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GL_EXT_separate_specular_color
EXT_separate_specular_color
Name Strings
GL_EXT_separate_specular_color
Version
$Date: 1997/10/05 00:16:23 $ $Revision: 1.3 $
Number
144
Dependencies
None
Overview
This extension adds a second color to rasterization when lighting is
enabled. Its purpose is to produce textured objects with specular
highlights which are the color of the lights. It applies only to
rgba lighting.
The two colors are computed at the vertexes. They are both clamped,
flat-shaded, clipped, and converted to fixed-point just like the
current rgba color (see Figure 2.8). Rasterization interpolates
both colors to fragments. If texture is enabled, the first (or
primary) color is the input to the texture environment; the fragment
color is the sum of the second color and the color resulting from
texture application. If texture is not enabled, the fragment color
is the sum of the two colors.
A new control to LightModel*, LIGHT_MODEL_COLOR_CONTROL_EXT, manages
the values of the two colors. It takes values: SINGLE_COLOR_EXT, a
compatibility mode, and SEPARATE_SPECULAR_COLOR_EXT, the object of
this extension. In single color mode, the primary color is the
current final color and the secondary color is 0.0. In separate
specular mode, the primary color is the sum of the ambient, diffuse,
and emissive terms of final color and the secondary color is the
specular term.
There is much concern that this extension may not be compatible with
the future direction of OpenGL with regards to better lighting and
shading models. Until those impacts are resolved, serious
consideration should be given before adding to the interface
specified herein (for example, allowing the user to specify a
second input color).
Issues
* Where is emissive included?
RESOLVED - Emissive is included with the ambient and diffuse
terms. Grouping emissive with specular (the "proper" thing) could
be implemented with a new value for the color control.
* Should there be two colors when not lighting or with index
lighting?
RESOLVED - The answer is probably yes--there should be two colors
when lighting is disabled and there could be an incorporation of
two colors with index lighting; but these are beyond the scope of
this extension. Further, attempts to accomplish these may not be
compatible with the future direction of OpenGL with respect to
high quality lighting and shading models.
* What happens when texture is disabled?
RESOLVED - The extension specifies to add the two colors when
texture is disabled. This is compatible with the philosophy of
"if texture is disabled, this mode does not apply".
None.
Accepted by the <pname> parameter of LightModel*, and also by the
<pname> parameter of GetBooleanv, GetIntegerv, GetFloatv, and
GetDoublev:
LIGHT_MODEL_COLOR_CONTROL_EXT 0x81F8
Accepted by the <param> parameter of LightModel* when <pname> is
LIGHT_MODEL_COLOR_CONTROL_EXT:
SINGLE_COLOR_EXT 0x81F9
SEPARATE_SPECULAR_COLOR_EXT 0x81FA
Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)
- (2.13, p. 40) Rework the second paragraph to acknowledge two
colors:
"Next, lighting, if enabled, produces either a color index or
primary and secondary colors. If lighting is disabled, the
current color index or color is used in further processing (the
current color is the primary color and the secondary color is 0).
After lighting, colors are clamped..."
- (Figure 2.8, p. 41) Change RGBA to primary RGBA and secondary RGB:
Ideally, there might be an RGB2 underneath RGBA (both places).
Alternatively, a note in the caption could clarify that RGBA
referred to the primary RGBA and a secondary RGB. (Speaking of
the caption, the part about "m is the number of bits an R, G, B,
or A component" could be removed as m doesn't appear in the
diagram.)
- (2.13.1, p. 42) Rework the opening of this section to not imply a
single color:
In the first sentence, change "a color" to "colors". Rephrase the
itemization of the two lighting states to:
"1. Lighting Off. In this state, the current color is assigned to
the vertex primary color. The vertex secondary color is 0.
2. Lighting On. In this state, the vertex primary and secondary
colors are computed from the current lighting parameters."
