Projector Brightnesses Dilemma

[Daveman]

I'm planning a show which requires rear-projection onto a 7m x 3m screen at the back of the stage. Due to us being an amateur theatre group, we're unable to pay for a high-quality, high-brightness projector. We may be able to borrow however, some business/standard brightness projectors, of about 2000 lumens. However I'm worried about the brightness level. We're projecting onto Rosco projection material. We wanted to use black material, but we may have to use white if the projection brightness is not high enough. I'm planning to do some testing and work out whether this is possible. However even with a white screen I'm worried the image won't be bright enough.

 

My potential idea for a solution to this would be to use two projectors with the edges aligned to create a larger image. Theoretically then, if one projector would provide 2000 lumens of brightness over a 7m x 3m area, using two projectors covering half the area each would provide 4000 lumens over the whole area.

 

Obviously not an ideal solution, as the edges will be tricky to line up and then the output to the projectors must be split over two display devices. But it seems to be the only thing I can think of at the moment.

 

This seems to make sense to me, however can anyone more experienced offer any criticisms of this idea or alternative solutions?

 

A concurrent post has been automatically merged from this point on.

 

Sorry for the triple post! Late night and a slow internet connection...

[steveo]

I am sure one of the video guys will be able to advise you better but my understanding is that you are attempting a soft edge blend without the software. I don't think this will double your output as you are still coming from a 2000 ansi lumen source.

 

Also why are you going for a black or white RP screen? If you are going to buy the material from Rosco have a look at their Grey RP material this will give a higher transmission than the White. Alternativley contact Harkness Screens and talk to them about their Translite range, explain what you are trying to do and they will advise on the best product to match, before you ask they will not advise their most expensive material they will go for what will work best.

 

My thuoghts are that you need a more powerful projector I am afraid but am sure other will advise better

 

Steve

[revbobuk]

Superimposing lower brightness projectors is possible - I did a conference this way a while back with two 3000 lumen 'office standard' projectors - but it is mighty hard work. As the projector lenses are offset, you need to adjust keystoning to compensate, and you need to get the images perfectly aligned over the whole of the acreen area. You do gain brightness, although unless you are pixel-perfect it won't double.

 

Use two identical projectors, and I found that the NEC image shape adjustments were more flexible than some other manufacturers.

 

But 2000 lumens is really quite a dim projector these days. You would be better off trying to borrow - or even hire - a brighter projector. 7m x 3m is a big screen - you'll clearly gain a lot if you can settle for a smaller area. It's also the case that most business projectors are 4:3 native format, so your screen would waste a lot of the image. If you must have 7x3, then hire a wide format projector!

[obsoperator]

Your project is routine for the professional "speaker support" business (i.e. meetings and conferences) in large auditoriums today. It has the added benefit of keeping the show going if one of the projectors fails, which is very important to the clients. I've seen it done regularly with stacked projectors weighing up to 200 pounds each.

 

Your challenge is to get the lenses of the two projectors as close together as possible (that's why stacking is often preferable to side-by-side), and to hold both of them steady. Years ago, Kodak Carousel stands were developed that permitted adjusting two or more physically close projectors independently, without disturbing one that isn't being tipped. You should try very hard to get two identical model projectors with identical lenses, or you will have a nasty time.

 

Also try to get a video generator with an alignment pattern. The fact is, you don't care about the edges of the images, you care about the center the most. I assume you have a way to replicate the signal.

[Daveman]

But 2000 lumens is really quite a dim projector these days. You would be better off trying to borrow - or even hire - a brighter projector. 7m x 3m is a big screen - you'll clearly gain a lot if you can settle for a smaller area. It's also the case that most business projectors are 4:3 native format, so your screen would waste a lot of the image. If you must have 7x3, then hire a wide format projector!

 

It would be great to use a proper wide format, high brightness projector. But our budget won't take that unfortunately. We've got to have 7 x 3 because that's the size of the gap between the two columns in the venue, and the projection is supposed to act as the scene backdrops. We'll be preparing the film and backdrops to the aspect ratio of 1:2.33 to entirely fill the space. 2000 lumens is fairly poor but it's a case of the best we can get.

 

Also why are you going for a black or white RP screen? If you are going to buy the material from Rosco have a look at their Grey RP material this will give a higher transmission than the White. Alternativley contact Harkness Screens and talk to them about their Translite range, explain what you are trying to do and they will advise on the best product to match, before you ask they will not advise their most expensive material they will go for what will work best.

