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| Name |
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| EXT_texture_lod_bias |
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| Name Strings |
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| GL_EXT_texture_lod_bias |
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| Notice |
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| Copyright NVIDIA Corporation, 1999, 2000. |
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| Contact |
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| Mark Kilgard, NVIDIA (mjk 'at' nvidia.com) |
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| Status |
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| Shipping since late 1999. |
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| The texture LOD bias functionality in OpenGL 1.4 is based on this |
| extension though the OpenGL 1.4 functionality added the ability to |
| specify a second per-texture object bias term. The OpenGL 1.4 enum |
| values match the EXT enum values. |
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| Version |
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| Last Modified Date: June 23, 2009 |
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| Number |
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| OpenGL Extension #186 |
| OpenGL ES Extension #60 |
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| Dependencies |
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| Written based on the wording of the OpenGL 1.2 specification. |
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| Affects ARB_multitexture. |
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| Interacts with OpenGL ES 1.1. |
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| Overview |
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| OpenGL computes a texture level-of-detail parameter, called lambda |
| in the GL specification, that determines which mipmap levels and |
| their relative mipmap weights for use in mipmapped texture filtering. |
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| This extension provides a means to bias the lambda computation |
| by a constant (signed) value. This bias can provide a way to blur |
| or pseudo-sharpen OpenGL's standard texture filtering. |
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| This blurring or pseudo-sharpening may be useful for special effects |
| (such as depth-of-field effects) or image processing techniques |
| (where the mipmap levels act as pre-downsampled image versions). |
| On some implementations, increasing the texture lod bias may improve |
| texture filtering performance (at the cost of texture bluriness). |
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| The extension mimics functionality found in Direct3D. |
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| Issues |
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| Should the texture LOD bias be settable per-texture object or |
| per-texture stage? |
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| RESOLUTION: Per-texture stage. This matches the Direct3D |
| semantics for texture lod bias. Note that this differs from |
| the semantics of SGI's SGIX_texture_lod_bias extension that |
| has the biases per-texture object. |
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| This also allows the same texture object to be used by two different |
| texture units for different blurring. This is useful for |
| extrapolating detail between various levels of detail in a |
| mipmapped texture. |
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| For example, you can extrapolate texture detail with |
| ARB_multitexture and EXT_texture_env_combine by computing |
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| (B0 - B2) * 2 + B2 |
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| where B0 is a non-biased texture (normal sharpness) and B2 is |
| the same texture but bias by 2 levels-of-detail (fairly blurry). |
| This has the effect of increasing the high-frequency information |
| in the texture. There are immediate Earth Sciences and medical |
| imaging applications for this technique. |
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| Per-texture stage control of the LOD bias is also useful for |
| allowing an application to control overall texture bluriness. |
| This can be used in games to simulate disorientation (note that |
| only textures will blur, not edges). It can also be used to |
| globally control texturing performance. An application may be |
| able to sustain a constant frame rate by avoiding texture fetch |
| stalls by using slightly blurrier textures. |
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| How does EXT_texture_lod_bias differ from SGIX_texture_lod bias? |
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| EXT_texture_lod_bias adds a bias to lambda. The |
| SGIX_texture_lod_bias extension changes the computation of rho (the |
| log2 of which is lambda). The SGIX extension provides separate |
| biases in each texture dimension. The EXT extension does not |
| provide an "directionality" in the LOD control. |
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| Does the texture lod bias occur before or after the TEXTURE_MAX_LOD |
| and TEXTURE_MIN_LOD clamping? |
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| RESOLUTION: BEFORE. This allows the texture lod bias to still |
| be clamped within the max/min lod range. |
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| Does anything special have to be said to keep the biased lambda value |
| from being less than zero or greater than the maximum number of |
| mipmap levels? |
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| RESOLUTION: NO. The existing clamping in the specification |
| handles these situations. |
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| The texture lod bias is specified to be a float. In practice, what |
| sort of range is assumed for the texture lod bias? |
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| RESOLUTION: The MAX_TEXTURE_LOD_BIAS_EXT implementation constant |
| advertises the maximum absolute value of the supported texture |
| lod bias. The value is recommended to be at least the maximum |
| mipmap level supported by the implementation. |
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| The texture lod bias is specified to be a float. In practice, what |
| sort of precision is assumed for the texture lod bias? |
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| RESOLUTION; This is implementation dependent. Presumably, |
| hardware would implement the texture lod bias as a fractional bias |
| but the exact fractional precision supported is implementation |
| dependent. At least 4 fractional bits is recommended. |
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| New Procedures and Functions |
