Name ARB_texture_border_clamp Name Strings GL_ARB_texture_border_clamp Contact Patrick R. Brown, Intel Corporation (patrick.r.brown 'at' intel.com) Notice Copyright (c) 2000-2013 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 the ARB, 20 June 2000 Version 1.0, 22 June 2000 Number ARB Extension #13 Dependencies OpenGL 1.0 is required. This extension is written against the OpenGL 1.2.1 Specification. This extension is based on and intended to replace GL_SGIS_texture_border_clamp. Overview The base OpenGL provides clamping such that the texture coordinates are limited to exactly the range [0,1]. When a texture coordinate is clamped using this algorithm, the texture sampling filter straddles the edge of the texture image, taking 1/2 its sample values from within the texture image, and the other 1/2 from the texture border. It is sometimes desirable for a texture to be clamped to the border color, rather than to an average of the border and edge colors. This extension defines an additional texture clamping algorithm. CLAMP_TO_BORDER_ARB clamps texture coordinates at all mipmap levels such that NEAREST and LINEAR filters return only the color of the border texels. IP Status No known IP issues. Issues (1) Is this formulation correct for higher-order texture filters (e.g., cubic or anisotropic filters)? RESOLVED: No. A more appropriate formulation would clamp the texture coordinates in texel space. New Procedures and Functions None. New Tokens Accepted by the parameter of TexParameteri and TexParameterf, and by the parameter of TexParameteriv and TexParameterfv, when their parameter is TEXTURE_WRAP_S, TEXTURE_WRAP_T, or TEXTURE_WRAP_R: CLAMP_TO_BORDER_ARB 0x812D Additions to Chapter 2 of the OpenGL 1.2.1 Specification (OpenGL Operation) None. Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization) Modify Table 3.17, p. 124, editing only the following lines: Name Type Legal Values ============== ======= ==================== TEXTURE_WRAP_S integer CLAMP, CLAMP_TO_EDGE, REPEAT, CLAMP_TO_BORDER_ARB TEXTURE_WRAP_T integer CLAMP, CLAMP_TO_EDGE, REPEAT, CLAMP_TO_BORDER_ARB TEXTURE_WRAP_R integer CLAMP, CLAMP_TO_EDGE, REPEAT, CLAMP_TO_BORDER_ARB Modify Section 3.8.4, Texture Wrap Modes, p.124 (add at the end of the section, p. 125) CLAMP_TO_BORDER_ARB clamps texture coordinates at all mipmaps such that the texture filter always samples border texels for fragments whose corresponding texture coordinate is sufficiently far outside the range [0,1]. The color returned when clamping is derived only from the border texels of the texture image, or from the constant border color if the texture image does not have a border. Texture coordinates are clamped to the range [min, max]. The minimum value is defined as min = -1 / 2N where N is the size (not including borders) of the one-, two-, or three-dimensional texture image in the direction of clamping. The maximum value is defined as max = 1 - min so that clamping is always symmetric about the [0,1] mapped range of a texture coordinate. Additions to Chapter 4 of the OpenGL 1.2.1 Specification (Per-Fragment Operations and the Frame Buffer) None. Additions to Chapter 5 of the OpenGL 1.2.1 Specification (Special Functions) None. Additions to Chapter 6 of the OpenGL 1.2.1 Specification (State and State Requests) None. Additions to Appendix A of the OpenGL 1.2.1 Specification (Invariance) None. Additions to the AGL/GLX/WGL Specifications None. GLX Protocol None. Errors None. New State Only the type information changes for these parameters. (table 6.13, p. 203) Initial Get Value Type Get Command Value Description Sec. Attribute --------- ---- ----------- ------- ----------- ---- --------- TEXTURE_WRAP_S 3+ x Z4 GetTexParameter REPEAT Texture wrap 3.8 texture TEXTURE_WRAP_T 3+ x Z4 GetTexParameter REPEAT Texture wrap 3.8 texture TEXTURE_WRAP_R 3+ x Z4 GetTexParameter REPEAT Texture wrap 3.8 texture Revision History 1.0, 06/22/2000 prbrown1: Added issue w.r.t. higher order filters. 0.2, 05/23/2000 prbrown1: Removed dependency on SGIS_texture_filter4 per ARB guidelines. 0.1, 05/02/2000 prbrown1: Initial revision -- mostly stolen from GL_SGIS_texture_border_clamp.