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updated documentation of effects

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Tim Moore 2009-11-22 00:19:43 +01:00
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docs-mini/README.effects
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@ -1,3 +1,6 @@
Effects
-------
Effects describe the graphical appearance of 3d objects and scenery in Effects describe the graphical appearance of 3d objects and scenery in
FlightGear. The main motivation for effects is to support OpenGL FlightGear. The main motivation for effects is to support OpenGL
shaders and to provide different implementations for graphics hardware shaders and to provide different implementations for graphics hardware
@ -10,13 +13,16 @@ values. Effects are read from files with a ".eff" extension or can be
created on-the-fly by FlightGear at runtime. An effect consists of a created on-the-fly by FlightGear at runtime. An effect consists of a
"parameters" section followed by "technique" descriptions. The "parameters" section followed by "technique" descriptions. The
"parameters" section is a tree of values that describe, abstractly, "parameters" section is a tree of values that describe, abstractly,
the graphical appearence of objects that use the effect. Techniques the graphical characteristics of objects that use the effect. Techniques
refer to these parameters and use them to set OpenGL state or to set refer to these parameters and use them to set OpenGL state or to set
parameters for shader programs. parameters for shader programs. The names of properties in the
parameter section can be whatever the effects author chooses, although
some standard parameters are set by FlightGear itself. On the other
hand, the properties in the techniques section are all defined by the
FlightGear.
Parameters can be declared to have a dynamic variance, which means Techniques
that if their value is changed the corresponding value in the ----------
technique will be changed too. [TO BE IMPLEMENTED]
A technique can contain a predicate that describes the OpenGL A technique can contain a predicate that describes the OpenGL
functionality required to support the technique. The first functionality required to support the technique. The first
@ -29,22 +35,131 @@ and, or, equal, less, less-equal
glversion - returns the version number of OpenGL glversion - returns the version number of OpenGL
extension-supported - returns true if an OpenGL extension is supported extension-supported - returns true if an OpenGL extension is supported
property - returns the boolean value of a property property - returns the boolean value of a property
shader-language - returns the version of GLSL supported, or 0 if there is none.
The proper way to test whether to enable a shader-based technique is:
<predicate>
<and>
<property>/sim/rendering/shader-effects</property>
<less-equal>
<value type="float">1.0</value>
<shader-language/>
</less-equal>
</and>
</predicate>
A technique can consist of several passes. A pass is basically an Open A technique can consist of several passes. A pass is basically an Open
Scene Graph StateSet. Ultimately all OpenGL and OSG modes and state Scene Graph StateSet. Ultimately all OpenGL and OSG modes and state
attributes will be setable in a technique. The following are attributes will be accessable in techniques. State attributes -- that
are currently implemented: is, technique properties that have children and are not just boolean
lighting - true, false modes -- have an <active> parameter which enables or disables the
shade-model - flat, smooth attribute. In this way a technique can declare parameters it needs,
but not enable the attribute at all if it is not needed; the decision
can be based on a parameter in the parameters section of the
effect. For example, effects that support transparent and opaque
geometry could have as part of a technique:
<blend>
<active><use>blend/active</use></active>
<source>src-alpha</source>
<destination>one-minus-src-alpha</destination>
</blend>
So if the blend/active parameter is true blending will be activated
using the usual blending equation; otherwise blending is disabled.
Values of Technique Attributes
------------------------------
Values are assigned to technique properties in several ways:
* They can appear directly in the techniques section as a
constant. For example:
<uniform>
<name>ColorsTex</name>
<type>sampler-1d</type>
<value type="int">2</value>
</uniform>
* The name of a property in the parameters section can be
referenced using a "use" clause. For example, in the technique
section:
<material>
<ambient><use>material/ambient</use></ambient>
</material>
Then, in the parameters section of the effect:
<parameters>
<material>
<ambient type="vec4d">
0.2 .2 0.2 1.0
</ambient>
</material>
</parameters>
It's worth pointing out that the "material" property in a
technique specifies part of OpenGL's state, whereas "material"
in the parameters section is just a name, part of a
hierarchical namespace.
* A property in the parameters section doesn't need to contain
a constant value; it can also contain a "use" property. Here
the value of the use clause is the name of a node in an
external property tree which will be used as the source of a
value. If the name begins with '/', the node is in
FlightGear's global property tree; otherwise, it is in a local
property tree, usually belonging to a model [NOT IMPLEMENTED
YET]. For example:
<parameters>
<chrome-light><use>/rendering/scene/chrome-light</use></chrome-light>
</parameters>
The type is determined by what is expected by the technique
attribute that will ultimately receive the value. [There is
no way to get vector values out of the main property system
yet; this will be fixed shortly.] Values that are declared
this way are dynamically updated if the property node
changes.
