> One more observation. Yesterday I was doing tweaks to the spin related > functions in my model and during spin tests I noticed that I get the same > affect when I am in a spin only the clouds are rotating about a vertical > axis > rather than a horizontal axis. Again it only appears to happen in > vertical > maneuvers. In a spin this is very disconcerting as the clouds spin in > the > same direction as the aircraft.
Yes - same reason - rapid change of the orientation of the view axis. The problem is generic: a real cloud is a 3-d distribution of droplets, i.e. for rendering purposes a function f(x,y,z) of opaqueness where projections like \int f(x,y,z) dz = c(x,y) determine the appearance of the cloud as seen from the z-direction. I don't quite know if one could render that in real time if enough interpolation is used (I'm no 3d rendering expert), but even coming up with a physically correct f(x,y,z) which projects in all directions into something like a cloud is a pretty hard task. So we have to fake it by using texture sheets projected onto something, and then there is always some situation in which the nature of the fake is apparent. I can given you clouds which look more or less realistic in level flight and normal airplane operation (by using rather high-resolution cloud images as done now) - but they rotate in an odd way when you do aerobatics. I can give you static cloud sheets which look great from far below or high above - but they look unrealistically flat when level with them (doesn't work for convective clouds though - I did some of the Cirrus sheets that way). I can give you clouds which are stable against the rotations you observe in aerobatics - but they behave in an odd way when you are close to them. I can give you clouds which are stable against aerobatics and have the right behaviour when you are close to them and look straight - but when you look out of the side window or take an external view they look odd. The problem isn't going to go away - you simply can't make a 2d sheet appear like a 3d object without the 2d nature being revealed somewhere, in some situation - it's like asking from a stage magician that you should be allowed to sit and observe behind the stage as well - if you want something which looks always and everywhere like a genuine 3d object, you need a genuine 3d object. All you can do is decide where you want the problem to be. And I guess that having clouds which are stable against an airplane flying through them are more desirable as clouds which look good from a spin... But you could simply edit the cloud shaders to insert the relative position vector to the object as the reference vector for the transform rather than the view axis just to see if you like that better - it's one line to edit I think. Stacking many 2d illusions into a 3d volume is a step towards a genuine 3d object, and Tim appears to think it can be done "> I've been looking at the clouds code again recently, which is oddly slow > on > my "monster" machine (Phenom II x6, GTX 460) . It has the same problem > that > the trees code did before a big makeover: it uses instancing techniques > on > geometry (flat quads) that is to far simple to be treated as an > instance. It > would be much better to take the same approach we do in trees and treat a > cloud as a list of quad or triangle polygons. The messy part is the > distance > sorting that is not optional for the cloud sprites." (I have no real idea as to what all this means... since I am not a 3d rendering expert). But then you run into a different problem - right now, the cloud code doesn't need to know any convective physics, because the cloud texture is a photograph and has all the information where a cloud looks how. If you break that and build the whole shape from 200 different elements, you need to input the whole information about where the texture should look how yourself, i.e. to get the same visual quality, the code to place the texture elements must have some rough idea about convection. There's no free lunch... So, I guess rather than more information where the current cloud schemes fail (which I at least know, I've spent 3 months with dreaming up and testing schemes adn trying to come up with workarounds), an opinion where you would like them to fail instead or how much performance you would be willing to throw at improved clouds would be more helpful to make progress :-) Or if you can come up with an even better scheme/transformation, I'd be very excited! Cheers, * Thorsten ------------------------------------------------------------------------------ Increase Visibility of Your 3D Game App & Earn a Chance To Win $500! Tap into the largest installed PC base & get more eyes on your game by optimizing for Intel(R) Graphics Technology. Get started today with the Intel(R) Software Partner Program. Five $500 cash prizes are up for grabs. http://p.sf.net/sfu/intelisp-dev2dev _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
