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Comment by huhtenberg | Hacker News Reader

Since the right people might be reading through the comments here -

I've been looking at making a hi-res ceiling projector for a living space. Perhaps not one, but a set of several projectors with an overlap, but the key point is that they need to be placed in a way so not to interfere with the normal flow of life. Nor to be easily shadowed by people walking around, etc. This (likely) mandates placing them around the room perimeter, possibly on the walls at some height. This in turn means that they will be projecting at an angle.

Do I understand correctly that it's not possible in principle to achieve proper focus on the target surface when projecting at an angle like this? That is if I am to project, say, a star field then it's not be possible to have stars close to the projector and far from it to be both in focus at the same time?

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In principle, it is possible. A tilt-shift camera can bring any plane in focus, so running it in reverse will project the image sharply on your ceiling.

See https://en.wikipedia.org/wiki/Tilt–shift_photography for cameras. https://www.projectorcentral.com/Understanding-Lens-Offset-a... learnt me that you can buy projectors that use this method. I suspect that back-projecting televisions (https://en.wikipedia.org/wiki/Rear-projection_television) often use it, too.

Thanks, these are good leads.

My older Epson 8350 projector has a feature they call "Lens shift." When setting it up you make sure that it is perpendicular to the screen surface. Then it has two mechanical adjustments to vertically and horizontally shift the image. Page 25 of this user manual shows that it can shift the image vertically up to one image height up or down. And horizontally up to one half image width either way. It makes setup a lot more flexible. I'm not sure how common the feature is though on newer more available projectors.

https://files.support.epson.com/pdf/plhc87u/plhc87uug.pdf

Look up "Short Throw Laser Projectors" it's somewhat possible with funky optics depending on how much distance you can afford.

Thanks. Can't seem to find how they work exactly except for "with mirrors!" remarks :)

Spherical mirrors and fisheye lenses do the trick.

Once you know something is possible, it becomes dramatically easier. :)

Current generation non-laser short throw projectors may already hit the price/performance you need, just mount them so they face the ceiling, on top of a bookshelf. (Laser projects are still quite expensive.) If they don't, then physics, specifically optics, has the equations to calculate the mirror specifications you need for your exact use-case. Custom-made mirrors aren't the cheapest, but what you're asking for sounds possible, though it's impossible to say without more information, specifically the dimensions and the brightness needed - you'll lose a lot of brightness from the mirrors.

The setup for Fishbug was two projectors facing a hemispherical mirror. https://www.youtube.com/watch?v=ThtF-FY28hM

I can't really answer your question directly, but check out teamLab Borderless[0] (also definitely go if you have the chance, it's an amazing time). They use 500+ hi-res projectors to cover the entire place in seamless imagery. Maybe you can find some more technical information regarding the installation.

[0] https://borderless.teamlab.art/

Haven't been there, but I've been to https://www.carrieres-lumieres.com/en and the sheer scale of it is absolutely mind-boggling.

As another commenter suggested, it's common for projectors to have a movable/adjustable lens (also called lens shift) to move the projected image without altering focus too much.

There are also ultra short throw projectors that can be mounted a few feet away from their target, designed for classrooms. They probably don't have the blackest black levels though.

A number of them support the unit being placed about 6in. from the wall. When you include the depth of the unit, that may be placing the mirror closer to 2ft. away, but that is still pretty darn close.

I’ve seen applications where they use redundancy with overlapping projections to avoid shadows of people walking in front. The image is still slightly interrupted, but the overlapping projections do a decent job filling it in - if done properly.

IIRC academy of science sf did this in one of the lines for an exhibit.

Most projectors are throwing at some angle. For example, the bottom of the image is further away than the top is. Like my projector lens is in line with the top edge of my screen, and works just fine. Most projectors have some way of compensating for this skew.

I would think if you mounted it vertically on a wall, it would be fine. You would still want it centered horizontally though.

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