4 Effects describe the graphical appearance of 3d objects and scenery in
5 FlightGear. The main motivation for effects is to support OpenGL
6 shaders and to provide different implementations for graphics hardware
7 of varying capabilities. Effects are similar to DirectX effects files
8 and Ogre3D material scripts.
10 An effect is a property list. The property list syntax is extended
11 with new "vec3d" and "vec4d" types to support common computer graphics
12 values. Effects are read from files with a ".eff" extension or can be
13 created on-the-fly by FlightGear at runtime. An effect consists of a
14 "parameters" section followed by "technique" descriptions. The
15 "parameters" section is a tree of values that describe, abstractly,
16 the graphical characteristics of objects that use the effect. Techniques
17 refer to these parameters and use them to set OpenGL state or to set
18 parameters for shader programs. The names of properties in the
19 parameter section can be whatever the effects author chooses, although
20 some standard parameters are set by FlightGear itself. On the other
21 hand, the properties in the techniques section are all defined by the
27 A technique can contain a predicate that describes the OpenGL
28 functionality required to support the technique. The first
29 technique with a valid predicate in the list of techniques is used
30 to set up the graphics state of the effect. A technique with no
31 predicate is always assumed to be valid. The predicate is written in a
32 little expression language that supports the following primitives:
34 and, or, equal, less, less-equal
35 glversion - returns the version number of OpenGL
36 extension-supported - returns true if an OpenGL extension is supported
37 property - returns the boolean value of a property
38 float-property - returns the float value of a property, useful inside equal, less or less-equal nodes
39 shader-language - returns the version of GLSL supported, or 0 if there is none.
41 The proper way to test whether to enable a shader-based technique is:
44 <property>/sim/rendering/shader-effects</property>
46 <value type="float">1.0</value>
52 There is also a property set by the user to indicate what is the level
53 of quality desired. This level of quality can be checked in the predicate
57 <property>/sim/rendering/shader-effects</property>
59 <value type="float">2.0</value>
60 <float-property>/sim/rendering/quality-level</float-property>
62 <!-- other predicate conditions -->
66 The range of /sim/rendering/quality-level is [0..5]
67 * 2.0 is the threshold for relief mapping effects,
68 * 4.0 is the threshold for geometry shader usage.
70 A technique can consist of several passes. A pass is basically an Open
71 Scene Graph StateSet. Ultimately all OpenGL and OSG modes and state
72 attributes will be accessable in techniques. State attributes -- that
73 is, technique properties that have children and are not just boolean
74 modes -- have an <active> parameter which enables or disables the
75 attribute. In this way a technique can declare parameters it needs,
76 but not enable the attribute at all if it is not needed; the decision
77 can be based on a parameter in the parameters section of the
78 effect. For example, effects that support transparent and opaque
79 geometry could have as part of a technique:
82 <active><use>blend/active</use></active>
83 <source>src-alpha</source>
84 <destination>one-minus-src-alpha</destination>
87 So if the blend/active parameter is true blending will be activated
88 using the usual blending equation; otherwise blending is disabled.
90 Values of Technique Attributes
91 ------------------------------
93 Values are assigned to technique properties in several ways:
95 * They can appear directly in the techniques section as a
96 constant. For example:
98 <name>ColorsTex</name>
99 <type>sampler-1d</type>
100 <value type="int">2</value>
102 * The name of a property in the parameters section can be
103 referenced using a "use" clause. For example, in the technique
106 <ambient><use>material/ambient</use></ambient>
108 Then, in the parameters section of the effect:
111 <ambient type="vec4d">0.2 0.2 0.2 1.0</ambient>
115 It's worth pointing out that the "material" property in a
116 technique specifies part of OpenGL's state, whereas "material"
117 in the parameters section is just a name, part of a
118 hierarchical namespace.
