Proof of Concept: DIY Pyramid shaped 3D Display

This is going to be my first post about one of my later projects (03/2011): The DIY 3D Display prototype. I don’t have anything documented about it, after all it was just a proof of concept. But the youtube Video generated enough comments that I will try to come up with a documentation in the next post.

In this post however I’m going to explain how I got to make this prototype and I will talk a little about how it works. For those only interested in the technical stuff head on to How it works.

History

I’d love to say that I came up with it all on my own, but that’d be a lie. I merely reverse engineered the concept from videos of the commercial versions.

The first Pyramid shaped 3D object that I saw was the Cheoptics 360. I watched the youtube videos of it closely, trying to figure out how they did it. From what I could see these were really just glass pyramids with each side being lit by a projector. Back then I didn’t act on it though, it was only later when I found the smaller versions of this type of display on Youtube, that I really got curious again. As I watched most of those videos I was able to discern four basic features:

  1. The pyramid sides are illuminated by small LCD screens on top of the pyramid
  2. The pyramids glass was darkened (rather than clear, as was the case with the Cheoptics 360)
  3. The inside of the pyramid was lit up
  4. The animations consisted of seperate objects, there was no visible background

Now the first thing I did was to simplify the design a bit.

  1. Instead of 3 seperate LCDs I used only one big LCD (an old Computer monitor)
  2. I only used clear plexiglass
  3. I left out the light

So after I picked the screen I measured its display area and made that the base area of my pyramid. I did some calculations to get the dimensions of each side of the pyramid, so I could send those to a plexiglass supplier on ebay who’d cut the sheets for me. I ordered 3mm thick plexiglass, as that was the cheapest. I also bought plexiglass glue. This special glue will only cure under UV light and when cured perfectly matches the plexiglass refractive index (it becomes invisible, unless you make a mess like I did 😀 ).

After I had the pyramid I only needed to build a support structure for the display. I made it out of some left over wood and metal sheet. A very crude but effective construction.

To give it a first test I just wrote a short script that would render a 3D cube and rotate it randomly. Of course my animation had a black background (remember point 4 of things I noticed about the originals).

The result can be seen in the video I uploaded to Youtube.

It’s already impressive how well it works at that point, but I quickly noticed a flaw: the reflection doubled. It dawned on me, that the light reflected off of the front of the plexiglass was also reflected on the back of the plexiglass. This is because the transparent plexiglass can’t reflect 100% of the light (then it would be a mirror and the 3D effect would be destroyed), so some of the light still passes through and gets partially reflected at the backside. This second reflection appeared weaker, so I immeadiently remembered that the original 3D pyramids had darkened glass. I didn’t know what level of tint they had, so I carefully watched the videos again and then went back to ebay to find a car window tint kit that matched the videos as closely as possible. I ended up buying something with I believe 20% transparency (I wrote about the exact values in one of the youtube comments, I’ll update this post once I find it again).

So the tint foil got rid of the double reflection issue, but the 3D effect suffered a bit. As it turns out the originals where lit up from the inside for a reason. Clearly seeing the space behind the glass is an important part of the illusion. So I added a light inside the pyramid (not too strong though, you don’t want to outshine the reflection) and got the perfect result, exactly like the one from the videos of the originals.

Now obviously I only uploaded the video of the first trial and I recently moved and still need to setup my new workspace, so it might be a while before I can set it up and make another video again. So for now let me just explain how it works.

How it works

The principle is so simple that it’s been a part of theater for a long time (since the 18th century). It was called “Pepper’s Ghost” and when you google it you’ll find a lot of interesting information about the effect. But let’s concentrate on the 21st century here. The depth perception of the reflected image comes from the fact that it appears to float inside the pyramid. And the reason why it appears to float inside the pyramid can be accounted to a basic law of optics. An object in a mirror will appear to stand as far inside the mirror as it is away from the mirrors surface. So the distance each pixel of the screen has to the point of reflection on the surface of the pyramid will appear as the virtual distance of that pixel from the pyramids surface into the pyramid.
virtual_reflection

As the crude illustration (I know the reflective surface isn’t really drawn at the correct angle for that reflection to occure) above shows, the 3D effect could already be achieved with only one surface. But then you couldn’t walk around the display, seeing the same image from different sides (which improves the illusion quite a bit).

