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Effect framework (view source)

Revision as of 05:40, 1 September 2019

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The effect framework as per version 2019.1
The effect framework as per version 2019.1
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.


=Default Effects in Terrain Materials and Models=
=Default Effects in Terrain Materials and Models=
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===predicate===
===predicate===
Condition for this technique to be used. If this fails it will try the next technique in the effect.
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
float-property - returns the float value of a property, useful inside equal, less or less-equal nodes
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:
<syntaxhighlight lang="xml">
<predicate>
  <and>
<property>/sim/rendering/shader-effects</property>
<less-equal>
  <value type="float">1.0</value>
  <shader-language/>
</less-equal>
  </and>
</predicate>
</syntaxhighlight>
 
There is also a property set by the user to indicate what is the level
of quality desired. This level of quality can be checked in the predicate
like this :
<syntaxhighlight lang="xml">
    <predicate>
      <and>
        <property>/sim/rendering/shader-effects</property>
<less-equal>
  <value type="float">2.0</value>
  <float-property>/sim/rendering/quality-level</float-property>
</less-equal>
<!-- other predicate conditions -->
      </and>
    </predicate>
</syntaxhighlight>
   
The range of /sim/rendering/quality-level is [0..5]
* 2.0 is the threshold for relief mapping effects,
* 4.0 is the threshold for geometry shader usage.


Example:
Example:
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===pass===
===pass===
What is passed to the shaders/OSG.
A technique can consist of several passes. A pass is basically an Open
(Not sure, but think multiple passes are possible.)
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:


Note that entries can use the <use> tags to enable parameters to specify the values.
<syntaxhighlight lang="xml">
  <blend>
<active><use>blend/active</use></active>
<source>src-alpha</source>
<destination>one-minus-src-alpha</destination>
  </blend>
</syntaxhighlight>
 
So if the blend/active parameter is true blending will be activated
using the usual blending equation; otherwise blending is disabled.
 
Values are assigned to technique properties in several ways:
 
* They can appear directly in the techniques section as a
constant. For example:
<syntaxhighlight lang="xml">
<uniform>
<name>ColorsTex</name>
<type>sampler-1d</type>
<value type="int">2</value>
</uniform>
</syntaxhighlight>
* The name of a property in the parameters section can be
referenced using a "use" clause. For example, in the technique
section:
<syntaxhighlight lang="xml">
<material>
<ambient><use>material/ambient</use></ambient>
</material>
</syntaxhighlight>
Then, in the parameters section of the effect:
<syntaxhighlight lang="xml">
<parameters>
<material>
<ambient type="vec4d">0.2 0.2 0.2 1.0</ambient>
</material>
</parameters>
</syntaxhighlight>
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:
<syntaxhighlight lang="xml">
<parameters>
<chrome-light><use>/rendering/scene/chrome-light</use></chrome-light>
</parameters>
</syntaxhighlight>
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.


====lighting====
====lighting====
Necolatis
574

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