/
fog_coord.txt
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fog_coord.txt
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Name
EXT_fog_coord
Name Strings
GL_EXT_fog_coord
Contact
Jon Leech, Silicon Graphics (ljp 'at' sgi.com)
Status
Shipping (version 1.6)
Version
$Date: 1999/06/21 19:57:19 $ $Revision: 1.11 $
Number
149
Dependencies
OpenGL 1.1 is required.
The extension is written against the OpenGL 1.2 Specification.
Overview
This extension allows specifying an explicit per-vertex fog
coordinate to be used in fog computations, rather than using a
fragment depth-based fog equation.
Issues
* Should the specified value be used directly as the fog weighting
factor, or in place of the z input to the fog equations?
As the z input; more flexible and meets ISV requests.
* Do we want vertex array entry points? Interleaved array formats?
Yes for entry points, no for interleaved formats, following the
argument for secondary_color.
* Which scalar types should FogCoord accept? The full range, or just
the unsigned and float versions? At the moment it follows Index(),
which takes unsigned byte, signed short, signed int, float, and
double.
Since we're now specifying a number which behaves like an
eye-space distance, rather than a [0,1] quantity, integer types
are less useful. However, restricting the commands to floating
point forms only introduces some nonorthogonality.
Restrict to only float and double, for now.
* Interpolation of the fog coordinate may be perspective-correct or
not. Should this be affected by PERSPECTIVE_CORRECTION_HINT,
FOG_HINT, or another to-be-defined hint?
PERSPECTIVE_CORRECTION_HINT; this is already defined to affect
all interpolated parameters. Admittedly this is a loss of
orthogonality.
* Should the current fog coordinate be queryable?
Yes, but it's not returned by feedback.
* Control the fog coordinate source via an Enable instead of a fog
parameter?
No. We might want to add more sources later.
* Should the fog coordinate be restricted to non-negative values?
Perhaps. Eye-coordinate distance of fragments will be
non-negative due to clipping. Specifying explicit negative
coordinates may result in very large computed f values, although
they are defined to be clipped after computation.
* Use existing DEPTH enum instead of FRAGMENT_DEPTH? Change name of
FRAGMENT_DEPTH_EXT to FOG_FRAGMENT_DEPTH_EXT?
Use FRAGMENT_DEPTH_EXT; FOG_FRAGMENT_DEPTH_EXT is somewhat
misleading, since fragment depth itself has no dependence on
fog.
New Procedures and Functions
void FogCoord[fd]EXT(T coord)
void FogCoord[fd]vEXT(T coord)
void FogCoordPointerEXT(enum type, sizei stride, void *pointer)
New Tokens
Accepted by the <pname> parameter of Fogi and Fogf:
FOG_COORDINATE_SOURCE_EXT 0x8450
Accepted by the <param> parameter of Fogi and Fogf:
FOG_COORDINATE_EXT 0x8451
FRAGMENT_DEPTH_EXT 0x8452
Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,
GetFloatv, and GetDoublev:
CURRENT_FOG_COORDINATE_EXT 0x8453
FOG_COORDINATE_ARRAY_TYPE_EXT 0x8454
FOG_COORDINATE_ARRAY_STRIDE_EXT 0x8455
Accepted by the <pname> parameter of GetPointerv:
FOG_COORDINATE_ARRAY_POINTER_EXT 0x8456
Accepted by the <array> parameter of EnableClientState and
DisableClientState:
FOG_COORDINATE_ARRAY_EXT 0x8457
Additions to Chapter 2 of the OpenGL 1.2 Specification (OpenGL Operation)
These changes describe a new current state type, the fog coordinate,
and the commands to specify it:
- (2.6, p. 12) Second paragraph changed to:
"Each vertex is specified with two, three, or four coordinates.
In addition, a current normal, current texture coordinates,
current color, and current fog coordinate may be used in
processing each vertex."
- 2.6.3, p. 19) First paragraph changed to
"The only GL commands that are allowed within any Begin/End
pairs are the commands for specifying vertex coordinates, vertex
colors, normal coordinates, texture coordinates, and fog
coordinates (Vertex, Color, Index, Normal, TexCoord,
FogCoord)..."