120 * A property in the parameters section doesn't need to contain
121 a constant value; it can also contain a "use" property. Here
122 the value of the use clause is the name of a node in an
123 external property tree which will be used as the source of a
124 value. If the name begins with '/', the node is in
125 FlightGear's global property tree; otherwise, it is in a local
126 property tree, usually belonging to a model [NOT IMPLEMENTED
129 <chrome-light><use>/rendering/scene/chrome-light</use></chrome-light>
131 The type is determined by what is expected by the technique
132 attribute that will ultimately receive the value. [There is
133 no way to get vector values out of the main property system
134 yet; this will be fixed shortly.] Values that are declared
135 this way are dynamically updated if the property node
141 The following attributes are currently implemented in techiques:
142 alpha-test - children: active, comparison, reference
143 Valid values for comparision:
144 never, less, equal, lequal, greater, notequal, gequal,
147 blend - children: active, source, destination, source-rgb,
148 source-alpha, destination-rgb, destination-alpha
149 Each operand can have the following values:
150 dst-alpha, dst-color, one, one-minus-dst-alpha,
151 one-minus-dst-color, one-minus-src-alpha,
152 one-minus-src-color, src-alpha, src-alpha-saturate,
153 src-color, constant-color, one-minus-constant-color,
154 constant-alpha, one-minus-constant-alpha, zero
156 cull-face - front, back, front-back
158 lighting - true, false
160 material - children: active, ambient, ambient-front, ambient-back, diffuse,
161 diffuse-front, diffuse-back, specular, specular-front,
162 specular-back, emissive, emissive-front, emissive-back, shininess,
163 shininess-front, shininess-back, color-mode
165 polygon-mode - children: front, back
174 geometry-vertices-out: integer, max number of vertices emitted by geometry shader
175 geometry-input-type - points, lines, lines-adjacency, triangles, triangles-adjacency
176 geometry-output-type - points, line-strip, triangle-strip
178 render-bin - (OSG) children: bin-number, bin-name
180 rendering-hint - (OSG) opaque, transparent
182 shade-model - flat, smooth
184 texture-unit - has several child properties:
185 unit - The number of an OpenGL texture unit
186 type - This is either an OpenGL texture type or the name of a
187 builtin texture. Currently supported OpenGL types are 1d, 2d,
188 3d which have the following common parameters:
195 The following built-in types are supported:
196 white - 1 pixel white texture
197 noise - a 3d noise texture
203 type - float, float-vec3, float-vec4, sampler-1d, sampler-2d,
206 vertex-program-two-side - true, false
208 vertex-program-point-size - true, false
213 One feature not fully illustrated in the sample below is that
214 effects can inherit from each other. The parent effect is listed in
215 the "inherits-from" form. The child effect's property tree is
216 overlaid over that of the parent. Nodes that have the same name and
217 property index -- set by the "n=" attribute in the property tag --
218 are recursively merged. Leaf property nodes from the child have
219 precedence. This means that effects that inherit from the example
220 effect below could be very short, listing just new
221 parameters and adding nothing to the techniques section;
222 alternatively, a technique could be altered or customized in a
223 child, listing (for example) a different shader program. An example
224 showing inheritance Effects/crop.eff, which inherits some if its
225 values from Effects/terrain-default.eff.
227 FlightGear directly uses effects inheritance to assign effects to 3D
228 models and terrain. As described below, at runtime small effects are
229 created that contain material and texture values in a "parameters"
230 section. These effects inherit from another effect which references
231 those parameters in its "techniques" section. The derived effect
232 overrides any default values that might be in the base effect's
238 Often shader effects need tangent vectors to work properly. These
239 tangent vectors, usually called tangent and binormal, are computed
240 on the CPU and given to the shader as vertex attributes. These
241 vectors are computed on demand on the geometry using the effect if
242 the 'generate' clause is present in the effect file. Exemple :
245 <tangent type="int">6</tangent>
246 <binormal type="int">7</binormal>
247 <normal type="int">8</normal>
250 Valid subnodes of 'generate' are 'tangent', 'binormal' or 'normal'.
251 The integer value of these subnode is the index of the attribute
252 that will hold the value of the vec3 vector.
254 The generate clause is located under PropertyList in the xml file.
256 In order to be available for the vertex shader, these data should
257 be bound to an attribute in the program clause, like this :
260 <vertex-shader>my_vertex_shader</vertex-shader>
262 <name>my_tangent_attribute</name>
266 <name>my_binormal_attribute</name>
271 attribute names are whatever the shader use. The index is the one
272 declared in the 'generate' clause. So because generate/tangent has
273 value 6 and my_tangent_attribute has index 6, my_tangent_attribute
274 holds the tangent value for the vertex.
276 Default Effects in Terrain Materials and Models
277 -----------------------------------------------
279 Effects for terrain work in this way: for each material type in
280 materials.xml an effect is created that inherits from a single default
281 terrain effect, Effects/terrain-default.eff. The parameters section of
282 the effect is filled in using the ambient, diffuse, specular,
283 emissive, shininess, and transparent fields of the material. The
284 parameters image, filter, wrap-s, and wrap-t are also initialized from
285 the material xml. Seperate effects are created for each texture
286 variant of a material.
288 Model effects are created by walking the OpenSceneGraph scene graph
289 for a model and replacing nodes (osg::Geode) that have state sets with
290 node that uses an effect instead. Again, a small effect is created
291 with parameters extracted from OSG objects; this effect inherits, by
292 default, from Effects/model-default.eff. A larger set of parameters is
293 created for model effects than for terrain because there is more
294 variation possible from the OSG model loaders than from the terrain
295 system. The parameters created are:
297 * material active, ambient, diffuse, specular, emissive,
298 shininess, color mode
299 * blend active, source, destination
303 * texture type, image, filter, wrap-s, wrap-t
305 Specifying Custom Effects
306 -------------------------
308 You can specify the effects that will be used by FlightGear as the
309 base effect when it creates terrain and model effects.
311 In the terrain materials.xml, an "effect" property specifies the name
314 In model .xml files, A richer syntax is supported. [TO BE DETERMINED]
316 Material animations will be implemented by creating a new effect
317 that inherits from one in a model, overriding the parameters that
323 The Effects directory contains the effects definitions; look there for
324 examples. Effects/crop.eff is a good example of a complex effect.
329 To apply an effect to a model or part of a model use:
332 <inherits-from>Effects/light-cone</inherits-from>
333 <object-name>Cone</object-name>
336 where <inherits-from> </inherits-from> contains the path to the effect you want to apply.
337 The effect does not need the file extension.
341 Chrome, although now implemented as an effect, still retains the old method of application:
345 <shader>chrome</shader>
346 <texture>glass_shader.png</texture>
347 <object-name>windscreen</object-name>
350 in order to maintain backward compatibility.