Jeff nous apporta ses lumieres ainsi en ce 2004-11-02 21:46... :

>After a hiatus of a couple of years, I am getting back into PovRay (3.6.1
>for Windows).
>
>I've read the FAQ both in the Help File and in the Online Documentation.
>There IS a clue, but not a hard answer ( " ... problems with very large
>numbers when scaling ... " or something). I've also searched the newsgroup.
>
>My main computer was destroyed in an accident, until I can afford a new one
>I am using (cough...) this old Pentium 166 with 128 Mb RAM and just a
>SuperVga chip for video. (I bought a cheap external serial port modem for
>this thing after I lost my GOOD computer - so at least I can connect to the
>Internet). This computer was originally just to run a few old legacy DOS
>games. MAYBE this computer (the CPU? FPU?) has something to do with my
>problem... maybe not, you never know... also I don't have a lot of time to
>experiment, takes LOTS of time to render.
>
>I am using *meters* as my basic scale, so a vector of <-3,2,-1> would be x=
>-3 meters, y = 2 meters, z= -1 meter. Right now I am back to working with
>old (tired?) favorites : spacecraft, space stations, planets etc. Working
>with meters is great for this.
>
>Currently I am trying to "improve" upon the "How to render a good starfield"
>concept, and as an extension to that, planets, stars, nebula etc.
>
>I am concerned with trying to render star systems as accurately *as
>possible*. Naturally this means lots of BIG things that are VERY far away.
>I already know that I can't accurately model, say, the sun (radius =
>695,500 Km, with the scale I am working in it would be sphere { <0, 0, 0>,
>695500000 } and translated to a position the approximate distance from the
>earth it would be 149,600,000 or a translate using meters might be
>translate <0, 0, 1.496E11> !!! These numbers are WAY too big, I know).
>
>Since I KNOW I can't do things accurately, I need to know "How much is TOO
>big and how much is TOO far?". Someone with a good knowledge of the innder
>workings of PoV, say, one of the programmers might help?
>
>Here are some clues:
>
>When using a textured Big Sphere to make stars, if the sphere has a radius
>of 10,000,000, the texture tends to *fail* below the x-z plane. When you
>start to exceed 10,000,000, the star texture fails completely and all you
>see is black. So far in my experiments a 9,999,999 meter Big Star Sphere
>seems to be okay, but I haven't played with moving around and looking in it
>yet (via changing the camera). When I get around to looking anywhere else
>besides directly into the z axis, or changing the camera Y, or changing my
>camera angle then 9,999,999 might fail too and I will have to "pull back".
>
>When trying to use 3D objects instead of The Big Sphere, such as triangles,
>discs, spheres, the failure distance seems to be much shorter (on the order
>of thousands rather than millions). The acceptable distance for spheres
>seems to be consistantly longer than for the finite patch primitives.
>
>Other than "Just try things out until they work!", can anyone offer any
>insight or *specific* useful information on this issue? Thanks. - Jeff
>
>
>
>  
>
Have you tried the good ol sky_sphere? It's "surface" is set at 
"infinity". You only have to find a good pattern for your stars (you 
don't scale a sky_sphere pattern like you do on la large sphere). For 
the other objects, the use of scaling can be used to fake a greater 
distance. A small or tiny object at a moderate distance will look like a 
large object very far away.

Alain

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