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.
- 1 Default Effects in Terrain Materials and Models
- 2 Specifying Custom Effects
- 3 Examples
- 4 Application
- 5 The xml tags of an effect
- 5.1 name
- 5.2 inherits-from
- 5.3 Parameters
- 5.4 Generate
- 5.5 Technique
- 5.5.1 predicate
- 5.5.2 pass
- 6 Uniforms passed to shaders outside the xml effect framework
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
- 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.
Material animations will be implemented by creating a new effect that inherits from one in a model, overriding the parameters that will be animated.
The $FGDATA/Effects directory contains the effects definitions; look there for examples. Effects/crop.eff is a good example of a complex effect.
To apply an effect to a model or part of a model use:
<effect> <inherits-from>Effects/light-cone</inherits-from> <object-name>Cone</object-name> </effect>
where <inherits-from> </inherits-from> contains the path to the effect you want to apply. The effect does not need the file extension.
Note that effects cannot be applied to groups, the objects must be specified explicit.
Parameters in model file
Parameters can be put into the model files effect application as well. But only bool, int, float, string can be used there.
Chrome old usage
Chrome, although now implemented as an effect, still retains the old method of application:
<animation> <type>shader</type> <shader>chrome</shader> <texture>glass_shader.png</texture> <object-name>windscreen</object-name> </animation>
in order to maintain backward compatibility.
The name of the effect
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.
Custom parameters that controls the effect.
Note that parameters can use the <use> tags to enable properties to specify the values.
Often shader effects need tangent vectors to work properly. These tangent vectors, usually called tangent and binormal, are computed on the CPU and given to the shader as vertex attributes. These vectors are computed on demand on the geometry using the effect if the 'generate' clause is present in the effect file. Exemple :
<generate> <tangent type="int">6</tangent> <binormal type="int">7</binormal> <normal type="int">8</normal> </generate>
Valid subnodes of 'generate' are 'tangent', 'binormal' or 'normal'. The integer value of these subnode is the index of the attribute that will hold the value of the vec3 vector.
The generate clause is located under PropertyList in the xml file.
In order to be available for the vertex shader, these data should be bound to an attribute in the program clause, like this :
<program> <vertex-shader>my_vertex_shader</vertex-shader> <attribute> <name>my_tangent_attribute</name> <index>6</index> </attribute> <attribute> <name>my_binormal_attribute</name> <index>7</index> </attribute> </program>
attribute names are whatever the shader use. The index is the one declared in the 'generate' clause. So because generate/tangent has value 6 and my_tangent_attribute has index 6, my_tangent_attribute holds the tangent value for the vertex.
A certain way of rendering this effect. Different pipelines typically have their own 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 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:
<predicate> <and> <property>/sim/rendering/shader-effects</property> <less-equal> <value type="float">1.0</value> <shader-language/> </less-equal> </and> </predicate>
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 :
<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>
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.
<predicate> <and> <property>/sim/rendering/shaders/quality-level</property> <property>/sim/rendering/shaders/model</property> <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> </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 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 0.2 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.
true or false
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
Children: active, source, destination, source-rgb, source-alpha, destination-rgb, destination-alpha
Children 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
<texture-unit> <active>true</active> <source>one-minus-dst-alpha</source> <destination>src-alpha-saturate</destination> </texture-unit>
flat or smooth
front, back, front-back, off
<texture-unit> <unit>3</unit> <image>Textures/Terrain/void.png</image> <type>2d</type> <filter>linear-mipmap-linear</filter> <mag-filter>linear-mipmap-linear</mag-filter> <wrap-s>repeat</wrap-s> <wrap-t>repeat</wrap-t> <wrap-r>repeat</wrap-r> <internal-format>normalized</internal-format> <mipmap-control> <function-r>average</function-r> <function-g>min</function-g> <function-b>sum</function-b> <function-a>product</function-a> </mitmap-control> <environment> <mode>decal</mode> <color>0.0 0.1 0.6 1.0</color> </environment> <point-sprite>true</point-sprite> <texenv-combine>operand0-rgb</texenv-combine> <texgen> <mode>S</mode> <planes>0.075, 0.0, 0.0, 0.5</planes> </texgen> </texture-unit>
true or false
children: front, back
Valid values: fill, line, point
Data accessible by shaders.
name: the name
type: bool, int, float, float-vec3, float-vec4, sampler-1d, sampler-2d, sampler-3d, sampler-1d-shadow, sampler-2d-shadow, sampler-cube
active: true, false
comparison: never, less, equal, lequal, greater, notequal, gequal, always
reference: 0 to 1
Sent to OSG.
bin-number: This is an integer defining the order stuff will be rendered in, it can be negative also.
bin-name: RenderBin, DepthSortedBin
Sent to OSG.
default, opaque, transparent
This basically just sets Renderbin:
opaque = bin 10, depthsortedbin
transparent = bin 0, renderbin
default = inherit renderbin details from parent node
- geometry-vertices-out: integer, max number of vertices emitted by geometry shader
- geometry-input-type - points, lines, lines-adjacency, triangles, triangles-adjacency
- geometry-output-type - points, line-strip, triangle-strip
<program> <vertex-shader n="0">Shaders/lcd.vert</vertex-shader> <fragment-shader n="0">Shaders/lcd.frag</fragment-shader> <fragment-shader n="1">Shaders/noise.frag</fragment-shader> <fragment-shader n="2">Shaders/filters-ALS.frag</fragment-shader> </program>
Uniforms passed to shaders outside the xml effect framework
|fg_ViewMatrix||mat4||In fullscreen pass only, view matrix used to transform the screen position to view direction|
|fg_ViewMatrixInverse||mat4||In fullscreen pass only, view matrix inverse used to transform the screen position to view direction|
|fg_ProjectionMatrixInverse||mat4||In fullscreen pass only, projection matrix inverse used to transform the screen position to view direction|
|fg_CameraPositionCart||vec3||Position of the camera in world space, expressed in cartesian coordinates|
|fg_CameraPositionGeod||vec3||Position of the camera in world space, expressed in geodesic coordinates (longitude in radians, latitude in radians, elevation in meters)|
|fg_DepthInColor||bool||Tells if the depth is stored in a depth texture or a color texture|
|fg_Planes||vec3||Used to convert the value of the depth buffer to a depth that can be used to compute the eye space position of the fragment|
|fg_BufferSize||vec2||Dimensions of the buffer, used to convert gl_FragCoord into the range [0..1][0..1]|
|osg_ViewMatrix||mat4||Defined by OSG, used only when working on actual geometry|
|osg_ViewMatrixInverse||mat4||Defined by OSG, used only when working on actual geometry|
|osg_SimulationTime||float||Defined by OSG|