OpenGL Attributes
-----------------
The following attributes are currently implemented in techiques:
alpha-test - children: active, comparison, reference
Valid values for comparision:
never, less, equal, lequal, greater, notequal, gequal,
always
blend - children: active, source, destination, source-rgb,
source-alpha, destination-rgb, destination-alpha
Each operand can have the following values:
dst-alpha, dst-color, one, one-minus-dst-alpha,
one-minus-dst-color, one-minus-src-alpha,
one-minus-src-color, src-alpha, src-alpha-saturate,
src-color, constant-color, one-minus-constant-color,
constant-alpha, one-minus-constant-alpha, zero
cull-face - front, back, front-back cull-face - front, back, front-back
rendering-hint - (OSG) opaque, transparent
render-bin - children: bin-number, bin-name lighting - true, false
material - children: ambient, ambient-front, ambient-back, diffuse,
material - children: active, ambient, ambient-front, ambient-back, diffuse,
diffuse-front, diffuse-back, specular, specular-front, diffuse-front, diffuse-back, specular, specular-front,
specular-back, emissive, emissive-front, emissive-back, shininess, specular-back, emissive, emissive-front, emissive-back, shininess,
shininess-front, shininess-back, color-mode shininess-front, shininess-back, color-mode
blend - true, false
alpha-test - true, false polygon-mode - children: front, back
Valid values:
fill, line, point
program
vertex-shader
fragment-shader
render-bin - (OSG) children: bin-number, bin-name
rendering-hint - (OSG) opaque, transparent
shade-model - flat, smooth
texture-unit - has several child properties: texture-unit - has several child properties:
unit - The number of an OpenGL texture unit unit - The number of an OpenGL texture unit
type - This is either an OpenGL texture type or the name of a type - This is either an OpenGL texture type or the name of a
@ -62,16 +177,17 @@ texture-unit - has several child properties:
environment environment
mode mode
color color
program
vertex-shader
fragment-shader
uniform uniform
name name
type - float, float-vec3, float-vec4, sampler-1d, sampler-2d, type - float, float-vec3, float-vec4, sampler-1d, sampler-2d,
sampler-3d sampler-3d
polygon-mode
front - fill, line, point vertex-program-two-side - true, false
back - fill, line, point
vertex-program-point-size - true, false
Inheritance
-----------
One feature not fully illustrated in the sample below is that One feature not fully illustrated in the sample below is that
effects can inherit from each other. The parent effect is listed in effects can inherit from each other. The parent effect is listed in
@ -83,164 +199,64 @@ precedence. This means that effects that inherit from the example
effect below could be very short, listing just new effect below could be very short, listing just new
parameters and adding nothing to the techniques section; parameters and adding nothing to the techniques section;
alternatively, a technique could be altered or customized in a alternatively, a technique could be altered or customized in a
child, listing (for example) a different shader program. Terrain child, listing (for example) a different shader program. An example
materials work in this way: for each material type in materials.xml showing inheritance Effects/crop.eff, which inherits some if its
an effect is created that inherits from a single default terrain values from Effects/terrain-default.eff.
effect. The parameters section of the effect is filled in using the
ambient, diffuse, specular, emissive, shininess, and transparent FlightGear directly uses effects inheritance to assign effects to 3D
fields of the material. Seperate effects are created for each texture models and terrain. As described below, at runtime small effects are
created that contain material and texture values in a "parameters"
section. These effects inherit from another effect which references
those parameters in its "techniques" section. The derived effect
overrides any default values that might be in the base effect's
parameters section.
Default Effects in Terrain Materials and Models
---------------------------------------
Effects for terrain work in this way: for each material type in
materials.xml an effect is created that inherits from a single default
terrain effect, Effects/terrain-default.eff. The parameters section of
the effect is filled in using the ambient, diffuse, specular,
emissive, shininess, and transparent fields of the material. The
parameters image, filter, wrap-s, and wrap-t are also initialized from
the material xml. Seperate effects are created for each texture
variant of a material. variant of a material.
Model effects are created by walking the OpenSceneGraph scene graph
for a model and replacing nodes (osg::Geode) that have state sets with
node that uses an effect instead. Again, a small effect is created
with parameters extracted from OSG objects; this effect inherits, by
default, from Effects/model-default.eff. A larger set of parameters is
created for model effects than for terrain because there is more
variation possible from the OSG model loaders than from the terrain
system. The parameters created are:
* material active, ambient, diffuse, specular, emissive,
shininess, color mode
* blend active, source, destination
* shade-model
* cull-face
* rendering-hint
* texture type, image, filter, wrap-s, wrap-t
Specifying Custom Effects
-------------------------
You can specify the effects that will be used by FlightGear as the
base effect when it creates terrain and model effects.
In the terrain materials.xml, an "effect" property specifies the name
of the model to use.
