Curved Display, JDome, Immersive 3D Display

[cybereality]

Nice thinking. I actually have a Papasan in my living room, but I never thought to use it for something like that. Very slick.

[cadcoke5]

Regarding your projection surface, let me describe two ideas that involve using a vacuum to hold the spherical shape of the projection surface. Picture covering both sides of your chair frame with a material that is not very porous to air. Then, use a fan to draw a light vacuum. It would suck in on all the surfaces, and this can be used to hold the shape of a cloth projection surface.

Things like fiberglass have been used to make spherical surfaces before, but I am thinking of something easier to fabricate. That is why I am thinking about a vacuum. For this to work, either the projection surface must be very stretchy, and use something like a latex sheet, or it must be made from a pattern.

Idea #1, Large white latex balloon.
I know they are available, but all the pictures I have seen of the white ones are fairly translucent when stretched. A 6ft dia balloon costs $22 at this site http://www.balloonbasics.com/JUMBO_BALLOONS.html" onclick="window.open(this.href);return false;. But, I suspect that is pretty stretched out. I am going to venture a wild guess and think a 4ft dia screen is viable. It would also be more opaque if it were inflated less than its quoted size. Latex would also have a limited life span, because it turns to goo.

Idea #2, Heat vacuum forming for a seamless surface;
Perhaps a heavy gauge while shower curtain liner would work. You don't need a curved form to draw the shape over over, because the material can be heated, and then placed over chair frame, with its back enclosure sealing the back side. A shop vacuum may be sufficient for the vacuum. However, the reason you can't do this at home is that you must heat the material evenly. It would mean that you have to go to a commercial sign maker that has a large oven to heat the material. Then the vacuum would have to be carefully monitored to hold the correct curve until the material cooled.

After the material has set, only a minor vacuum would be necessary to hold the shape. Perhaps a computer muffin fan would be sufficient.

Idea #3, Giant white beach ball;
There are very large 5ft dia., and 7ft dia. white beach balls availble for $74 and $125 respectively [http://www.beachballs.com/shopdisplaypr ... each+Balls] Note that I don't think you want the entire diameter, unles you plan to use a spherical mirror to spread the projector light. But, if you get the 7ft diameter, you can just use a portion of the circle, and it may be a viable good projection surface.

I just did a bit of geometry work, and see that if a 4' diameter circle is cut off of a 7' dia ball, the resulting field of vision from the center is 66 degrees. If the same 4' dia circle is cut from the 5' dia ball, the results are 94 degrees. Note that multiple screens could be cut from one beach ball.

One drawback to the beach ball over latex is that vinyl is not nearly as stretchable. So, with a low vacuum, the screen will not really pull to a sphere, but tend to be more like 3 portions of a cylinder. You also will see the seams.

I have some more ideas about this whole vacuum thing, but won't post here unless there is interest. Then, perhaps we should form a new thread.


Joe Dunfee

[3dvison]

Hi PalmerTech,
The way it sounds from your post, is with the use of the Papasan chair, you would be using front projection not rear projection is that right ?

I guess I'm just wondering, where and how, you have the ShowWX mounted in relation to the screen ?

If it is front projection, does your head get in the way of the projectors light beam, when you move in two feet from the screen ?

[PalmerTech]

Yes, I am using front projection. The ShowWX has a very short throw ratio, 1:1! It also has a pretty strong upward projection bias; That is, if you set it flat on a table, the image is projected a little upwards, so as the clear the table surface without having to tilt the unit up, which would cause distortion.

Right now, I have it set facing towards me, from the bottom edge of the dome, and then reflecting off a 12" or so normal flat mirror. I might try a magnifying mirror next to see what that does to a laser projector! But for now, my setup fills almost the entire dome, and I am considering a double mirror setup, to make the throw distance even less, so I can sit even closer than 2 feet from the dome.

Joe, I definitely have some interest in your idea, especially as rear projection is really the best way to do this, in an ideal world. Like I said, I am going to try paper mache for front projection, as fabrication cannot get much easier than that. I was doing that all the way back in preschool!

[cadcoke5]

When you mentioned the paper mache, I was wondering how you could make a smooth sphere over the framework of the chair. Perhaps your experiment with taping the cloth inside showed that the method is tolerant of a lot of deviation from a sphere? Is that the case?

Joe Dunfee

[PalmerTech]

I can get something very, very close to perfectly smooth by using fabric. Once I make that framework, forming a paper mache shell in it should be very easy. It will not be perfectly flat to start, since paper mache is usually a bit wrinkly, but like I said, I will be using Bondo filler putty, sanding it smooth, then painting over it.

And actually, up till just now, I was planning on doing the paper mache inside the dome. I just realized that it would probably be easier to lay it on the ground curved side up, and make the shell on top of it! Silly me.

I should be able to get something that is very, very close to a sphere.

