Non-Salient AprilTags

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In the following page we introduce you to the non-salient tags that were utilized throughout this project for localization. The purpose of using non-salient tags was to allow for changes to the area we were localizing in to be unnoticeable.


How the Non-Salient Tags Work

A visible tag conveys data through two colors, black and white. We allow an infrared camera to see these colors in the NIR spectrum by shining an NIR emitter at the non-salient tag we wish to view. The retro-reflective parts of the tag shine light back to the source (with camera being very close to the source) such that the retro-reflective parts appear white. All other areas of the tag appear black. Therefore the tag appears as it should through the camera's view in order for the AprilTags module to recognize it. The following picture exhibits the retro-reflectivity of the non-salient tags with the left hand image showing the non-salient tags in the visible spectrum and the right hand image showing how the reflectivity of the tags allow the non-salient tags to appear just as a regular AprilTag would:


Here you can see what part of the spectrum the tags appear salient in, the NIR spectrum is not within the visible spectrum which allows for non-salience:


Our system utilizes a PS Eye camera modified for near infrared (NIR) vision with an IR LED ring that emits the IR light which is seen by the camera.


Using Non-Salient Tags

POLARIS can be utilized with both visible and non-salient tags. In order to use POLARIS with non-salient tags, one must have completed the following prerequisites:

Connect a Near-Infrared (NIR) Camera to the System

The camera must be capable of only viewing the NIR spectrum and must have an NIR emitter facing outward in the same direction that the camera is pointed attached to it. See the following tutorial for converting a visible PSEye to view NIR: Conversion of the PS Eye Camera to NIR

Create Non-Salient Tags

In order to create a non-salient tag, print an inverted AprilTag on a piece of paper with the color of the background that you wish to place the tag on. Put retro-reflective tape of the same color on the parts of the inverted tag that have black printed on them. Note that this method has been tested for the creation of white non-salient tags, therefore it is recommended that the user sticks to lighter colors. Here is an example of a finished non-salient tag for a white background:


Here one can see how non-salient tags look when interpreted using the Infrared PSEye Camera:


Run the Localization Module

Please see the following tutorial: Using the Localization Module