The most complicated part of this project then becomes to write the software that will display an object from three different angles onto the three sides of the pyramid in a way that will not let the viewer notice that it’s really three different images he’s seeing.

This can be accomplished by rendering the object in 3D space and then set up 3 camera perspectives corresponding to the sides of the pyramid in the real world. Those three camera perspectives then need to be rendered onto a 2D projection of the pyramid that perfectly aligns with the physical pyramid. However I will discuss the software in another post, as this one has already gotten quite lengthy. I will see to it that there will be source code available for download as well.

That’s it for now, be sure to check back for the rest of the documentation.

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25 thoughts on “Proof of Concept: DIY Pyramid shaped 3D Display

  1. Argyris

    Hello!
    I really liked your work! I am making an academic project on holograms and I am building a big size pyramid, using a projector instead of a screen. Can I ask you a few questions by email?

    Reply
  2. Martín

    Hi mate, nice work! i have an old display with the same size, can you provide me the measures of your pyramid?

    Reply
  3. Arash

    hello dude

    my name is Arash and i live in Tehran. it’s been a while that i’m trying to build one of these pyramids but it ended up to deadend every time. i used transparent clear Plexi glasses and use a wide 16″ laptop LCD with a dongle which can show any type of inputs into 40pin LCD the result was unclear picture and i wonder why it didn’t work because i saw this in one company and its has clear vision in pyramid but mine doesn’t . so as you said i saw they glasses was a bit darker than mine so i searched for car window tint with 30% darkness after installing that, the result got better but not that i expected so i searched whole internet for that until i see your page in my opinion my pyramid difference with yours would be in type of window tint and the light which you used in pyramid i didn’t use light so i’ll be really appreciated if you help me to have clear view in my pyramid pictures

    Reply
  4. Vincent

    Hello, I live in France, I’ve a some difficulties to find a 20% glass tint film. Those I’ve found are dedicated for cars, and “20%” mean 20% of transparency (80% of opacity). Where can I found a 80% transparency film (20% opacity)?

    Reply
    1. D P

      Hey,
      sorry for the late reply.
      I’ve found all my glass tint films on eBay (where else), but you might also try car tuning suppliers (the film I used was dedicated to cars as well).
      However, 80% opacity won’t hurt you much. You might have to get a dedicated light source for the inside of the pyramid though.

      Reply
  5. Sreyas s Sreedharan

    I want to the max size of pyramid i can make with my samsung j7 mobile screen i want the hologram big can you help me with calculations. Screen small but i need hologram big what are the factors that makes it big :cs4u13@gmail.com

    Reply
    1. D P

      Your image has to cover the whole surface area of the Pyramid.
      So you either need a huge display, multiple smaller displays or use projection.
      That being said, I don’t think your smartphones screen has enough brightness to project an image onto a bigger pyramid.

      Reply
  6. Jeanne

    Hi, so if I understood correctly, there is no reflexion but only refraction ? It’s our brain which imagine that the object is inside the pyramid ?
    Thank you for your article.
    I”m not English but it really heps me !
    Waiting for your answer.

    Reply
    1. D P

      No, it’s actually reflection. As you know, when you look into a mirror your face appears twice as far away as the mirrors surface.
      And when you reflect an image on a semi transparent surface the image will appear to be coming from behind that surface, creating the illusion of a hologram.
      The fact that the pyramid has three sides where you project three different images just makes the illusion stronger, since you can walk around the pyramid and actually see your image from different perspectives.

      Reply
  7. bifi

    Hey there,

    I’m doing this for my science fair. Since the HOLHO uses darken glass, I was wondering if I used darken glass too. What do you would happen if I use darken glass? Will it also work in light areas and will the projection be better and clearer?

    Thanks a bunch

    Reply
  8. SABARI

    Hello..
    I wanted to know if there are any possibilites to replace the monitor with a projector….
    please let me know the dimension ratio…if it is possible…

    Reply
    1. D P

      It’s even easier with a projector, since you can adjust the projected area.
      So basically with a projector you can more freely chose the size of your pyramid.

