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This is probably going to be too detailed for some but I'm trying to avoid the forgotten feature syndrome. I find that when I get comfortable with a new (to me) feature I tend to move on to the next thing with no documentation. When I want to re-use that feature sometime in the future (a month? a year?) I have to re-learn it all over again (I'm looking at you Stride Length). So this thread is mostly a tutorial for my future self. There's a zipped file at the end that some may find useful.


In v19 Steffen added the ability to add a series of images to sprites so this seemed like an ideal time for my steam locomotive actually steam. The following image encapsulates everything I've tried so far which barely scratches the surface considering how many sprite variables there are.


To improve render turnarounds I used a stripped down version of my locomotive and tender, just enough to get the smoke plume's shape, scale, colour and shadows to be convincing.The first smoke sprite image I tried came on the Extras DVD with the Particles tutorial. Using this image I learned that sprites only accept images with an alpha channel and that the image works best if the image's colour channel is just a simple grey field. Using this image I was also able to determine a simple way to get a parabolic shape to the plume emitted from a fast moving object. The trick was to have the emitters be a series of flat angled louvers within the smokestack which are aimed forward at about 45 deg. A high enough "Inital velocity" and a bit of "drag" yield a reasonable plume shape. There's also a conical force named "wind" constrained to the loco to further shape the plume, add some turbulence and provide a cross-wind so the shadows fall on the side of the tender.


A web search yielded a number of smoke images that I thought were slightly more believable along with a 6 image series showing the development of a puff of smoke. These images are shown in the collage on the lower left numbered 1, 5, 9, 13, 17 and 21. It seemed like a 6 step evolution would be too abrupt to be believable so I used variable percentage layers in PSPro to generate in-betweens to get a three step dissolve between each original keyframe.


To see if this was worth the effort I set up a test of just my smokestack with one emitting louver active and a strong wind. To "calibrate" my plume I generated 21 numbered images that were also colour coded in groups (red, orange, yellow, green, magenta, blue) to really show the distribution of the images. This is the center test plume labelled 21 frame distribution. Replacing the numbered image sequence with the 6 original keyframes produces the upper test plume. Using my 21 smoke image sequence produces the lower test plume. The difference is not obvious for a fast moving plume but it might be more convincing in a calmer environment.


The image in the upper corner is a still from attached test clip. The locomotive smokestack uses five emitting surfaces. The front, left and right emitters use a material with the center image to the right of the still with a light grey colour. The center emitter uses the 21 image sequence with a medium grey colour. The rear emitter uses the bottom image and a darker grey to suggest the steam's self shadowing.


The mp4 files is a 16 sec clip of my test locomotive including the initlal emission of the plume so you can see how its shadow develops. Considering how little I've tweaked all the parameters, the result is pretty convincing.


smoke_frames.zip contains all 21 tga smoke frames shown in the image with alpha channels. As I said, in my opinion these work best as sprites if the colour channel is one or two simple shades of grey.



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Very cool- thanks for the sequence!


Your smoke looks good... my eye likes to see the particles continually expand and lose opacity over life, with a random factor controlling the lifetime/size/opacity so they don't all follow-suit. I'll have to look at these new settings!

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  • 10 months later...



2 min. (90Mb) clip of locomotive and tender covering 1 mi. taken by a low flying drone camera. Music soundtrack is a jazz tune from 1930 called "Choo Choo".

As an example of A:M animation it's just a repeating walk cycle with stride length so nothing really happens. It was just a test to see;

1) Will Netrender choke on a square mile (1.6 x 1.6 km) of scenery?

2) Can I cheat on the level of detail of the tracks and the rock ballast when the camera is not too close and are the transitions noticeable?
3) How much "passing meadow" can I build with just a material? There are flashes of it in the first half but a reasonable material kicks in for the last half after the close-up on the loco.

4) Does the smoke and steam withstand 2 min. of scrutiny?


Adding trees and other details to the landscape is underway.

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Nice one Rodger!


The smoke and steam stands up fine for me.

I did notice my mind started to wander just a little as the general repetition at the beginning didn't change much. The music may have been what held my attention until I began to sense the imagery changing.

A few changes in scenery likely would have welcome... and occasional bush or patch of dirt etc.


I was very surprised not to see some strobing of the tracks as they came more clearly into view.

That worked quite well.

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That looks great, Rodger!

I like the bit of ground where the flowers show.

The steam coming from the pistons gets over bright when the particles stack up. Is there a non-additive switch that would prevent that? Or perhaps the transparency could be animated to compensate?

Perhaps one extra you might add is some side to side wobble as if the tracks are not perfectly perfect.

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