Background replacement with a miniature with fine details

Hey everyone!  I'm getting ready to shoot a stop motion short and need help figuring out what to do with the background.  The main character of the film is a cactus- and he has many, many, fine little spines all over his body.  I did a blue screen test that was very unsuccessful.  The spines are so fine that they all ended up blue in the image, and cleaning them up is impossible.  So now I'm trying to figure out how to do the background physically.  A matte painting or photograph seems like the best way to go, but these are methods I can't find much information about.  Any suggestions out there for a low budget solution?  The setting is a desert, so the background needs to be a big sky and some little mesas in the distance.  Thanks everyone!

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My usual setup for outdoor scenes is to have a background painting.  Like this time-lapse of building and shooting a desert scene.  It's an Australian desert, so no mesas or cactus, but may still be of some help for how I set it up.

That is a pretty large painting, on a frame 2.4 metres (8 ft) high x 4.8 (16 ft) metres wide.  It has 4 panels, each one with an 8 x 4 sheet of 3mm ply on a pine frame, to staple the canvas onto.  But a single sheet 8 ft wide x 4 ft high would be plenty, I've shot a few on smaller backdrops than that.  I staple canvas on a frame so I can take the painting off the frame and roll it up to use again.  If it's a one-off, you could paint directly on the ply, or a wall, or whatever you can find.    Canvas does need to be stapled onto something, and get primed first.  I paint with flat acrylic wall paint.  Here is a different backdrop being painted on the same frame, showing how I lay in the different shades of blue for the sky:  https://youtu.be/zpeRCeiUwEU

In some ways it is similar to shooting with a blue or green screen, you want a fairly even flat light on the sky painting and enough distance so you don't get shadows from the puppet or set on it.

If you have the capability, projecting an image/picture onto something for your background could work.

I did use to do Front Projection with a slide projector, but to get  nice bright image I had to use the Scotchlite material, like the reflective material on road signs, and a half-silvered mirror at 45 degrees to shoot through so the projector could line up with the camera lens.  

The other method, Rear Projection, requires a translucent screen, and room behind it for the throw of the projector.  I haven't tried either of these since going to digital cameras.  

Frankly, painting a backdrop is less fussing about.

Frontlight-Backlight is another method I've used, but I suspect the fine spines on the cactus would be burnt out by the white screen on the silhouette pass, giving you the same problem that you get with blue screen.  And you have to shoot twice, with a lighting change for each frame, so it's tedious.  It is helpful when you can't avoid the blue or green keying colours in the subject, but probably not for this.

One thing that always helps me key fine detail is a strong backlight -- combine a solid rim (slightly yellow, to help protect the spines from blue spill) with a really smooth, evenly lit blue screen (the less you have to clamp down your key, the more detail you can preserve), and shoot with the highest quality format your camera can handle, and you might have a fighting chance.

Of course, a straight practical solution like a painted backdrop will save a ton of work in post.  When in doubt, do whatever Nick tells you to :)

Backlighting is a great idea. Make that cactus glow.
This is just an idea, maybe the other guys have some insight, but if it doesn't ruin your aesthetic, what about painting the needles some bold color? I'm not speaking from experience, but I feel a reflective silver might make them more prominent against the backdrop or a deep dark black with thicker spines if you want to reconstruct them. That with the backlight might be enough, but again, your aesthetic is going to change. Thorough camera tests would be necessary.

Great to pick up that tip about backlighting. I did a project earlier this year where the scene was a savannah. I wanted to convey the sense of space in the African plains, so tracked through a miniature set towards a green screen, into which I put my After Effects 3D landscape. Unfortunately the grasses on the back edge of the set just wouldn't key out cleanly and I got a fair bit of boiling.

Now I know - backlight! Thanks, guys.

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