Our Build

Sandbox

The sandbox itself should have a 4:3 aspect ratio, to match the fields-of-view of the Kinect camera and the projector. The size of the sandbox is limited by the Kinect camera’s minimum and maximum sensing distances, and the desired sandbox resolution. Due to the Kinect camera’s approximately 90° field-of-view, the Kinect camera has to be mounted about as high above the sand surface as the sandbox is wide. The Kinect camera should be mounted directly above the sandbox’s center point, looking straight down (see Figure 1).

Figure 1: Typical arrangement of projector and Kinect camera above a 40″x30″ (1mx0.75m) AR Sandbox. The short-throw projector is mounted at the same height as the Kinect camera, but above to the rear long edge of the sandbox to account for its above-axis projection.

We recommend a sandbox size of 40″x30″ or 1mx0.75m, leading to a Kinect camera mounting height of approximately 40″ or 1m. At this height, the camera’s nominal horizontal resolution is 1.56mm, and its effective horizontal resolution is high enough to resolve features on the order of 5mm. Vertical resolution at the same height is 2.79mm. Increasing the size of the sandbox increases the required height of the camera/projector mount by the same factor, and not only reduces horizontal resolution, but also vertical resolution. In a 2mx1.5m sandbox, for example, nominal horizontal resolution is 3.12mm, and vertical resolution drops to 11.16mm (vertical resolution is roughly proportional to height squared).

Sand

The sandbox should be filled with sand to a depth of around 4″ or 10cm. At 40″x30″, this totals 2.77 cubic feet or 75dm³ or 75l of sand, weighing approximately 198lb or 98kg. We recommend Sandtastik White Play Sand, for between $15 and $25 per 25lb, or $120-$200 total. Sandtastik sand has excellent projection properties, but a shallow angle of repose when dry. We recommend keeping the sand slightly moist to make it moldable. Adding 1 cup or 0.25l of water to 198lb or 98kg of sand and mixing thoroughly is sufficient.

Health Concerns

Regular sand is basically crystalline silica, primarily in the form of quartz. While silica is non-toxic when ingested orally, inhaling very fine silica dust can cause adverse health effects.

The recommended Sandtastik play sand does not, according to the manufacturer, contain or release fine silica dust. Neither should alternatives such as Moon Sand or Kinetic Sand, as they are either not made from actual sand, mixed with a binding agent, or surface-treated with a polymer. Regular sand, such as bought in bulk from hardware or home improvement stores, should be washed before use to reduce the amount of fine dust particles contained in it. Here is a how-to guide on washing play sand (step 5, baking, is optional).

Projector

The projector is a simple Kinect gaming system component. We purchased it off of Amazon.