I've used a couple of different makes of projection screen wall paint. The best has been
Screen Goo.
The types of projectors I was using it with were very prone to hot-spoting. This a bright reflection in the image. You get the same effect where a light can be seen in reflection on a glossy TV or PC screen. No matter where you move the bright reflection is always there. Screen Goo is the closest I've found so far to a proper projection surface where it is matt-enough to minimise hot-spotting but still gives good reflective properties for the image. Don't ask me how it does that. I think is must be some kind of sorcery
. Other things I noticed were that the colours were very good (no colour shift in the paint) and the viewing angle was pretty wide like a decent fibreglass screen.
I had to do a big painted screen for a lecture theatre installation. For that I used Smarter Surfaces paint as it fit the budget for the size of area to be covered.
In both cases when the projector is off and the wall viewed under normal lighting it's pretty obvious where the screen area is painted compared to the rest of a freshly painted white wall. The right reflected off the surface has a different quality compared to normal matt or eggshell paint. I mention this because if you want the wall to look "normal" then be prepared to cost up for painting the whole surface rather than just the portion that will be the screen area.
There's a few different commercial screen paints so have a look and read up. You'll also come across the DIYers using Dulux Ice Storm or some similar name as a cheap solution. By all means give it a try if you're on a budget. Bear in mind though that it's not designed to be a screen paint, so it won't look as sharp or reflect the light as accurately, but that might not matter so much if you're just doing projection as a "big telly alternative". You might want to check out though how it works with ultra-short-throw projectors because that could change things and not for the better.
Regarding projectors, there are different classes of lens. What you saw at CES was probably an ultra-short-throw machine (UST). They sit within a couple of feet from the wall/screen and throw a 100" diagonal 16:9 image. The lens arrangement required to make this happen isn't cheap. It has to maintain sharp focus and even image brightness over such a large area, so the cost of the lenses is high if you want something very close to the wall that also does a big projected image at HD or 4K UHD resolutions. Here's the
Epson EH-LS500B at roughly £2400 (YT video of projector in use being tested by HDTVTest)
No doubt you'll search out some other UST options, and in doing so you'll find machines under a grand. These are mostly for education use in classrooms with smart board. They're a teaching aid; a projected touchscreen product with a typical screen size of 60". The projector resolutions are low by home video standards. They're as low as XGA (1024 x 768) or WXGA (1280 x 768). This isn't what you want for home entertainment. Also the lenses are made to a cost to work with these smaller classroom screen sizes, or the brightness uniformity and sharpness isn't so critical on a smaller image but would start to look poor if you tried them at a larger size. The bottom line is you get what you pay for.
The next lens class up is Short Throw projectors (ST). This is typified by the gaming and home entertainment projectors. You'll find quite a bit of choice here between £300 and £2000. The Epson EF-11 and Optoma HD29HST are both a decent place to start getting to know the capabilities. Each is £800. They require more throw distance than the USTs, but the lenses don't cost as much so you get more for your money.
When you're looking at these you'll notice there's a number for throw ratio. This can be used to work out the screen size for any given throw distance. The EF-11 has a throw ratio of 1:1. That means for every metre back from the wall the screen width is a metre wider. Two mtrs back = 2m wide image, 3m = 3m and so on. The Optoma is 0.49:1, but lets just call it 0.5 for easy maths. For 1m back the image width is 1/0.5 which gives 2m. In simpler terms it's throw distance x2 = image width. e.g. 2m throw = 4m width
Image width is the distance from side to side parallel to the floor, like you measure a room length and width. This is
not diagonal. To work out the diagonal, take your image width and then first divide by 16, then multiply that result by 18.35. (If you want to know the image height, take the width then divide by 16 and multiply by 9.)
Converting centimetres to inches is done by dividing by 2.54
Once you have a grasp of using the throw ratio number you can work out the placement of a projector or its image size. This goes for projectors with a zoom lens too. The zoom lens can be used to adjust either throw distance or image size. As such, the specs for a projector with a zoom lens will read something like this: 1.1~1.4:1 This gives the minimum and maximum throw ratio. In other word you do two sums, one for each end of the range. In this example the image size range at 3m throw would be 3/1.1 = 2.72m and 3/1.4 = 2.14m.
Standard throw and long throw projectors are the ones where you'll start to get useful set-up features such as lens shift. This means that a projector can be positioned towards the back of the room and high up but throw an image which is positioned lower on the opposite wall.
