first commit
This commit is contained in:
Your Name
360
docs-mini/README.effects
Normal file
360
docs-mini/README.effects
Normal file
@@ -0,0 +1,360 @@
|
||||
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
|
||||
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.
|
||||
|
||||
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 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.
|
||||
|
||||
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
|
||||
geometry-shader
|
||||
fragment-shader
|
||||
attribute
|
||||
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
|
||||
|
||||
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
|
||||
mipmap-control - controls how the mipmap levels are computed.
|
||||
Each color channel can be computed with different functions
|
||||
among average, sum, product, min and max. For example :
|
||||
<function-r>average</function-r>
|
||||
<function-a>min</function-a>
|
||||
function-r - function for red
|
||||
function-g - function for green
|
||||
function-b - function for blue
|
||||
function-a - function for alpha
|
||||
The following built-in 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.
|
||||
|
||||
Generate
|
||||
--------
|
||||
|
||||
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.
|
||||
|
||||
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.
|
||||
|
||||
Application
|
||||
-----------
|
||||
|
||||
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:
|
||||
|
||||
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.
|
||||
|
||||
Reference in New Issue
Block a user