[3dvison]

What about that foam that comes in a spary can used to make car seats fitted to your shape of insulation foam in a can.
Could you spray out a bunch of that into a tub and then press a balloon into it, let dry then pop balloon ? Or spray around outside of balloon like you were going to shave it with shaving cream, let dry..and POP balloon ?

[cybereality]

You guys have some great ideas. But what if you buy one of those big beach balls (doesn't have to be white) and then do the paper mache over that. You could make an endless supply of curved screens all from the same beach ball. And that should be pretty affordable.

[3dvison]

The only reason I said use foam, was because I thought it would be very light and not have enough wheight to change the shape of the balloon or ball and be very smooth.

[tritosine5G]

Military guys seem to use this with laser:
http://www.freepatentsonline.com/7342719.html" onclick="window.open(this.href);return false;

similar to my proposal. Notice they talk about projectors getting smaller and speckle artefacts going up in exchange( sounds like laser ). .

Also found a nice post on DIY RP screens:

To all of you interested to know what Good Rear Projection TV's use as a screen materail, it's simple, it's ground glass, sometimes a thin sheet, sometimes it varies in thickness from edge to center, ground glass has a texture so the viewing angle is wider because when the light hits it it is diffused, and there is less hot spotting, though if you have the F-Lengths correct/optimal with regard to lens hot spotting is not so much of an issue anyway. Ground glass is also used on telecine transfer machines for putting film onto video tape/computer.

Cheaper rear projection tv's have a cheaper Bulk diffuser screen, which uses plastic that has been impregnated with small
particles of material that have a different index of refraction from
the plastic. These particles are dispersed randomly throughout
the plastic, hence the name bulk diffusers. Bulk diffusers have
very high-resolution capabilities, but limited contrast control
relying on tinting to reduce ambient light reflections. This tinting
reduces the projected light through the screen. Bulk diffuser can
have asymmetric viewing angles with the use of a lenticular, but
the addition of a lenticular allows the possibility of moiré
patterns occurring.

Another method used in cheaper rear projection tvs/screens is Beaded Diffusers, Beaded screens use a sheet of transparent beads imbedded into a black plastic substrate. These beads are pushed through the black plastic so that just the tip of the bead is clear through on the viewer side of the screen. By doing this, the ambient light hits the mostly black surface and is absorbed. This allows the beaded screen to display the blackest blacks and hence the best contrast. The back of the bead focuses the projected light through the clear tip of the bead, keeping projected light losses at
a minimum. The beads are small enough for this screen to have high resolution. This screen suffers from the inability to have separate viewing angles in the vertical and horizontal directions, limiting the screen gain or brightness of the display. Because
this is not a lenticular screen, moiré noise is not a big factor, but uneven distribution of the beads can give a mottled look that can be disturbing to the viewer. Speckle is still an issue to be resolved.

The third type often used, again on cheaper tv's/screens are Surface Relief Diffusers (SRD’s). Surface relief diffusers are engineered perturbations in the surface of the plastic that refract the projected light into a diffusive pattern. This type of screen has very fine features allowing for high resolution. The screens can be engineered to have different viewing angles in the vertical and horizontal directions. Since the patterns are random, there is no periodicity to create moiré patterns. Two advantages of SRD’s are high efficiency and a high degree of flexibility in defining viewing angles. Because of their efficiency, SRD’s can have very high gain, however, for good contrast control tinting is sometimes necessary, which negates some of brightness advantage.

Top of the range rear projection tv's use glass-beaded screens, and tinted surface relief diffusers as well as good old fashioned ground glass. I would have to say, IMO the only type of screen that someone can make that will be anywhere near a professional screen is ground glass (sand blasted glass is effectively just the same). Just buy some grinding paste and off you go with some 1500 grade W&D sandpaper, making sure that there is more relief in the centre of the screen if you suffer from very bad hot spotting, if hot spotting is not really a problem, grind the glass evenly all over. If you do use ground glass, you don't need a frsnell, as the ground glass is doing the necessory diffusing.


Hope this helps you all.

http://www.diyaudio.com/forums/diy-proj ... post314747" onclick="window.open(this.href);return false;

[tritosine5G]

I have a finnish projection screen technician on Skype now. Is there anything you want me to ask ?

[mayaman]

Palmer no pics? I have to see this.

[ERP]

What about that foam that comes in a spary can used to make car seats fitted to your shape of insulation foam in a can.
Could you spray out a bunch of that into a tub and then press a balloon into it, let dry then pop balloon ? Or spray around outside of balloon like you were going to shave it with shaving cream, let dry..and POP balloon ?

I Actually really like this idea, and might give it a try, you'd be better off using a decent 2 part foam though, the problem with the 1 part canned foams is they never really stop expanding.
You'd still have to use bondo or something to get a smooth surface, the cell density is usually too uneven to get a smooth finish from the foam itself.

[PalmerTech]

Palmer no pics? I have to see this.

I am out of town at the moment, I left a few days ago, no time to properly set it up. I promise to take some pictures when I get back, though.