- (2.7, p. 20) Insert the following paragraph following the third
paragraph describing current normals:
" The current fog coodinate is set using
void FogCoord[fd]EXT(T coord)
void FogCoord[fd]vEXT(T coord)."
The last paragraph is changed to read:
"The state required to support vertex specification consists of
four floating-point numbers to store the current texture
coordinates s, t, r, and q, one floating-point value to store
the current fog coordinate, four floating-point values to store
the current RGBA color, and one floating-point value to store
the current color index. There is no notion of a current vertex,
so no state is devoted to vertex coordinates. The initial values
of s, t, and r of the current texture coordinates are zero; the
initial value of q is one. The initial fog coordinate is zero.
The initial current normal has coordinates (0,0,1). The initial
RGBA color is (R,G,B,A) = (1,1,1,1). The initial color index is
1."
- (2.8, p. 21) Added fog coordinate command for vertex arrays:
Change first paragraph to read:
"The vertex specification commands described in section 2.7
accept data in almost any format, but their use requires many
command executions to specify even simple geometry. Vertex data
may also be placed into arrays that are stored in the client's
address space. Blocks of data in these arrays may then be used
to specify multiple geometric primitives through the execution
of a single GL command. The client may specify up to seven
arrays: one each to store edge flags, texture coordinates, fog
coordinates, colors, color indices, normals, and vertices. The
commands"
Add to functions listed following first paragraph:
void FogCoordPointerEXT(enum type, sizei stride, void *pointer)
Add to table 2.4 (p. 22):
Command Sizes Types
------- ----- -----
FogCoordPointerEXT 1 float,double
Starting with the second paragraph on p. 23, change to add
FOG_COORDINATE_ARRAY_EXT:
"An individual array is enabled or disabled by calling one of
void EnableClientState(enum array)
void DisableClientState(enum array)
with array set to EDGE_FLAG_ARRAY, TEXTURE_COORD_ARRAY,
FOG_COORDINATE_ARRAY_EXT, COLOR_ARRAY, INDEX_ARRAY,
NORMAL_ARRAY, or VERTEX_ARRAY, for the edge flag, texture
coordinate, fog coordinate, color, color index, normal, or
vertex array, respectively.
The ith element of every enabled array is transferred to the GL
by calling
void ArrayElement(int i)
For each enabled array, it is as though the corresponding
command from section 2.7 or section 2.6.2 were called with a
pointer to element i. For the vertex array, the corresponding
command is Vertex<size><type>v, where <size> is one of [2,3,4],
and <type> is one of [s,i,f,d], corresponding to array types
short, int, float, and double respectively. The corresponding
commands for the edge flag, texture coordinate, fog coordinate,
color, color, color index, and normal arrays are EdgeFlagv,
TexCoord<size><type>v, FogCoord<type>v, Color<size><type>v,
Index<type>v, and Normal<type>v, respectively..."
Change pseudocode on p. 27 to disable fog coordinate array for
canned interleaved array formats. After the lines
DisableClientState(EDGE_FLAG_ARRAY);
DisableClientState(INDEX_ARRAY);
insert the line
DisableClientState(FOG_COORDINATE_ARRAY_EXT);
Substitute "seven" for every occurence of "six" in the final
paragraph on p. 27.
- (2.12, p. 41) Add fog coordinate to the current rasterpos state.
Change the first sentence of the first paragraph to read
"The state required for the current raster position consists of
three window coordinates x_w, y_w, and z_w, a clip coordinate
w_c value, an eye coordinate distance, a fog coordinate, a valid
bit, and associated data consisting of a color and texture
coordinates."
Change the last paragraph to read
"The current raster position requires six single-precision
floating-point values for its x_w, y_w, and z_w window
coordinates, its w_c clip coordinate, its eye coordinate
distance, and its fog coordinate, a single valid bit, a color
(RGBA color and color index), and texture coordinates for
associated data. In the initial state, the coordinates and
texture coordinates are both (0,0,0,1), the fog coordinate is 0,
the eye coordinate distance is 0, the valid bit is set, the
associated RGBA color is (1,1,1,1), and the associated color
index color is 1. In RGBA mode, the associated color index
always has its initial value; in color index mode, the RGBA
color always maintains its initial value."
