 The magazine of the Melbourne PC User Group Going Loco with POV-Ray Ken Holmes |
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It pains me to think that there are still among you those who have not yet had a go at POV-Ray, the freeware ray-tracing program. It really is a great program which can produce very satisfying pictures with a minimum of effort or artistic ability. Its documentation is extensive and helpful, and it comes with a large number of prewritten script files from which it can produce a wide variety of ray tracings at your command. You can modify these files and, hence, the pictures, to learn how it all happens before launching into something original which might reflect your other hobbies or interests. Searching under "POV" on our BBS will turn up a dozen files which are either the program itself, add-on utilities or pictures produced from it.
While seeking new subjects for ray-tracing, the word "locomotives" sprang to mind; while I haven't indulged in model trains, the real thing never fails to attract. Who can walk up to a steaming, panting, hissing monster and maintain equanimity? Who hasn't chased Puffing Billy through the hills, waving madly to the kids on board at every crossing? A quick perusal of the local libraries faced me with about 47 varieties that I wanted to reproduce, but the winner had to be Stephenson's "Locomotion," which, in 1825, hauled the first public rail service, on the Stockton and Darlington line, delivering coal. It's chunky with its working bits, hung all over the outside; I just had to see it in stereo animation. With three photographs and one small drawing available, the main elements of its works can be guessed at, but if anyone has more information on the valving and control, I would be grateful to see it.
The front wheels led the rear by 90 degrees to give a smoother drive by the two double acting steam cylinders buried in the top of the boiler. They were connected by horizontal crossbars and vertical connecting rods. The jointed side rods connecting front and rear wheels need a little thought to understand their exact functioning. If the wheels remained at 90° phase difference, the longer rod was always horizontal and the knuckle joint was in the same position relative to the rear wheel as the front crankpin was to the front wheel; the short rod spanned a 90° segment of the rear wheel. But the knuckle joint could not be connected directly to the wheel since the crankpin and conrod had to pass between - so, what stopped the rods just folding and allowing the wheels to lose correct phasing? When the front crankpin was 45° past top dead centre, the rear crankpin was 45° behind TDC and the rods were stretched out straight. This stopped the rear lagging by more than 90° at this stage. Likewise, with the front 45° past BDC and the rear 45° still 45° from BDC, the rods were folded completely in line and, again, the rear could not lag by more than 90°. But, what stopped the rear catching up to reduce its lag? There must have been a bias in the control valving to make the rear wheels hang back the full 90°.
The top crossbars operated, via two butterfly frames, the control levers of four steam valves up among the gubbins on top so that the position of the front cylinder controlled the steam supply to the rear cylinder, and vice versa; see those diagonal steam lines! When one cylinder was at TDC or BDC, the other was half way up, or down, its stroke, thus smoothing out the drive. The extra rod beside the front, right conrod operated a linkage which disappeared under the boiler, to connect up with a linkage under the left, driver's, side. There, two long rods came up to operate control valves near the steam chests near each cylinder; the linkages were adjustable via two handles available to the driver who stood on the left platform. Doubtless, these allowed him to go slow, go slower, reverse and use retardation. Perhaps there was a parking brake - or a brick under the wheels? The stack was very tall to encourage natural draught and exhaust steam was injected into the stack via the horizontal pipes to assist. The fireman sat on the tender with the water tank and a supply of coal underneath.
The loco wheel structure was interesting. Twelve sectors consisted each of inner and outer castings with holes for lightening or to lessen casting cracks, or both; the tyres were heated and shrunk onto the assembly. They presented a nice little exercise in programming; the ray tracing slows down noticeably when it gets to the wheels.
At the inauguration, a single coach (described as a shed on wheels) carried the Directors, and the local populace, or nearly 600 of them, piled onto 21 wagons. With a dozen coal-laden wagons, the total load was 69 tons and, on occasions, 15 mph was reached during the 21 mile journey. It must have been exciting with most passengers waiting (hoping?) for the loco to explode!
Picture Resolution
Once you have the POV-Ray script file, it can be rendered for any resolution screen you desire; the left- and right-eye pictures here were each calculated at 1024 x 768 resolution to get the sharpest image for printing. A limitation of the mirror system for stereo is that each picture is only half the width of the screen. I usually choose subjects which can use the full screen height, but, when you come to natural subjects like trains, you really are confined to the 4/3 width/height format and so you only use half the screen height as well. With a 2 MB video card, I can view the 1280 x 1024 screen, so the two views can be calculated at 640 x 512 and spliced into a viewable 1280 x 512 GIF file. In making an animation for the BBS, it is necessary to compromise as members have various video cards; a 800 x 300 version was chosen as it can be viewed on the 800 x 600 screen with a 512 KB SVGA card. The .FLC stereo animation file can be downloaded as LOCOMOTN.LZH [240 KB]
To get a feeling of "closeness," to make the loco bigger, the eyes' position used in these pictures was about 10 cm from the page (or screen), but it is more comfortable to view from about 30 cm; so there is some distortion of the perspective. Eye separation was chosen so that a 30 cm wide mirror would give a normal apparent depth; it isn't critical, but other widths will vary the apparent depth proportionately. For those who haven't seen previous articles.
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Place mirror with one edge down centreline, facing left and at right angles to page. With nose on far edge, look at the right picture with the right eye, and at the reflection of the left picture in the mirror with the left eye. You will see the stereo reconstruction on the right side. Ignore the direct view of the left picture.
Version 3 of POV-Ray could appear this year, with many embellishments such as new shapes, textures and lighting effects, "while" loops, "if" statements, calculation of sine and cosine, exponentiation, random numbers, modular, vector operations etc. "While" loops would be useful for repeated items such as rings or rows of rivets, sectors of wheels, or assemblies of piston/cylinder in multi-cylinder engines. For multiple frame calculations for animations with the current Version 2, it is necessary to use batch files to write "include" files to feed in all the variables for each frame as only the four arithmetic operations are available inside the script file. Version 3 offers animation (but not stereo), externally or internally; "external" means you supply, via a batch file, the frame number and/or a "clock" value and you program calculations within to define the changing positions of objects; "internal" means that the program repeats itself to produce all the frames requested (ie. no batch file). One feature not provided, which would be useful, would be being able to preload arrays into the script for object positioning. For stereo, one could, of course, do runs for each eye and then splice corresponding frames.
Still, don't wait for Version 3; get Version 2 today. I feel a little selfish as I seem to be having all the fun; Id like to think more were joining me. Perhaps we could run a competition for Heath Robinsonish animations, optional stereo? To encourage your interest in POV-Ray,
LOCOMOTN.LZH will include the POV-Ray script file (.POV), the batch files used to run the frame calculations and splice them in stereo pairs into .GIF files, and the methods of creating and viewing the animation. There are always many ways to do things and I expect to receive helpful feedback on other approaches and resources available.
Reprinted from the August 1996 issue of PC Update, the magazine of Melbourne PC User Group, Australia