- (Table 2.7, p.44) Add new entry (at the bottom):
Parameter Type Default Value Description
--------- ---- ---------------- ------------------------------
c_es enum SINGLE_COLOR_EXT controls computation of colors
- (p. 45, top of page) Rephrase the first line and equation:
"Lighting produces two colors at a vertex: a primary color c_1 and
a secondary color c_2. The values of c_1 and c_2 depend on the
light model color control, c_es (note: c_es should be in italics
and c_1 and c_2 in bold, so this really won't be as confusing as
it seems). If c_es = SINGLE_COLOR_EXT, then the equations to
compute c_1 and c_2 are (note: the equation for c_1 is the current
equation for c):
c_1 = e_cm
+ a_cm * a_cs
+ SUM(att_i * spot_i * (a_cm * a_cli
+ dot(n, VP_pli) * d_cm * d_cli
+ f_i * dot(n, h_i)^s_rm * s_cm * s_cli)
c_2 = 0
If c_es = SEPARATE_SPECULAR_COLOR_EXT, then:
c_1 = e_cm
+ a_cm * a_cs
+ SUM (att_i * spot_i * (a_cm * a_cli
+ (n dot VP_pli) * d_cm * d_cli)
c_2 = SUM(att_i * spot_i * (f_i * (n dot h_i)^s_rm * s_cm * s_cli)
- (p. 45, second paragraph from bottom) Clarify that A is in the
primary color:
After the sentence "The value of A produced by lighting is the
alpha value associated with d_cm", add "A is always associated
with the primary color c_1; c_2 has no alpha component."
- (Table 2.8, p. 48) Add a new entry (at the bottom):
Parameter Name Number of values
--------- ----------------------------- ----------------
c_es LIGHT_MODEL_COLOR_CONTROL_EXT 1
- (2.13.6, p. 51) Clarify that both primary and secondary colors are
clamped:
Replace "RGBA" in the first line of the section with "both primary
and secondary".
- (2.13.7, p. 52) Clarify what happens to primary and secondary
colors when flat shading--reword the first paragraph:
"A primitive may be flatshaded, meaning that all vertices of the
primitive are assigned the same color index or primary and
secondary colors. These come from the vertex that spawned the
primitive. For a point, these are the colors associated with the
point. For a line segment, they are the colors of the second
(final) vertex of the segment. For a polygon, they come from a
selected vertex depending on how the polygon was generated. Table
2.9 summarizes the possibilities."
- (2.13.8, p. 52) Rework to not imply a single color:
In the second sentence, change "If the color is" to "Those" and ",
it is" to "are". In the first sentence of the next paragraph,
change "the color" to "two colors".
Additions to Chapter 3 of the 1.0 Specification (Rasterization)
- (Figure 3.1, p. 55) Add a box between texturing and fog called
"color sum".
- (3.8, p. 85) In the first paragraph, second sentence, insert
"primary" before RGBA. Insert after this sentence "Texturing does
not affect the secondary color."
- (new section before 3.9) Insert new section titled "Color Sum":
"At the beginning of this stage in RGBA mode, a fragment has two
colors: a primary RGBA color (which texture, if enabled, may have
modified) and a secondary RGB color. This stage sums the R, G,
and B components of these two colors to produce a single RGBA
color. If the resulting RGB values exceed 1.0, they are clamped
to 1.0.
In color index mode, a fragment only has a single color index and
this stage does nothing."
Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations
and the Frame Buffer)
None.
Additions to Chapter 5 of the 1.0 Specification (Special Functions)
- (5.3, p. 137) Specify that feedback returns the primary color by
changing the last sentence of the large paragraph in the middle
of the page to:
"The colors returned are the primary colors. These colors and the
texture coordinates are those resulting from the clipping operations
as described in section 2.13.8."
Additions to Chapter 6 of the 1.0 Specification (State and State Requests)
- (Table 6.9, p. 157) Add:
Get Value - LIGHT_MODEL_COLOR_CONTROL_EXT
Type - Z2
Get Cmnd - GetIntegerv
Initial Value - SINGLE_COLOR_EXT
Description - color control
Sec. - (whatever it ends up as)
Attribute - lighting
Additions to the GLX Specification
None.
GLX Protocol
None.
Errors
None.
New State
(see changes to table 6.9)
Implementation Support
List of OpenGL implementations supporting the GL_EXT_separate_specular_color extension
Original File
Original text file for the GL_EXT_separate_specular_color extension
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