TITLE: Planetary Nebula NAME: Simon Smith COUNTRY: UK EMAIL: simon_smith@zen.co.uk WEBPAGE: http://www.simon-smith.org/raytracing/index.html TOPIC: Mist and Fog COPYRIGHT: I SUBMIT TO THE STANDARD RAYTRACING COMPETITION COPYRIGHT. JPGFILE: nebula1.jpg ZIPFILE: nebula1.zip RENDERER USED: PoVRay 3.00a.riscos.cc5.06 (unofficial version) TOOLS USED: Trial and error, a text editor and 2D drawing tool RENDER TIME: 1360x1024 took about 28 hours HARDWARE USED: 600MHz Iyonix (30321 ARM processor) No FPU -( IMAGE DESCRIPTION: This is a mock-up astronomical image intended to be compared with views of the Cat's Eye Nebula, the Hourglass Nebula, the Ant Nebula and others. I have provided two views - a greyscale version and a 'false colour' version. The 'false colours' used were based on those in this picture of the Cat's Eye Nebula; http://www.astro.washington.edu/balick/WFPC2/catseye.jpeg, although I did decide to use them in different order in my scene. I actually think I like the greyscale version slightly better. :-) DESCRIPTION OF HOW THIS IMAGE WAS CREATED: I anticipated that most of the submissions for this round's topic would be earthbound scenes - early morning mists and the like. And given how expensive fog is to render, I never seriously intended to enter this month's competition, simply because my home PC lacks the computing power. But with 18 days to go to the deadline, inspiration struck; a planetary nebula would be a legitimate subject - it just happens to be an atmospheric effect for a stellar atmosphere rather than a planetary atmosphere. And I knew that these nebulae tend to throw off their atmospheres in concentric 'shells' which could be modelled reasonably well using standard PoVray textures. Of course, 18 days, a slow computer, and other commitments made it impossible to do as much work as I would have liked to have done. I lost a couple of Saturdays and a couple of weekday evenings which I really could have used. A complaint with which every other raytracing hobbyist will be thoroughly familiar. :-) In fact, most of my early test renders just used some overlapping ellipses and they rendered quickly and looked quite reasonable. It was always the textures that were going to be the more important part of this scene. It was only quite late on that I switched to a more complex, organic nebula shape based on blobs. By that time, the layered textures were slowing things down beautifully, and using complex blobs instead of simple ellipses made little difference. The colours in the nebula are based on the Cat's Eye nebula picture mentioned above. Its shape was mostly my own creation, but I was certainly inspired by NGC7009 (see the Wikipedia link below), the Hourglass Nebula, and M2-9, the Wings of a Butterfly Nebula (APOD 2005 June 02 http://antwrp.gsfc.nasa.gov/apod/image/0506/m2-9b_hst_big.jpg) (hm, looks more like a squid to me) and others besides. When producing this scene I did about 150 test renders. Of those, about a dozen were focused on getting the shapes right - all the rest were tests and refinements of the textures. This is one of the most 'faked' scenes I have ever produced. I would have liked to have made the star ultra-bright so that it really did burn through the nebula shell in the direction of the camera. The current setup uses a wood texture with a transparent core in which the star sits. This produces much the same effect, but would require the wood texture to be rotated if the camera was moved. The blobby nebula shape started out perfectly symmetrical - there is a sample render in the zip file. I then randomly added or subtracted a small amount to or from each parameter, and this distorted it by about the right amount. I also made sure that it was scaled and rotated by varying amounts each time it was used in the scene, and the textures were given similar treatment. One does not need big displacements when doing this sort of thing - a 2-5% linear distortion or 1 or 2 degree rotation in each direction seems plenty. A similar method was used for the nebula ring. In early test renders it was just a torus. I have no idea whether it would be possible for a real planetary nebula to produce a ring structure like this. When testing the various wood textures, I draw a couple of ellipses in a 2D graphics package and interpolated them with ten colour bands. I could then overlay the nebula shape and see roughly where the colour transitions would occur. (My colour maps were set using using 0.1 point 'steps'.) This was very helpful and saved a lot of tedious test renders. But it does make it doubly annoying that I still managed to lose the central star at the last minute. Oh well. There's a test render in the zip file showing how it was supposed to appear. (ring.png) The final image, a greyscale and colour montage, is the same image file saved twice, once in full colour and once in greyscale. AFAICS saving in greyscale seems to be a standard feature of the JPEG format, even if not widely used. For the purists, my attached zip file contains nebulabw.pov which simply averages the RGB components of each colour. This may not look exactly the same as the greyscale image in my submission, but I can honestly say I have not been able to percieve any significant difference. OMISSIONS: You may note that there are no background stars. I did have some originally (and what's more, I had made a point of ensuring that they were different sizes and colours rather than all shades of grey) but the main sample nebula images I was working from were often so closely focused on their target that they did not have any background stars visible. So in the end I decided to take them out of my own scene too. Lens flare: I had decided not to use any 'lens flare' on the central star, even though this is quite common among nebula images. I'm assuming I have an idealised camera that is not subject to such distortions. This was decision was made moot by the fact that a final texture change made the central star completely invisible anyway. See below. One annoying and unintentional omission is the central star from my own scene. The smaller test renders tended not to pick it up anyway, because it is quite small, and it was only too late that I found a texturing oversight had erased it (again, darnit!) from my final high-quality render. That's why there's an ugly black blob in the foreground 'ring' structure. The central star is supposed to have 'burned through' the thin shell of gas there. Unfortunately it's been mugged by an error in the inner nebula texture, I suspect, so it isn't visible after all. I will be correcting this when I get a chance to re-render and re-post the image on my own web site. Probably in a week or two. LINKS: For how a false-colour image of a real planetary nebula might be built up, see here: http://hubblesite.org/sci.d.tech/behind_the_pictures/meaning_of_color/catseye.sht ml For a wide variety of planetary nebulae see: http://www.astro.washington.edu/balick/WFPC2/ Image montage in pdf format at: http://www.astro.washington.edu/balick/WFPC2/plneb.pdf Photo gallery of images at: http://oposite.stsci.edu/pubinfo/PR/97/pn/photo-gallery.html See also the Astronomy picture of the day at: http://antwrp.gsfc.nasa.gov/apod/planetary_nebulae.html And planetary nebulae have reasonable coverage in the Wikipedia at: http://en.wikipedia.org/wiki/Planetary_nebula