      Reply
  9. Ata

    Thank you,
    you mister, are my hero! I’ve been looking for DIY pyramids all over the internet . Your technique and description of that blasted double reflections saved me. SAVED MY LIFE… sorta.
    SO AGAIN THANK YOU VERY MUCH ! ! !

    Reply
  10. RomBass

    Hi there

    I’d like to build a similar display for a pyramid with a 150 x 150 base in order to put it on stage while playing with my band.
    I’m really bad at calculus, and I don’t know what to start with.

    I understand that I need à 45° angle between the base and face, in order to have the right effect.
    What would be the other dimensions of the face triangles in order to get this 45° angle when I put together the four sides. And furthermore, I’d like to know what would be the size of my LCD screen with such a pyramid to get best result, as I plan to use a HD TV LED display.

    Thank for your answers, and forgive my english. If I’m not clear enough, don’t hesitate to say it !

    Rom

    Reply
    1. D P

      Hey there Rom,

      one problem I see is that you want to use an HD-TV set. HD TVs (or most kinds of displays these days) are widescreen (16:9 & 16:10), which doesn’t really work well with a square base pyramid (well it does, but you’d be wasting a lot of screen space).
      Basically you want your pyramid base to be the same size as your display, so it makes sense to pick a display first and go from there.

      You want to put it on a stage while playing with your band, so I’m wondering if you’d even need a pyramid, as I suspect your audience would only be able to see the front face of it.
      The effect works really well with only one face (if you’re not trying to walk around it and view it from other sides). And if you only have one face it’d be much easier to set up. You’ll only need one sheet of acrylic with the same width as the screen. The height of this sheet of acrylic depends on the height of the display (though it’s not the same).

      If you come back with the exact measurements of your display I’ll calculate both versions for you.

      Reply
  11. Massimo

    Hi, finally I’ve developed a mock up in clear glass for 22″ LCD.

    All it’s ok, except an issue caused from the glass, the hologram is duplicated of the exact thickness of the glass . eg : the glass of 5mm produce an hologram doubled of 5mm.

    I’ve also tested on clear and dark plexy but with the same results.

    Have you an idea to fix this ?

    Reply
    1. D P

      The Software:
      Panda3D
      Some IDE or simple Editor, like Notepad++; However I use Komodo Edit on Windows.

      The How:
      By recording a scene from three different angles and rendering the result to a texture.
      See this part of the Panda3D Manual to find out how one can render to texture.

      The basic idea is simple. Record a virtual space that is the equivalent of the actual space inside the pyramid (thus recording from the three angles). Then project it onto a 2D plane (the monitor – using render to texture) and then adjusting that virtual projection until the real world image on the pyramid lines up nicely for each side, so that a coherent impression of a 3D image can be achieved.

      Reply
  12. Massimo

    “So after I picked the screen I measured its display area and made that the base area of my pyramid. I did some calculations to get the dimensions”

    Can you explain me how calculate the pyramid dimension ? Eg: ipad screen or 42″ tv ?

    Thanks in advance

    Reply
    1. D P

      Imagine it like this:
      You take the base dimensions of your pyramid and add the desired height. You get a box. Add a 45° angle towards the center to 3 of the sides of that box. “Cut off” the excess and you’re left with your pyramid.
      Other than that it’s just a^2 + b^2 = c^2 … a being the dimensions you measured, b being the height you decided to use and c being the side you need to calculate.

      Reply
    2. mark

      the Peppers Ghost pyramid gets its illusion from the angle at which the light is reflected to the viewer. The best angle from the base, transitioning to the sides is 54 degrees. so whatever size your screen is, is how wide the base will be. draw lines at a 54 degree angle from the base, and eventually they will intersect. thats the top of the pyramid. so just draw it out on cardboard. OOORRRR you can just calculate a smaller version and do it on paper.

      Reply
  13. Kai Norden

    Hi there,

    We have a much smaller scale model. We are searching for a way to create a moving image with a black background similar to the multicolored cube you used. Anyway you would share your video? or explain how we could make such a moving image?

    Thank you so much.

    Reply
  14. Leslie Wang

    nice post. I really want one by myself. Looking forward to your next post on software part.

    Reply

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