jll wrote:...May be the best way will be to define mesh with numerous very transparent "layers" ?
Perhaps we would gain on various aspects :
- more gas effect
- to make the structure less visible from side view
- the colors would be obtained by addition of the various layers
Yep, that's it; your third point will not be easy to control but I think a serious try could be done

jll wrote:...On the other side :
- add-ons are heavier
- more complex to define (need to split texture into different component)
I don't think the size is a big problem if the work is really nice and yes, the work on the texture need some special attentions in that case...
jll wrote:...I tried this in M42 and M27 (with only 2 layers but without very transparent texture) : it's not so pretty Sad ,
and with your transparent texture (from a previous post) with 3 layers (a smaller mesh inside and bigger outside the main mesh ... again I wasn't satisfied Sad...
The main problem is those nebula are a volume, so trying to create a real 3D representation will not be possible using only 2D picture. We generally only have 1 point of view, so no one can say how will look X or Y nebula from 90?° left, right, up or down... I think there is a big part of interpretation so the final product will never be really accurate; now how you will work will depend on what exactly you want to do. If you want an accurate view then IMO it won't be possible with the actual possibilities; if you want to create some nice personal interpretation based on the reals documents, then trying with the above points can result in some nice addons. In that case, I think you should try not to use the same texture on the meshes to avoid some irrealistic "double views" of this texture, and use your imagination to create the rest of the meshes...
I think the result is good when you go through the nebula and have a nice 3D effect...
If I have some time (quite hard for now), I will do more test to give you (if I can) some ideas for this hard task
Bye.