressions other than the one above. In particular, a "poor-man's" |
| shadow mode can be implemented by setting TEXTURE_FAIL_VALUE_ARB to |
| the scene's ambient intensity and the texture environment mode to |
| GL_MODULATE. This is the intention of the original |
| GL_SGIX_shadow_ambient extension. |
| |
| (3) What are the downsides of this extension? |
| |
| This extension places some arithmetic in the texture sampling logic |
| which would otherwise be better implemented in the texture combine |
| logic. |
| |
| Also, there probably aren't many (if any) other uses for this logic |
| other than shadow ambient term. |
| |
| Those who already implement advanced texture combine modes probably |
| won't want to implement this extension. |
| |
| (4) What should the "shadow ambient" variable be named: |
| |
| RESOLUTION: TEXTURE_COMPARE_FAIL_VALUE_ARB since this extension |
| specifies the value used when the texture comparison function fails. |
| |
| (5) Is GL_ARB_shadow_ambient a good name for this extension? |
| |
| RESOLUTION: Probably. Even though this extension is phrased in |
| general terms, it's going to be used almost exclusively for shadow |
| mapping applications. |
| |
| (6) Should TEXTURE_COMPARE_FAIL_VALUE_ARB be an alias for |
| GL_SHADOW_AMBIENT_SGIX (from GL_SGIX_shadow_ambient)? |
| |
| RESOLUTION: Yes. It serves the same purpose and avoids adding |
| yet more state to texture objects. |
| |
| New Procedures and Functions |
| |
| None |
| |
| New Tokens |
| |
| Accepted by the <pname> parameter of TexParameterf, TexParameteri, |
| TexParameterfv, TexParameteriv, GetTexParameterfv, and GetTexParameteriv: |
| |
| TEXTURE_COMPARE_FAIL_VALUE_ARB 0x80BF (same as SHADOW_AMBIENT_SGIX) |
| |
| 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_FAIL_VALUE_ARB float any value in [0, 1] |
| |
| Section 3.8.4, Texture Parameters, append last paragraph with: |
| |
| "The value set by TEXTURE_COMPARE_FAIL_VALUE_ARB is clamped to |
| lie in [0, 1]." |
| |
| Modify section 3.8.13.1, defined in the GL_ARB_shadow specification, |
| as follows: |
| |
| "3.8.13.1 Depth Texture Comparison Mode |
| |
| [...] |
| else if TEXTURE_COMPARE_MODE_ARB = COMPARE_R_TO_TEXTURE_ARB |
| |
| if TEXTURE_COMPARE_FUNC_ARB = LEQUAL |
| |
| { 1.0, if R <= Dt |
| r = { |
| { TEXTURE_COMPARE_FAIL_VALUE_ARB, if R > Dt |
| |
| else, if TEXTURE_COMPARE_FUNC_ARB = GEQUAL |
| |
| { 1.0, if R >= Dt |
| r = { |
| { TEXTURE_COMPARE_FAIL_VALUE_ARB, if R < Dt |
| |
| 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 and texture R |
| coordinate. The details of this are implementation-dependent |
| but r should be a value in the range |
| [TEXTURE_COMPARE_FAIL_VALUE_ARB, 1] and 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, change the paragraph introduced by the |
| ARB_shadow extension to be: |
| |
| "The texture compare mode, function, result format and comparison |
| failure value may be queried by calling GetTexParameteriv or |
| GetTexParameterfv with <pname> set to TEXTURE_COMPARE_MODE_ARB, |
| TEXTURE_COMPARE_FUNC_ARB, TEXTURE_COMPARE_RESULT_ARB, or |
| TEXTURE_COMPARE_FAIL_VALUE_ARB, respectively." |
| |
| Additions to the GLX Specification |
| |
| None |
| |
| Errors |
| |
| None. |
| |
| New State |
| |
| In table 6.16, Texture Objects, p. 224, add the following: |
| |
| Get Value Type Get Command Init Val Description Sec. Attribute |
| ------------------------------ ------ -------------------- -------- -------------------------- ------ --------- |
| TEXTURE_COMPARE_FAIL_VALUE_ARB R[0,1] GetTexParameter[if]v 0.0 texture compare fail value 3.8.13 texture |
| |
| New Implementation Dependent State |
| |
| None |
| |
| Revision History |
| |
| 16 April 2001 |
| - initial version |
| 22 April 2001 |
| - rename TEXTURE_FAIL_LUMINANCE_ARB to TEXTURE_FAIL_VALUE_ARB. |
| 23 April 2001 |
| - minor tweaks |
| 16 November 2001 |
| - rename TEXTURE_FAIL_VALUE_ARB to TEXTURE_COMPARE_FAIL_VALUE_ARB |
| 17 November 2001 |
| - adjusted 3.8.7.1 section number |
| 18 November 2001 |
| - Clarify that TEXTURE_COMPARE_FAIL_VALUE_ARB is clamped to [0,1] |
| - added issue 6 |
| - let TEXTURE_COMPARE_FAIL_VALUE_ARB be the same as SHADOW_AMBIENT_SGIX |
| 12 December 2001 |
| - rewritten against the OpenGL 1.3 spec |
| - fixed some typos |
