is the"dummy" means a ..human body model?

No, it's not a crash test dummy or something... It's called "dummy", because it's not a real (i.e. complete) Object3D. It has no polygons, no vertices and no properties (well, it has them but there are ignored). It doesn't have to be added to an instance of World either. All it really is, is a kind of container for a transformation matrix. This is due to the fact that jPCT is not a pure scene-graph based API but borrows some elements from the scene-graph concept.
Anyway, what is it good for? Ok, imagine a planet spinning around the sun and a moon spinning around the planet. The planet itself is spinning around its vertical axis too. So the basic transformation the planet does is the rotation around its own center while the moon should rotate around the planet. Two problems arise here: The moon should follow the planet if it's moving (i.e. spinning around the sun, which i'll cover later) but it can't be a child of it, because it must not inherit the planet's rotation around its center. The other problem is, that the planet is spinning around itself, but not around the sun. A dummy object is a convienient way to solve this. You create a dummy object and place it into the center of the planet. This dummy object has its rotation pivot in the center of the sun. This way, the dummy rotates around the sun, which is what the planet should do to. So you attach the planet as a child of the dummy. The same goes for the moon: Set the rotation pivot to the center of the planet and attach it as a child to the dummy. This way, both objects will follow the dummy around the sun while performing their own, independent rotations too.
Have a look at the downloadable jPCT-demo. There is a room with a planet and a moon rotating around a pillar that illustrate exactly (almost...) this effect.
Somewhat clearer now?

i got it.so if i have the need to rotate and transform my arm fixed to my body at the same time,i need to asemble object by hand,and use addchild/add parent to make they have hierachie relation.then i use transform to the main body, other arm/leg/head....will  transform as the same.is it correct?^^||.sorry it seem i am somewhat suck in 3d progaming/_\

Yes. A child object inherits the transformations (rotation/translation) of its parent. There can be more than one parent, but i would try to organize my scene in a way that i get away with max. one parent per object. I don't even know anymore, how multiple parents are implemented exactly...
Example: You make the arm a child of the body and the hand a child of the arm. This way, hand and arm are transformed if the the body is and the hand inherits the transformations of the arm in addition. Sometimes, it's usefull to use "virtual" objects to build up your hierachie. You can achieve this by using "dummy"-object (see: Object3D.createDummyObj()).
Hope this helps.

hello everyone,i am a fresh guy programming 3d.^^
this engine is very cool:lol: ..can't imagine that before,making 3d game is not hard work.
i have a question that if i load objects from a 3ds file including head,leg,body,arm....which each is a object
how do  i assemble and fix the arm to the "correct position" ?can i make it is jpct?

I'm not quite sure if i'm understanding your problem correctly. Do the objects reside in different 3ds-files or are they all in one? The the later case, the positions of the parts should be fine, because jPCT will import them as they are placed in the orginal model. In the former case, you would need to do the positioning "by hand" by finding the right position for every part.
If this wasn't the question, then maybe you are referring to a possibility to group seperate objects into a single entity. What you can't do (not yet) in jPCT is merging objects into a new one. What you can do, is building up a hierarchie between them by using the addChild() and/or addParent()-methods from Object3D. With them, you may add all body-parts to the chest (or to whatever is appropiate in your case...maybe a dummy object...whatever...) and then the child-objects will inherit all transformations of the parent object.
Does this answer your question somehow?

You are basically correct. The only problem is, that using portals won't be possible that way, because the geometry won't be prepared accordingly. If you intend to use portals, you would have to load the level "by hand". None of the loaders of jPCT support them ATM. Anyway, Octrees are supported on such a level and depending on the level, they could very well be sufficient (or even preferable).
Just give it a try. Oh, and let me know if you find a good converter for this task...

Wow, downloaded and going to test it tomorrow...    I am sorry for this hard work I am causing, I hope many will take advantage of these features!

Did it work? Anyway, it's in the new 0.89 release...

FYI: A new version of LWJGL (the lib jPCT uses for the OpenGL binding) is out, but jPCT 0.88 won't work with this version. 0.89 will.

If you are interested in LWJGL, grab it here: http://java-game-lib.sourceforge.net/

Before the days of the OpenGL renderer, this would have been rather simple. You can access the Image-instance that holds the pixels generated by the software renderer and blit it into whatever and whereever you want. But though it's possible, it's not very convenient. Therefor, i once added an option to jPCT (that isn't available via the API at the moment) that "shifts" the projected 2D coordinates in any direction, because this is, what's actually needed to avoid the problem: If your sidebar is for example 100 pixels, you would like to start the rendering not at x=0 but at x=100. This is no problem with the software renderer, because i have full access to every pixel and can do whatever i want to. However, i didn't find a working way to tell OpenGL not to start rendering at (0,0) but somewhere else. That's why the option to do this is not publicly available.
Manipulating the matrices doesn't help btw, because what you want is basically a 2D operation...just tell the stupid polygon to start at (x+a,y+b) instead of (x,y) on screen (the position in 3D wouldn't change). I just don't know how to achieve this in OpenGL ATM...maybe i'll have a closer look again one day (glViewport comes to mind...but i tried that once and it failed somehow...well, maybe i'll give it a shot again...). For now, i suggest to take it as feature. Maybe you want to make the sidebar removable... :wink:

Edit: About the filtering: Maybe it hurts performance on PowerPC's and their VMs more than it does on Intel...can't wait to see what happens when LWJGL comes out for Mac (byteorder anyone... :roll: ?).

Edit2: I forgot to mention this in the docs of 0.88: The coords the projectCenter...stuff returns are backbuffer coords, i.e. if you are using 1.5 or 2 times oversampling, you have to divide them by 1.5 (or 2) to get the actual coords of the frontbuffer (for blitting and stuff). 0.89's docs will mention this.