| Name |
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| NV_fog_distance |
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| Name Strings |
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| GL_NV_fog_distance |
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| Contact |
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| Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com) |
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| Notice |
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| Copyright NVIDIA Corporation, 1999, 2000, 2001. |
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| IP Status |
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| NVIDIA Proprietary. |
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| Status |
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| Shipping (version 1.0) |
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| Version |
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| NVIDIA Date: January 18, 2001 |
| Version: 1.0 |
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| Number |
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| 192 |
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| Dependencies |
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| Written based on the wording of the OpenGL 1.2 specification. |
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| Overview |
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| Ideally, the fog distance (used to compute the fog factor as |
| described in Section 3.10) should be computed as the per-fragment |
| Euclidean distance to the fragment center from the eye. In practice, |
| implementations "may choose to approximate the eye-coordinate |
| distance from the eye to each fragment center by abs(ze). Further, |
| [the fog factor] f need not be computed at each fragment, but may |
| be computed at each vertex and interpolated as other data are." |
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| This extension provides the application specific control over how |
| OpenGL computes the distance used in computing the fog factor. |
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| The extension supports three fog distance modes: "eye plane absolute", |
| where the fog distance is the absolute planar distance from the eye |
| plane (i.e., OpenGL's standard implementation allowance as cited above); |
| "eye plane", where the fog distance is the signed planar distance |
| from the eye plane; and "eye radial", where the fog distance is |
| computed as a Euclidean distance. In the case of the eye radial |
| fog distance mode, the distance may be computed per-vertex and then |
| interpolated per-fragment. |
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| The intent of this extension is to provide applications with better |
| control over the tradeoff between performance and fog quality. |
| The "eye planar" modes (signed or absolute) are straightforward |
| to implement with good performance, but scenes are consistently |
| under-fogged at the edges of the field of view. The "eye radial" |
| mode can provide for more accurate fog at the edges of the field of |
| view, but this assumes that either the eye radial fog distance is |
| computed per-fragment, or if the fog distance is computed per-vertex |
| and then interpolated per-fragment, then the scene must be |
| sufficiently tessellated. |
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| Issues |
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| What should the default state be? |
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| IMPLEMENTATION DEPENDENT. |
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| The EYE_PLANE_ABSOLUTE_NV mode is the most consistent with the way |
| most current OpenGL implementations are implemented without this |
| extension, but because this extension provides specific control |
| over a capability that core OpenGL is intentionally lax about, |
| the default fog distance mode is left implementation dependent. |
| We would not want a future OpenGL implementation that supports |
| fast EYE_RADIAL_NV fog distance to be stuck using something less. |
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| Advice: If an implementation can provide fast per-pixel EYE_RADIAL_NV |
| support, then EYE_RADIAL_NV is the ideal default, but if not, then |
| EYE_PLANE_ABSOLUTE_NV is the most reasonable default mode. |
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| How does this extension interact with the EXT_fog_coord extension? |
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| If FOG_COORDINATE_SOURCE_EXT is set to FOG_COORDINATE_EXT, |
| then the fog distance mode is ignored. However, the fog |
| distance mode is used when the FOG_COORDINATE_SOURCE_EXT is |
| set to FRAGMENT_DEPTH_EXT. Essentially, when the EXT_fog_coord |
| functionality is enabled, the fog distance is supplied by the |
| user-supplied fog-coordinate so no automatic fog distance computation |
| is performed. |
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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 <pname> parameters of Fogf, Fogi, Fogfv, Fogiv, |
| GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev: |
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| FOG_DISTANCE_MODE_NV 0x855A |
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| When the <pname> parameter of Fogf, Fogi, Foggv, and Fogiv, is |
| FOG_DISTANCE_MODE_NV, then the value of <param> or the value pointed |
| to by <params> may be: |
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| EYE_RADIAL_NV 0x855B |
| EYE_PLANE |
| EYE_PLANE_ABSOLUTE_NV 0x855C |
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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 3.10 "Fog" |
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| Add to the end of the 3rd paragraph: |
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| "If pname is FOG_DISTANCE_MODE_NV, then param must be, or params |
| must point to an integer that is one of the symbolic constants |
| EYE_PLANE_ABSOLUTE_NV, EYE_PLANE, or EYE_RADIAL_NV and this symbolic |
| constant determines how the fog distance should be computed." |
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| Replace the 4th paragraph beginning "An implementation may choose |
| to approximate ..." with: |
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| "When the fog distance mode is EYE_PLANE_ABSOLUTE_NV, the fog |
| distance z is approximated by abs(ze) [where ze is the Z component |
| of the fragment's eye position]. When the fog distance mode is |
| EYE_PLANE, the fog distance z is approximated by ze. When the |
| fog distance mode is EYE_RADIAL_NV, the fog distance z is computed |
| as the Euclidean distance from the center of the fragment in eye |
| coordinates to the eye position. Specifically: |
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| z = sqrt( xe*xe + ye*ye + ze*ze ); |
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| In the EYE_RADIAL_NV fog distance mode, the Euclidean distance |
| is permitted to be computed per-vertex, and then interpolated |
| per-fragment." |
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| Change the last paragraph to read: |
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| "The state required for fog consists of a three valued integer to |
| select the fog equation, a three valued integer to select the fog |
| distance mode, three floating-point values d, e, and s, and RGBA fog |
| color and a fog color index, and a single bit to indicate whether |
| or not fog is enabled. In the initial state, fog is disabled, |
| FOG_MODE is EXP, FOG_DISTANCE_NV is implementation defined, d = |
| 1.0, e = 1.0, and s = 0.0; Cf = (0,0,0,0) and if = 0." |
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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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| None |
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| Additions to the AGL/GLX/WGL Specifications |
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| None |
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| GLX Protocol |
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| None |
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| Errors |
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| INVALID_ENUM is generated when Fog is called with a <pname> of |
| FOG_DISTANCE_MODE_NV and the value of <param> or what is pointed |
| to by <params> is not one of EYE_PLANE_ABSOLUTE_NV, EYE_PLANE, |
| or EYE_RADIAL_NV. |
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| New State |
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| (table 6.8, p198) add the entry: |
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| Get Value Type Get Command Initial Value Description Sec Attribute |
| -------------------- ---- ----------- --------------- ----------- ----- --------- |
| FOG_DISTANCE_MODE_NV Z3 GetIntegerv implementation Determines how 3.10 fog |
| dependent fog distance |
| is computed |
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| New Implementation State |
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| None |
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| Revision History |
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| None |
