Hey all. I’m going to help my friend decorate a pixel mapped Christmas tree this year. I’m just trying to figure out specs for the lights I want to get. I think I’ll be getting 12V Pebble Lights with green wire from Ray Wu. Just wondering if I should get 10 cm or 5 cm pixel spacing (or smaller…?). I’d be happy for guidance from someone whose created a mapped tree before. Thank you!
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Depends on the density you want. For reference I had 600 pixels in my tree last year and it was awesome. There was still plenty of space for other ornaments. Knowing you, double that wouldn’t be too many but you might want to ask your friend if they are going for - no wait, they asked you to help of course they want all the LEDs 
Also 5v is probably ok for 100-200 LEDs if you dial brightness down slightly. On my tree I had 3 runs of 200 pixels of the fixed address super thin wire stuff, each with their input connected to power, and you can barely see the color shift when they all go white during parts of cubefire. A solid white would show it, but not any rainbow at the brightness level I was running.
12v will run hotter than 5v if it’s ws2811 and not ws2815/gs8208 and even then when doing single colors. IMO 5v is better if you can inject power and/or run a few independent strings. Hot lights can dehydrate a tree and make it go crispy before it’s time.
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Sounds like fun!
Here’s a planning worksheet that helps you get an equal density for the number of pixels you choose.
The number of lights to use depends on the tree height. I bought a 9’ tall artificial tree years ago before I was into addressable LEDs. It came with 900 incandescents and I replaced them with 1000 x 12 V WS2811 bullet-style. Here’s what it looks like (and I still recommend the concentric ring mapping approach mentioned):
At that density / conical surface volume, I’d recommend the 10cm spacing unless you want to buy 2X as much. I injected 12V at the 0, 370, and 700 pixel points (which was driven more by the breaks in the artificial tree segments, since I leave them attached between years). The pebbles have 22 AWG vs the 18 AWG on bullet bulbs, so you still need to inject every 200-300 pixels, which is kind of of annoying since less injection work is one of the prime motivations for using 12V!
They do run hotter than the 5V, but not uncomfortably so. It’s not the peak driving heat so much as the quiescent current that keeps them at a warmer baseline than typical 5V pixels. The pebbles you mention will be much cooler than my bullets (41mA max vs 8mA max per pixel). I don’t have quiescent measurements. I run them well below 50% max brightness for aesthetic reasons, and like all WS281X, I’m annoyed at the low-intensity chunky color transitions to off.
If redoing it today, I’d used either the pebble strand you refer to (less bright, likely less noticeable 0-to-1/255 transitions), or for the classic bulb look, they finally have 12V GS8208 bullets which have great low-end transitions. I don’t think you’d notice the black-vs-green wire in a tree to be honest, whereas I cringe at the low-intensity color inaccuracy and chunky transitions to black every year.
Using 1000 WS2811s in one serial line will result in most patterns running around 30FPS. Use an output expander from the center, or 2 PBs in sync mode to double that.
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I’ve settled on 800 pixels of the fairy wire LEDs, at 10cm spacing. 600 wasn’t quite dense enough, and 1000 seems like overkill. They’re 5v, and run through an output expander, 200 LEDs per channel.
Mapping has been banned from our tree. For some reason, people think I’ll do a terrifying, spinning volumetric Santa head or something. No idea why they’d think that. So I stick to slow changing, masses-of-color patterns.
Jeff is absolutely right about the awful colors at low intensities, and the very, very ‘notchy’ fades at low levels with these LEDs, but I just program around it for the most part for a relaxing, warm looking tree.
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Thank you @wizard, @jeff, and @zranger1 for the advice! It’s very helpful. @jeff, your worksheet is amazing! I’ll have to find out how big my friend’s tree will be before making the final decision on lights, but you’ve given me a lot to work with. It’s never to soon to get into the Christmas season!