+Effects
+-------
+
Effects describe the graphical appearance of 3d objects and scenery in
FlightGear. The main motivation for effects is to support OpenGL
shaders and to provide different implementations for graphics hardware
created on-the-fly by FlightGear at runtime. An effect consists of a
"parameters" section followed by "technique" descriptions. The
"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
-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
-that if their value is changed the corresponding value in the
-technique will be changed too. [TO BE IMPLEMENTED]
+Techniques
+----------
A technique can contain a predicate that describes the OpenGL
functionality required to support the technique. The first
glversion - returns the version number of OpenGL
extension-supported - returns true if an OpenGL extension is supported
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
Scene Graph StateSet. Ultimately all OpenGL and OSG modes and state
-attributes will be setable in a technique. The following are
-are currently implemented:
-lighting - true, false
-shade-model - flat, smooth
+attributes will be accessable in techniques. State attributes -- that
+is, technique properties that have children and are not just boolean
+modes -- have an <active> parameter which enables or disables the
+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
+
+lighting - true, false
+
+material - children: active, ambient, ambient-front, ambient-back, diffuse,
+ diffuse-front, diffuse-back, specular, specular-front,
+ specular-back, emissive, emissive-front, emissive-back, shininess,
+ shininess-front, shininess-back, color-mode
+
+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
-render-bin - children: bin-number, bin-name
-material - children: ambient, ambient-front, ambient-back, diffuse,
-diffuse-front, diffuse-back, specular, specular-front,
-specular-back, emissive, emissive-front, emissive-back, shininess,
-shininess-front, shininess-back, color-mode
-blend - true, false
-alpha-test - true, false
+
+shade-model - flat, smooth
+
texture-unit - has several child properties:
unit - The number of an OpenGL texture unit
type - This is either an OpenGL texture type or the name of a
environment
mode
color
-program
- vertex-shader
- fragment-shader
uniform
name
type - float, float-vec3, float-vec4, sampler-1d, sampler-2d,
sampler-3d
-polygon-mode
-front - fill, line, point
-back - fill, line, point
+
+vertex-program-two-side - true, false
+
+vertex-program-point-size - true, false
+
+Inheritance
+-----------
One feature not fully illustrated in the sample below is that
effects can inherit from each other. The parent effect is listed in
effect below could be very short, listing just new
parameters and adding nothing to the techniques section;
alternatively, a technique could be altered or customized in a
-child, listing (for example) a different shader program. Terrain
-materials work in this way: for each material type in materials.xml
-an effect is created that inherits from a single default terrain
-effect. The parameters section of the effect is filled in using the
-ambient, diffuse, specular, emissive, shininess, and transparent
-fields of the material. Seperate effects are created for each texture
+child, listing (for example) a different shader program. An example
+showing inheritance Effects/crop.eff, which inherits some if its
+values from Effects/terrain-default.eff.
+
+FlightGear directly uses effects inheritance to assign effects to 3D
+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.
+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
that inherits from one in a model, overriding the parameters that
will be animated.
+Examples
+--------
-<PropertyList>
- <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>
+The Effects directory contains the effects definitions; look there for
+examples. Effects/crop.eff is a good example of a complex effect.
projects / flightgear.git / commitdiff