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
of varying capabilities. Effects are similar to DirectX effects files
and Ogre3D material scripts.

An effect is a property list. The property list syntax is extended
with new "vec3d" and "vec4d" types to support common computer graphics
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
"parameters" section followed by "technique" descriptions.  The
"parameters" section is a tree of values that describe, abstractly,
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. 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. 

Techniques
----------

A technique can contain a predicate that describes the OpenGL
functionality required to support the technique. The first
technique with a valid predicate in the list of techniques is used
to set up the graphics state of the effect. A technique with no
predicate is always assumed to be valid. The predicate is written in a
little expression language that supports the following primitives:

and, or, equal, less, less-equal
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 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

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
        builtin texture. Currently supported OpenGL types are 1d, 2d,
        3d which have the following common parameters:
		image (file name)
                filter
                mag-filter
                wrap-s
                wrap-t
                wrap-r
        The following builtin types are supported:
                white - 1 pixel white texture
                noise - a 3d noise texture
        environment
                mode
                color
uniform
        name
        type - float, float-vec3, float-vec4, sampler-1d, sampler-2d,
        sampler-3d

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
the "inherits-from" form. The child effect's property tree is
overlaid over that of the parent. Nodes that have the same name and
property index -- set by the "n=" attribute in the property tag --
are recursively merged. Leaf property nodes from the child have
precedence.  This means that effects that inherit from the example
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. 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
--------

The Effects directory contains the effects definitions; look there for
examples. Effects/crop.eff is a good example of a complex effect.