AprilTag Metadata

Introduction

A Library tag stores Metadata, a collection of at least 4 fields (of these Blocks/Java types):

  • ID code (number/int)

  • tag name (text/String)

  • tag size (number/double)

  • unit for tag size and estimated position (DistanceUnit INCH, MM, CM, METER)

Two optional Metadata fields are described at the Advanced Use page.

The full use of Metadata Blocks and Java methods is covered at the Library page. For now it’s enough to know the 4 basic elements of Metadata.

Tag Contents

The SDK 8.2 Sample OpModes use AprilTags with these Metadata values:

  • 583, Nemo, 4, DistanceUnit.INCH

  • 584, Jonah, 4, DistanceUnit.INCH

  • 585, Cousteau, 6, DistanceUnit.INCH

  • 586, Ariel, 6, DistanceUnit.INCH

These four are available with the getSampleTagLibrary() Block or Java method.

After Kickoff in September 2023, the CENTERSTAGE game tags will be available with getCenterStageTagLibrary(). That Library currently contains 3 placeholder tags: MEOW (ID 0), WOOF (ID 1) and OINK (ID 2).

Before and after Kickoff, a call to getCurrentGameTagLibrary() will provide both sets of tags.

These three Libraries are discussed further at the Library page.

Tag Names

A tag name, whether default or custom, can be retrieved as follows:

Tag Names

Example of Reading AprilTag Names

Example of retrieving AprilTag Name

AprilTagDetection myAprilTagDetection;
String myAprilTagName;
myAprilTagName = myAprilTagDetection.metadata.name;

As with tag ID code, the tag name is usually retrieved inside a for() loop, for immediate processing or stored for later use. See the Initialization page for sample for() loop code.

Unlike tag ID code, a detected AprilTag might have no tag name – if it was not placed into the Library by default or with the custom Builder pattern.

To avoid logic errors, an OpMode can check the Metadata for a null condition before attempting to process a tag name. This is illustrated in these Sample OpModes:

  • Blocks: ConceptAprilTag

  • Java: ConceptAprilTag.java

Questions, comments and corrections to westsiderobotics@verizon.net