Reply To: BenQ MX631S projection angle to sandbox

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Oliver Kreylos

The 105% figure means that, if your projected image is 1m tall, its bottom edge will be 5cm (5% of 1m) above the imaginary center line of the projector.

The projector lens is designed such that the image will be in focus everywhere if projected onto a flat screen that is at a right angle to that imaginary center line. Concretely, if you put the projector completely level onto a table and project onto a vertical wall, the image will sit 5% above the horizontal plane going through the lens center, and the entire image will be in focus.

The downside of above-centerline projection for AR Sandboxes is that the light at the front edge of the sandbox, where most users will stand, comes in at a fairly low angle, which reduces brightness, and can lead to shadowing when there are steep slopes facing away from the projector. A centerline projector directly above the sandbox’s center point would improve that.

The ECHO, TERC, and LHS setups use a compromise where the projector is shifted towards the center of the box, and tilted out to move the projection image back to the center. This was necessary because the projector hood was designed too small.

There are two problems when a projection surface is not orthogonal to the projector’s centerline: keystone distortion and focus. Keystone distortion is effectively taken care of by the AR Sandbox’s calibration procedure, but focus remains. With a tilted projector, only a single horizontal line is in perfect focus; lines above and below that become increasingly blurry. It’s not a huge effect, but it is noticeable. Enough that I don’t recommend building an AR Sandbox with a tilted projector.

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