Step 6: Mount the projector above the sandbox

Just like with the Kinect camera, the Augmented Reality Sandbox is capable of dealing with arbitrary projector alignments. As long as there is some overlap between the Kinect camera's field-of-view and the projector's projection area, the two can be calibrated with respect to each other. However, for several reasons, it is best to align the projector carefully such that it projects perpendicularly to the flattened average sand surface. The main reason is pixel distortion: if the projection is wildly off-axis, the size of projected pixels will change sometimes drastically along the sand surface. While the Augmented Reality Sandbox can account for overall geometric distortion, it cannot change the size of displayed pixels, and the projected image looks best if all pixels are approximately square and the same size.

Some projectors, especially short-throw projectors, have off-axis projections, meaning that the image is not centered on a line coming straight out of the projection lens. In such cases, perpendicular projection does not imply that the projector is laterally centered above the sandbox; in fact, it will have to be mounted off to one side. The criterion to judge perpendicular projection is that the projected image appears as a rectangle, not a trapezoid.

We strongly recommend against using any built-in keystone correction a particular projector model might provide. The Augmented Reality Sandbox corrects for keystoning internally, and projector-based keystone correction works on an already pixelated image, meaning that it severely degrades image quality. Never use keystone correction. Align the projector as perpendicularly as possible, and let the Augmented Reality Sandbox handle the rest.

The second reason to aim for perpendicular projections is focus. Projector images are focused in a plane perpendicular to the projection direction, meaning that only a single line of the projected image will be in correct focus when a non-perpendicular projection is chosen. Either way, after the projector has been mounted, we recommend to focus it such that the entirety of the flattened average sand surface is as much in focus as possible.

Heads up!

On a tangential note, we also strongly recommend to only run projectors at their native pixel resolutions. Most projector models will support a wide range of input video formats to accommodate multiple uses, but using any resolution besides the one corresponding to the projector's image generator is a very bad idea because the projector will have to rescale the input pixel grid to its native pixel grid, which causes severe degradation in image quality. Some projectors "lie" about their capabilities to seem more advanced, resulting in a suboptimal resolution when using plug & play or automatic setups. It is always a good idea to check the projector's specification for its native resolution and ensure that the graphics card uses that resolution when the projector is connected.