262 lines
10 KiB
Text
262 lines
10 KiB
Text
Effects
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-------
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Effects describe the graphical appearance of 3d objects and scenery in
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FlightGear. The main motivation for effects is to support OpenGL
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shaders and to provide different implementations for graphics hardware
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of varying capabilities. Effects are similar to DirectX effects files
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and Ogre3D material scripts.
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An effect is a property list. The property list syntax is extended
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with new "vec3d" and "vec4d" types to support common computer graphics
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values. Effects are read from files with a ".eff" extension or can be
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created on-the-fly by FlightGear at runtime. An effect consists of a
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"parameters" section followed by "technique" descriptions. The
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"parameters" section is a tree of values that describe, abstractly,
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the graphical characteristics of objects that use the effect. Techniques
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refer to these parameters and use them to set OpenGL state or to set
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parameters for shader programs. The names of properties in the
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parameter section can be whatever the effects author chooses, although
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some standard parameters are set by FlightGear itself. On the other
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hand, the properties in the techniques section are all defined by the
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FlightGear.
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Techniques
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----------
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A technique can contain a predicate that describes the OpenGL
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functionality required to support the technique. The first
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technique with a valid predicate in the list of techniques is used
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to set up the graphics state of the effect. A technique with no
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predicate is always assumed to be valid. The predicate is written in a
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little expression language that supports the following primitives:
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and, or, equal, less, less-equal
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glversion - returns the version number of OpenGL
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extension-supported - returns true if an OpenGL extension is supported
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property - returns the boolean value of a property
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shader-language - returns the version of GLSL supported, or 0 if there is none.
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The proper way to test whether to enable a shader-based technique is:
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<predicate>
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<and>
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<property>/sim/rendering/shader-effects</property>
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<less-equal>
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<value type="float">1.0</value>
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<shader-language/>
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</less-equal>
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</and>
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</predicate>
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A technique can consist of several passes. A pass is basically an Open
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Scene Graph StateSet. Ultimately all OpenGL and OSG modes and state
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attributes will be accessable in techniques. State attributes -- that
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is, technique properties that have children and are not just boolean
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modes -- have an <active> parameter which enables or disables the
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attribute. In this way a technique can declare parameters it needs,
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but not enable the attribute at all if it is not needed; the decision
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can be based on a parameter in the parameters section of the
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effect. For example, effects that support transparent and opaque
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geometry could have as part of a technique:
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<blend>
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<active><use>blend/active</use></active>
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<source>src-alpha</source>
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<destination>one-minus-src-alpha</destination>
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</blend>
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So if the blend/active parameter is true blending will be activated
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using the usual blending equation; otherwise blending is disabled.
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Values of Technique Attributes
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------------------------------
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Values are assigned to technique properties in several ways:
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* They can appear directly in the techniques section as a
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constant. For example:
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<uniform>
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<name>ColorsTex</name>
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<type>sampler-1d</type>
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<value type="int">2</value>
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</uniform>
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* The name of a property in the parameters section can be
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referenced using a "use" clause. For example, in the technique
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section:
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<material>
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<ambient><use>material/ambient</use></ambient>
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</material>
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Then, in the parameters section of the effect:
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<parameters>
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<material>
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<ambient type="vec4d">
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0.2 .2 0.2 1.0
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</ambient>
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</material>
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</parameters>
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It's worth pointing out that the "material" property in a
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technique specifies part of OpenGL's state, whereas "material"
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in the parameters section is just a name, part of a
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hierarchical namespace.
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* A property in the parameters section doesn't need to contain
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a constant value; it can also contain a "use" property. Here
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the value of the use clause is the name of a node in an
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external property tree which will be used as the source of a
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value. If the name begins with '/', the node is in
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FlightGear's global property tree; otherwise, it is in a local
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property tree, usually belonging to a model [NOT IMPLEMENTED
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YET]. For example:
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<parameters>
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<chrome-light><use>/rendering/scene/chrome-light</use></chrome-light>
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</parameters>
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The type is determined by what is expected by the technique
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attribute that will ultimately receive the value. [There is
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no way to get vector values out of the main property system
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yet; this will be fixed shortly.] Values that are declared
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this way are dynamically updated if the property node
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changes.
