Tim Moore
> Hello,
> A couple of weeks ago I was asked for a sample of an effects file that
> uses my
> proposed changes to the property system; here it is. The syntax differs
> from
> current property system syntax in two ways: it uses vector types for some
> properties, and some properties can have a variance="dynamic" attribute
> that
> indicates that a parameter can be animated and that graphics state that
> depends on it may need to be updated.
>
> Many people seem unconvinced by my arguments for new property types.
> That's fine;
> I'm still convinced that they are desirable :) One implementation reason
> that I
> would like to treat values like colors as vectors is that the underlying
> OSG type
> is a vector, so if the components are individual properties that can be
> written
> individually, the implementation choices for updating the OSG state are
> unattractive:
>
> 1) Write the full vector every time a component is changed. This means
> potentially 4
> times the memory traffic to change a color, and leaves the values stored
> in OSG in
> an inconsistent state for a time.
>
> 2) Update all effects with dynamic components every frame. We have a lot
> of material
> animations, and their management is currently eats a lot of time in the
> update phase,
> so I'd like to avoid this. On the other hand, we may be updating most
> animations
> every frame anyway.
>
> 3) Record which effects have been changed, then update their OSG state.
> Complicated
> bookkeeping...
>
> If people really don't like the effects syntax, I might be willing to hold
> my nose
> and use the existing property implementation. I'm also not committed to
> having the
> effects properties be of class SGPropertyNode; they might be a subtype.
>
> <?xml version="1.0" encoding="utf-8">
> <!--
> An effect consists of parameters and techniques. The "parameters"
> section of an effect is a tree of values that describe, abstractly,
> the graphical appearence 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. Parameters can be declared to
> have a dynamic variance, which means that if their value is changed
> the corresponding value in the technique will be changed too.
>
> 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.
>
> A technique can consist of several passes, which are run in
> sequence.
>
> 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. This means that effects that
> inherit from the example "default 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. Material animations will be implemented by creating a new
> effect that inherits from one in a model, overriding the parameters
> that will be animated.
> -->
> <PropertyList>
> <effect>
> <name>default-effect</name>
> <!-- <inherits-from>another-effect</inherits-from> -->
> <parameters>
> <material>
> <ambient type="vec4d">
> 0.0 0.0 0.0 1.0
> </ambient>
> <diffuse type="vec4d">
> .5 .5 .5 1.0
> </diffuse>
> <specular type="vec4d">
> 0.3 0.3 0.3 1.0
> </specular>
> <emissive type="vec4d" variance="dynamic">
> 0.0 0.0 0.0 1.0
> </emissive>
> <shininess>1.2</shininess>
> </material>
> <texture0>
> <texture2d>
> <image>city.png</image>
> <filter>linear-mipmap-linear</filter>
> <!-- also repeat -->
> <wrap-s>clamp</wrap-s>
> <wrap-t>clamp-to-edge</wrap-t>
> <!--
> <wrap-r>clamp-to-border</wrap-r>
> -->
> <!-- float, signed-integer, integer -->
> <internal-format>normalized</internal-format>
> </texture2d>
> </texture0>
> <texture1>
> <texture2d>
> <image>detail.png</image>
> <filter>linear-mipmap-linear</filter>
> <!-- also repeat -->
> <wrap-s>clamp</wrap-s>
> <wrap-t>clamp-to-edge</wrap-t>
> <!--
> <wrap-r>clamp-to-border</wrap-r>
> -->
> <!-- float, signed-integer, integer -->
> <internal-format>normalized</internal-format>
> </texture2d>
> </texture1>
> <bump-height type="double">.05</bump-height>
> <pattern-rotation type="vec4d">0 0 1 1.5708</pattern-rotation>
> </parameters>
> <technique>
> <predicate>
> <or>
> <less-equal>
> <value>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>
> </predicate>
> <pass>
> <lighting>true</lighting>
> <material>
> <ambient><use>material/ambient</use></ambient>
> <diffuse><use>material/diffuse</use></diffuse>
> <specular><use>material/specular</use></specular>
> <shininess><use>material/shininess</use></shininess>
> </material>
> <texture-unit>
> <texture2d><use>texture0/texture2d</use></texture2d>
> </texture-unit>
> <texture-unit>
> <texture2d><use>texture1/texture2d</use></texture2d>
> </texture-unit>
> <shader-program>
> <uniform>
> <name>bumpHeight</name>
> <type>float</type>
> <use>bump-height</use>
> </uniform>
> <uniform>
> <name>patternRotation</name>
> <type>vec4d</type>
> <use>pattern-rotation</use>
> </uniform>
> <vertex-shader>
> "Shaders/util.vert"
> </vertex-shader>
> <vertex-shader>
> "Shaders/foo.vert"
> </vertex-shader>
> <fragment-shader>
> "Shaders/foo.frag"
> </fragment-shader>
> </shader-program>
> </pass>
> </technique>
> <technique>
> <pass>
> <lighting>true</lighting>
> <material>
> <ambient><use>material/ambient</use></ambient>
> <diffuse><use>material/diffuse</use></diffuse>
> <specular><use>material/specular</use></specular>
> </material>
> <texture-unit>
> <texture2d><use>texture0/texture2d</use></texture2d>
> </texture-unit>
> </pass>
> </technique>
> </effect>
> </PropertyList>
>
Wow! Complicated or what? I can live with the colour stuff - just. But I
think there's stuff there which will for ever catch me out:
<less-equal>? We use <less-than-equals> everywhere else. Unless this has the
sense "some animals are more equal than others"
<texture0>, <texture1>. I would expect <texture>, <texture>. Unless these
tags have totally different meaning, in which case they should be named
differently.
<ambient><use>material/ambient</use></ambient> ? I would expect, perhaps
wrongly:
<material>
<use>
<ambient>material/ambient</ambient>
<diffuse><use>material/diffuse></diffuse>
<specular><use>material/specular</specular>
<use>
</material>
I think you would have to be a very dedicated developer to wade into this.
Nevertheless, the access to shader programs looks very worthwhile. In fact I
would accept (almost) anything to get that facility on board.
Vivian
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