Cylindrical Markers

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Cylindrical markers are easily fabricated from cardboard mailing tubes and construction paper. The most basic type is the bi-color marker. Using two vertically aligned distinct color strips helps to distinguish markers from random background clutter.

Where to get supplies

You can purchase 2.5 or 3 inch diameter white mailing tubes from office supply stores (Staples, Office Depot), from FedEx or UPS stores, or from art supply stores. A 2.5"x24" tube costs around $2 and can be cut in half to make two landmarks.

Construction paper can be purchased from art supply stores, or Amazon.com. The important thing is to choose paper that is not shiny, because specular reflection makes shiny paper look white to the robot's camera. One brand we know works well is Rainbow Construction Paper manufactured by Pacon Corporation; a 10 color assortment can be purchased from Amazon for around $7.

Assembly

Cut the mailing tube to a length of 10 inches. Throw away the endcaps.

Cut 2 inch wide strips of construction paper for each color. If you're using a 2.5 inch diameter tube, your strips should be at least 8.25 inches long; for a 3 inch diameter tube the strips should be 10 inches long.

Wrap the strip tightly around the tube; the ends should overlap by about half an inch. Tape the ends together using clear tape. The strip should be tight enough to hold its position on the tube, but you should still be able to slide it up or down with gentle pressure. Attach the second strip above or below the first one on the tube.

For the BiColorMarkerData class, the widths of the color strips don't matter, but what is crucial is the location where the two strips meet. This point should be exactly 7 inches from the bottom end of the tube. You can change this by modifying the markerHeights[biColorMarkerType] entry of your MapBuilderRequest, or the static variable MapBuilderRequest::defaultMarkerHeight.

Testing

Use the SegCam viewer to check that your color bands are visible to the robot.

Use Root Control > Mode Switch > Navigation Demos > LookForBiColorMarkers to try out marker detection. Open the camera-space SketchGUI to see the markers. Control-click on camFrame and rawY. Note that the top color strip determines the color of the marker symbol displayed in the SketchGUI.

Use the world space SketchGUI to check that the robot is estimating distance correctly based on the height of the point where the two color strips meet.