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OpenGL Attributes
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-----------------
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The following attributes are currently implemented in techiques:
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alpha-test - children: active, comparison, reference
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Valid values for comparision:
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never, less, equal, lequal, greater, notequal, gequal,
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always
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blend - children: active, source, destination, source-rgb,
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source-alpha, destination-rgb, destination-alpha
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Each operand can have the following values:
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dst-alpha, dst-color, one, one-minus-dst-alpha,
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one-minus-dst-color, one-minus-src-alpha,
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one-minus-src-color, src-alpha, src-alpha-saturate,
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src-color, constant-color, one-minus-constant-color,
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constant-alpha, one-minus-constant-alpha, zero
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cull-face - front, back, front-back
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lighting - true, false
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material - children: active, ambient, ambient-front, ambient-back, diffuse,
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diffuse-front, diffuse-back, specular, specular-front,
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specular-back, emissive, emissive-front, emissive-back, shininess,
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shininess-front, shininess-back, color-mode
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polygon-mode - children: front, back
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Valid values:
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fill, line, point
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program
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vertex-shader
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fragment-shader
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render-bin - (OSG) children: bin-number, bin-name
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rendering-hint - (OSG) opaque, transparent
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shade-model - flat, smooth
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texture-unit - has several child properties:
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unit - The number of an OpenGL texture unit
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type - This is either an OpenGL texture type or the name of a
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builtin texture. Currently supported OpenGL types are 1d, 2d,
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3d which have the following common parameters:
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image (file name)
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filter
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mag-filter
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wrap-s
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wrap-t
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wrap-r
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The following builtin types are supported:
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white - 1 pixel white texture
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noise - a 3d noise texture
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environment
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mode
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color
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uniform
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name
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type - float, float-vec3, float-vec4, sampler-1d, sampler-2d,
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sampler-3d
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vertex-program-two-side - true, false
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vertex-program-point-size - true, false
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Inheritance
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-----------
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One feature not fully illustrated in the sample below is that
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effects can inherit from each other. The parent effect is listed in
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the "inherits-from" form. The child effect's property tree is
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overlaid over that of the parent. Nodes that have the same name and
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property index -- set by the "n=" attribute in the property tag --
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are recursively merged. Leaf property nodes from the child have
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precedence. This means that effects that inherit from the example
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effect below could be very short, listing just new
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parameters and adding nothing to the techniques section;
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alternatively, a technique could be altered or customized in a
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child, listing (for example) a different shader program. An example
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showing inheritance Effects/crop.eff, which inherits some if its
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values from Effects/terrain-default.eff.
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FlightGear directly uses effects inheritance to assign effects to 3D
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models and terrain. As described below, at runtime small effects are
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created that contain material and texture values in a "parameters"
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section. These effects inherit from another effect which references
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those parameters in its "techniques" section. The derived effect
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overrides any default values that might be in the base effect's
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parameters section.
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Default Effects in Terrain Materials and Models
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---------------------------------------
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Effects for terrain work in this way: for each material type in
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materials.xml an effect is created that inherits from a single default
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terrain effect, Effects/terrain-default.eff. The parameters section of
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the effect is filled in using the ambient, diffuse, specular,
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emissive, shininess, and transparent fields of the material. The
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parameters image, filter, wrap-s, and wrap-t are also initialized from
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the material xml. Seperate effects are created for each texture
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variant of a material.
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Model effects are created by walking the OpenSceneGraph scene graph
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for a model and replacing nodes (osg::Geode) that have state sets with
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node that uses an effect instead. Again, a small effect is created
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with parameters extracted from OSG objects; this effect inherits, by
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default, from Effects/model-default.eff. A larger set of parameters is
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created for model effects than for terrain because there is more
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variation possible from the OSG model loaders than from the terrain
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system. The parameters created are:
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* material active, ambient, diffuse, specular, emissive,
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shininess, color mode
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* blend active, source, destination
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* shade-model
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* cull-face
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* rendering-hint
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* texture type, image, filter, wrap-s, wrap-t
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Specifying Custom Effects
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-------------------------
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You can specify the effects that will be used by FlightGear as the
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base effect when it creates terrain and model effects.
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In the terrain materials.xml, an "effect" property specifies the name
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of the model to use.
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In model .xml files, A richer syntax is supported. [TO BE DETERMINED]
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Material animations will be implemented by creating a new effect
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that inherits from one in a model, overriding the parameters that
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will be animated.
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Examples
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--------
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The Effects directory contains the effects definitions; look there for
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examples. Effects/crop.eff is a good example of a complex effect.
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