In model .xml files, A richer syntax is supported. [TO BE DETERMINED]
Material animations will be implemented by creating a new effect Material animations will be implemented by creating a new effect
that inherits from one in a model, overriding the parameters that that inherits from one in a model, overriding the parameters that
will be animated. will be animated.
Examples
--------
<PropertyList> The Effects directory contains the effects definitions; look there for
<effect> examples. Effects/crop.eff is a good example of a complex effect.
<name>city</name>
<!-- <inherits-from>another-effect</inherits-from> -->
<parameters>
<material>
<ambient type="vec4d">
0.0 0.0 0.0 1.0
</ambient>
<diffuse type="vec4d">
.5 .5 .5 1.0
</diffuse>
<specular type="vec4d">
0.3 0.3 0.3 1.0
</specular>
<emissive type="vec4d" variance="dynamic">
0.0 0.0 0.0 1.0
</emissive>
<shininess>1.2</shininess>
</material>
<texture n="0">
<image>city.png</image>
<filter>linear-mipmap-linear</filter>
<!-- also repeat -->
<wrap-s>clamp</wrap-s>
<wrap-t>clamp-to-edge</wrap-t>
<!--
<wrap-r>clamp-to-border</wrap-r>
-->
<!-- float, signed-integer, integer -->
<internal-format>normalized</internal-format>
</texture>
<texture n="1">
<image>detail.png</image>
<filter>linear-mipmap-linear</filter>
<!-- also repeat -->
<wrap-s>clamp</wrap-s>
<wrap-t>clamp-to-edge</wrap-t>
<!--
<wrap-r>clamp-to-border</wrap-r>
-->
<!-- float, signed-integer, integer -->
<internal-format>normalized</internal-format>
</texture>
<bump-height type="double">.05</bump-height>
<pattern-rotation type="vec4d">0 0 1 1.5708</pattern-rotation>
</parameters>
<technique>
<predicate>
<!-- This is the general way to test for shader support -->
<or>
<less-equal>
<value type="float">2.0</value>
<glversion/>
</less-equal>
<and>
<extension-supported>GL_ARB_shader_objects</extension-supported>
<extension-supported>GL_ARB_shading_language_100</extension-supported>
<extension-supported>GL_ARB_vertex_shader</extension-supported>
<extension-supported>GL_ARB_fragment_shader</extension-supported>
</and>
</or>
</predicate>
<pass>
<lighting>true</lighting>
<material>
<ambient><use>material/ambient</use></ambient>
<diffuse><use>material/diffuse</use></diffuse>
<specular><use>material/specular</use></specular>
<shininess><use>material/shininess</use></shininess>
</material>
<texture-unit>
<unit>0</unit>
<image><use>texture[0]/image</use></image>
<filter><use>texture[0]/filter</use></filter>
<wrap-s><use>texture[0]/wrap-s</use></wrap-s>
<wrap-t><use>texture[0]/wrap-t</use></wrap-t>
<internal-format>
<use>texture[0]/internal-format</use>
</internal-format>
</texture-unit>
<texture-unit>
<unit>1</unit>
<image><use>texture[1]/image</use></image>
<filter><use>texture[1]/filter</use></filter>
<wrap-s><use>texture[1]/wrap-s</use></wrap-s>
<wrap-t><use>texture[1]/wrap-t</use></wrap-t>
<internal-format>
<use>texture[1]/internal-format</use>
</internal-format>
</texture-unit>
<uniform>
<name>bumpHeight</name>
<type>float</type>
<use>bump-height</use>
</uniform>
<uniform>
<name>patternRotation</name>
<type>float-vec4</type>
<use>pattern-rotation</use>
</uniform>
<uniform>
<name>baseTexture</name>
<type>sampler-2d</type>
<value>0</value>
</uniform>
<uniform>
<name>detailTexture</name>
<type>sampler-2d</type>
<value>1</value>
</uniform>
<shader-program>
<!-- These two vertex shaders are linked together -->
<vertex-shader>
"Shaders/util.vert"
</vertex-shader>
<vertex-shader>
"Shaders/foo.vert"
</vertex-shader>
<fragment-shader>
"Shaders/foo.frag"
</fragment-shader>
</shader-program>
</pass>
</technique>
<!-- This technique is always valid -->
<technique>
<pass>
<lighting>true</lighting>
<material>
<ambient><use>material/ambient</use></ambient>
<diffuse><use>material/diffuse</use></diffuse>
<specular><use>material/specular</use></specular>
</material>
<texture-unit>
<unit>0</unit>
<image><use>texture[0]/image</use></image>
<filter><use>texture[0]/filter</use></filter>
<wrap-s><use>texture[0]/wrap-s</use></wrap-s>
<wrap-t><use>texture[0]/wrap-t</use></wrap-t>
<internal-format>
<use>texture[0]/internal-format</use>
</internal-format>
</texture-unit>
</pass>
</technique>
</effect>
</PropertyList>