 

Ideally we want a black screen because the projected images are mostly going to be sketchy white lines, and we wanted the contrast between them and the black background. Thanks for the tip though, I'll get in touch with Harkness and see if they have anything that would work better and fit within our budget.

[revbobuk]

Your project is routine for the professional "speaker support" business (i.e. meetings and conferences) in large auditoriums today. It has the added benefit of keeping the show going if one of the projectors fails, which is very important to the clients. I've seen it done regularly with stacked projectors weighing up to 200 pounds each.

 

But there's a big difference in the projectors. Business class projectors often have this capability designed in, whereas the linearity of cheaper low-end projectors leaves a lot to be desired. Often, higher end projectors have lens-shift capabilities, which makes the whole thing so much easier.

 

Having said all that, it's not impossible, particularly if all you want to project is a backdrop, rather than stuff (like text) which needs to be kept very sharp.

 

And I'd agree - stacked one above the other is much easier than side-by-side. I built a support frame myself to do just this, and it worked well.

[stillwave]

Another tip is to have a look in the menu of the projector, some have a feature referred to as 'Eco mode' turning this off will gain you a few extra lumens... but sacrificing lamp life a little!

 

I would second overlaying projectors, we have done this but to compare projectors to convince someone that the projector they had been given had a terrible contrast ratio.

 

Regards,

 

Ben Wainwright

[Daveman]

I may not have been especially clear on this: what I was suggesting was instead of having one projector filling the whole 7 x 3 m area, having two projectors each filling half of the area. This would give us double the horizontal resolution. Imagine joining two 4:3 screens together. I was not suggesting overlaying the projectors images completely, although this perhaps could be an additional possibility. Any ideas on which method would work better?

[David Lee]

I read this as edge blending the two projectors, not doubling them. With edge blending you need to have some kit to blend the two projectors together or you will see the join. Just worked on one yesterday where a Diventix was used as it can blend by making the preview a 2nd output.

 

As said elsewhere and will apply with a blend as well - 2 x identical projectors are needed (pref with similar lamp life) so as to aid getting colour balance, geometry right.

 

As also stated above, 2000 lumens isn't a lot of output, make sure your stage lights don't hit the screen.

[lobba]

I may not have been especially clear on this: what I was suggesting was instead of having one projector filling the whole 7 x 3 m area, having two projectors each filling half of the area. This would give us double the horizontal resolution. Imagine joining two 4:3 screens together. I was not suggesting overlaying the projectors images completely, although this perhaps could be an additional possibility. Any ideas on which method would work better?

 

 

Hi Daveman,

 

What you are suggesting does have a lot of merit in terms of brightness and resolution, however you will come up with some fundamental problems. When you 'butt' up two images, even from the same graphics card (I presume your source is a PC) you get synchronicity problems. Imagine a horizontal line running up and down the whole image, where the outputs 'butt' together the line never quite meets unless it is still, one side is always behind the other, it is unfortunately a physical limitation of all graphics cards. You also lose the brightness of the part of the projected image that will end up off the side of the screen as two 4:3 images don't quite fit your screen, you will lose about 25% of the brightness which seem critical in this scenario.

 

The most common way of getting rid of this is soft edge blending which is why most of the guys on the forum have mentioned it. The trouble is with soft edge blending as you have pointed out is budget, most pieces of kit that can generate a soft edge and overlap are very expensive. Watchout, Spyder, Encore etc all run into many thousands of pounds. You do have an option to use the workstation Nvidia Quadro FX graphics cards though. Again these can be very expensive but you can get them second hand...

 

http://www.amazon.co.uk/gp/offer-listing/B...;condition=used

 

Here is a compatible card second hand for only £150, this will do your overlap and softedge for very little spend. This means you can use the extra brightness and resolution of two projectors.

 

As an example.... If you use 2x 1024x768 projectors

You have a screen height of 3m giving you a single screen width of 4m. Fitting 2 4m images on your 7m wide screen give you a very acceptable overlap of 1m or 25%.

This give you an on screen resolution of 1792x768.

 

It is also worth noting that soft edge blending can hide or at least reduce a whole load of line up issues if you don't have anyone very experienced with projection.