- (3.10, p. 139) Change the second and third paragraphs to read
"This factor f may be computed according to one of three
equations:"
f = exp(-d*c) (3.24)
f = exp(-(d*c)^2) (3.25)
f = (e-c)/(e-s) (3.26)
If the fog source (as defined below) is FRAGMENT_DEPTH_EXT, then
c is the eye-coordinate distance from the eye, (0 0 0 1) in eye
coordinates, to the fragment center. If the fog source is
FOG_COORDINATE_EXT, then c is the interpolated value of the fog
coordinate for this fragment. The equation and the fog source,
along with either d or e and s, is specified with
void Fog{if}(enum pname, T param);
void Fog{if}v(enum pname, T params);
If <pname> is FOG_MODE, then <param> must be, or <param> must
point to an integer that is one of the symbolic constants EXP,
EXP2, or LINEAR, in which case equation 3.24, 3.25, or 3.26,,
respectively, is selected for the fog calculation (if, when 3.26
is selected, e = s, results are undefined). If <pname> is
FOG_COORDINATE_SOURCE_EXT, then <param> is or <params> points to
an integer that is one of the symbolic constants
FRAGMENT_DEPTH_EXT or FOG_COORDINATE_EXT. If <pname> is
FOG_DENSITY, FOG_START, or FOG_END, then <param> is or <params>
points to a value that is d, s, or e, respectively. If d is
specified less than zero, the error INVALID_VALUE results."
- (3.10, p. 140) Change the last paragraph preceding section 3.11
to read
"The state required for fog consists of a three valued integer
to select the fog equation, three floating-point values d, e,
and s, an RGBA fog color and a fog color index, a two-valued
integer to select the fog coordinate source, and a single bit to
indicate whether or not fog is enabled. In the initial state,
fog is disabled, FOG_COORDINATE_SOURCE_EXT is
FRAGMENT_DEPTH_EXT, FOG_MODE is EXP, d = 1.0, e = 1.0, and s =
0.0; C_f = (0,0,0,0) and i_f=0."
Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization)
None
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 Specification (State and State
Requests)
None
Additions to Appendix A of the OpenGL 1.2.1 Specification (Invariance)
None
Additions to the GLX / WGL / AGL Specifications
None
GLX Protocol
Two new GL rendering commands are added. The following commands are
sent to the server as part of a glXRender request:
FogCoordfvEXT
2 8 rendering command length
2 4124 rendering command opcode
4 FLOAT32 v[0]
FogCoorddvEXT
2 12 rendering command length
2 4125 rendering command opcode
8 FLOAT64 v[0]
Errors
INVALID_ENUM is generated if FogCoordPointerEXT parameter <type> is
not FLOAT or DOUBLE.
INVALID_VALUE is generated if FogCoordPointerEXT parameter <stride>
is negative.
New State
(table 6.5, p. 195)
Get Value Type Get Command Initial Value Description Sec Attribute
--------- ---- ----------- ------------- ----------- --- ---------
CURRENT_FOG_COORDINATE_EXT R GetIntegerv, 0 Current 2.7 current
GetFloatv fog coordinate
(table 6.6, p. 197)
Get Value Type Get Command Initial Value Description Sec Attribute
--------- ---- ----------- ------------- ----------- --- ---------
FOG_COORDINATE_ARRAY_EXT B IsEnabled False Fog coord array enable 2.8 vertex-array
FOG_COORDINATE_ARRAY_TYPE_EXT Z8 GetIntegerv FLOAT Type of fog coordinate 2.8 vertex-array
FOG_COORDINATE_ARRAY_STRIDE_EXT Z+ GetIntegerv 0 Stride between fog coords 2.8 vertex-array
FOG_COORDINATE_ARRAY_POINTER_EXT Y GetPointerv 0 Pointer to the fog coord array 2.8 vertex-array
(table 6.8, p. 198)
Get Value Type Get Command Initial Value Description Sec Attribute
--------- ---- ----------- ------------- ----------- --- ---------
FOG_COORDINATE_SOURCE_EXT Z2 GetIntegerv, FRAGMENT_DEPTH_EXT Source of fog 3.10 fog
GetFloatv coordinate for
fog calculation
Revision History
* Revision 1.6 - Functionality complete
* Revision 1.7-1.9 - Fix typos and add fields to bring up to date with
the new extension template. No functionality changes.