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| None |
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| New Tokens |
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| Accepted by the <target> parameters of GetTexEnvfv, GetTexEnviv, |
| TexEnvi, TexEnvf, Texenviv, and TexEnvfv: |
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| TEXTURE_FILTER_CONTROL_EXT 0x8500 |
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| When the <target> parameter of GetTexEnvfv, GetTexEnviv, TexEnvi, |
| TexEnvf, TexEnviv, and TexEnvfv is TEXTURE_FILTER_CONTROL_EXT, then |
| the value of <pname> may be: |
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| TEXTURE_LOD_BIAS_EXT 0x8501 |
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| Accepted by the <pname> parameters of GetBooleanv, GetIntegerv, |
| GetFloatv, and GetDoublev: |
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| MAX_TEXTURE_LOD_BIAS_EXT 0x84FD |
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| Additions to Chapter 2 of the 1.2 Specification (OpenGL Operation) |
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| None |
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| Additions to Chapter 3 of the 1.2 Specification (Rasterization) |
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| -- Section 3.8.5 "Texture Minification" |
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| Change the first formula under "Scale Factor and Level of Detail" to read: |
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| "The choice is governed by a scale factor p(x,y), the level of detail |
| parameter lambda(x,y), defined as |
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| lambda'(x,y) = log2[p(x,y)] + lodBias |
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| where lodBias is the texture unit's (signed) texture lod bias parameter |
| (as described in Section 3.8.9) clamped between the positive and negative |
| values of the implementation defined constant MAX_TEXTURE_LOD_BIAS_EXT." |
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| -- Section 3.8.9 "Texture Environments and Texture Functions" |
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| Change the first paragraph to read: |
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| "The command |
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| void TexEnv{if}(enum target, enum pname, T param); |
| void TexEnv{if}v(enum target, enum pname, T params); |
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| sets parameters of the texture environment that specifies how texture |
| values are interepreted when texturing a fragment or sets per-texture |
| unit texture filtering parameters. The possible target parameters |
| are TEXTURE_ENV or TEXTURE_FILTER_CONTROL_EXT. ... When target is |
| TEXTURE_ENV, the possible environment parameters are TEXTURE_ENV_MODE |
| and TEXTURE_ENV_COLOR. ... When target is TEXTURE_FILTER_CONTROL_EXT, |
| the only possible texture filter parameter is TEXTURE_LOD_BIAS_EXT. |
| TEXTURE_LOD_BIAS_EXT is set to a signed floating point value that |
| is used to bias the level of detail parameter, lambda, as described |
| in Section 3.8.5." |
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| Add a final paragraph at the end of the section: |
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| "The state required for the per-texture unit filtering parameters |
| consists of one floating-point value." |
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| Additions to Chapter 4 of the 1.2 Specification (Per-Fragment Operations |
| and the Frame Buffer) |
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| None |
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| Additions to Chapter 5 of the 1.2 Specification (Special Functions) |
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| None |
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| Additions to Chapter 6 of the 1.2 Specification (State and State Requests) |
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| -- Section 6.1.3 "Texture Environments and Texture Functions" |
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| Change the third sentence of the third paragraph to read: |
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| "The env argument to GetTexEnv must be either TEXTURE_ENV or |
| TEXTURE_FILTER_CONTROL_EXT." |
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| Additions to the GLX Specification |
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| None |
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| Dependencies on OpenGL ES 1.1 |
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| If the GL is OpenGL ES 1.1, omit reference to GetDoublev. |
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| Errors |
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| INVALID_ENUM is generated when TexEnv is called with a <pname> of |
| TEXTURE_FILTER_CONTROL_EXT and the value of <param> or what is pointed to |
| by <params> is not TEXTURE_LOD_BIAS_EXT. |
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| New State |
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| (table 6.14, p204) add the entry: |
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| Get Value Type Get Command Initial Value Description Sec Attribute |
| ----------------------- ---- ----------- -------------- --------------- ----- --------- |
| TEXTURE_LOD_BIAS_EXT R GetTexEnvfv 0.0 Biases texture 3.8.9 texture |
| level of detail |
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| (When ARB_multitexture is supported, the TEXTURE_LOD_BIAS_EXT state is per-texture unit.) |
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| New Implementation State |
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| (table 6.24, p214) add the following entries: |
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| Get Value Type Get Command Minimum Value Description Sec Attribute |
| -------------------------- ---- ----------- ------------- ----------------- ------ -------------- |
| MAX_TEXTURE_LOD_BIAS_EXT R+ GetFloatv 4.0 Maximum 3.8.9 - |
| absolute texture |
| lod bias |
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| Revision History |
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| 6/23/09 (Jon Leech) - assign OpenGL ES extension number. |
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| 4/29/09 (Benj Lipchak) - add interaction with OpenGL ES 1.1. |
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| 8/27/03 - updated status to mention OpenGL 1.4 functionality. |
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| 8/26/03 - fixed incorrect enum name (TEXTURE_FILTER_CONTROL_EXT is |
| correct) in the Errors section. |
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| 6/2/00 - add spec language to allow GetTexEnv to accept |
| TEXTURE_FILTER_CONTROL_EXT. |
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