 

Here is a link to edge blending with Nvidia cards

http://www.nvidia.com/object/IO_29134.html

 

That is probably the way I would approach your project with a limited budget.

 

Hope that helps

Lobba

[Daveman]

Thanks everyone for your advice. I've looked into soft-edge blending and clearly this is what I need to do. Unfortunately I'm currently planning to run the show off my macbook pro, so the Nvidia card isn't an easy option. I could potentially try and beg/borrow a PC from somewhere, but it's another thing to worry about.

 

I've seen quite a few software implementations of soft-edge blending, for example

. This is using the matrox dual head2go. This would be a better option for me, as its something that I would be willing to personally buy and therefore doesn't have to come out of the show budget. This hardware uses the host computers graphics card to do its processing, but for this show the projection is only showing images and the occasional video, there is no difficult 3D processing.

 

Would this solution work?

[quinch]

What you are suggesting does have a lot of merit in terms of brightness and resolution, however you will come up with some fundamental problems. When you 'butt' up two images, even from the same graphics card (I presume your source is a PC) you get synchronicity problems. Imagine a horizontal line running up and down the whole image, where the outputs 'butt' together the line never quite meets unless it is still, one side is always behind the other, it is unfortunately a physical limitation of all graphics cards.

 

If you're seeing this problem from a PC, I think you might find that Windows plays its part as well! Normally Windows will configure a multi-monitor setup as individual displays, each with independent control of bit depth/resolution etc. Under these circumstances you will nearly always find a noticeable lag or frame-dropping from the secondary output. You can usually coax Windows into running the card in a 'Stretched' or 'Horizontal span' mode where you end up with one desktop spread over the two outputs and this performs a lot better. This mode is not without its annoyances (dialogs appearing split across two screens etc) but at least you get both outputs running more or less in sync.

 

s.

[sam.hunt]

I would definitely suggest looking into Dual/Triple Head2Go's. I believe that QLab should work fine for a dual screen output, although I have never actually used it for such (I'm a Catalyst kinda guy). Grab it at HERE and give it a go.

 

As for edge-blending, you have a number of options. Firstly, there is the cheap and bodge job of using tape in front of the projector lenses to blur the edge of the image. Cheap, but not especially easy or even reliable.

 

If you decide to go down the QLab route, then you have two choices for edge-blending. You can render a feather in the sides of the clips - this would require a fair amount of trial and error to get the correct position and amount of blend. A much more elegant solution would to be to create a feathered image file - a PNG file that has a gradient from black to transparent. You can then have this on the top layer of video, meaning it would feather everything underneath it, and it is fully adjustable within QLab, so it shouldn't take too long to get it in the right place once it's all set up.

 

Personally, I'd go for the DualHead2Go and using a feather in QLab to achieve the blend. This solution should be a little under £300, and is completely reusable for any future projects. QLab also do rental options, which would cut down the cost. If you were willing to personally buy the DualHead, and use a rental license for QLab, there'd be very little required from the show budget.

[Daveman]

Wow, I hadn't seen QLab before. This looks like exactly what I need! I wasn't sure what software I was going to use as a video server but this will definitely work very well. Financially, I can get an analog dualhead2go for about £110, and then QLab rental would cost me a maximum of about £16. So that's perfectly affordable.

 

I notice that QLab supports Quartz Composer custom compositions. I found a composition to do edge blending (http://local.wasp.uwa.edu.au/~pbourke/text...lour/edgeblend/), so this is probably the easiest way to apply the edge blend.

 

One thing I'm thinking about though: I'm going to prepare the images and video to an aspect ratio the size of the screen (7x3m). The way the dualhead2go seems to work is by acting as one double-wide display and using hardware to split the image and direct each half to a different output. Because edge-blending is being used, there needs to be an overlap between the two projected images. So I need to get QLab to output one set of display data (to be split by the dualhead2go), which includes the edge blend and the overlap. How would I do that?

 

Thanks a lot for your advice so far I think I'm finally getting towards a solution here.

[Daveman]

Update and new query: I've managed to make a quartz composer patch to do the video edge-blend processing so that problem is sorted. Now I'm thinking about the screen again.

 

Someone suggested to me that instead of using a black screen, I could use a grey or white screen with a black gauze hanging in front. This would give the same visual appearance, but with a wider viewing angle and increased light transmission. Any comments on this idea? Would it work/be better than using a black screen?