JSON Dump Specification

Limelight’s JSON results output is a collection of arrays containing targeting results. Separate arrays exist for retroreflective, fiducial, neural classifier, and neural detector results. JSON is human-readable and easy to parse in any language on any platform, so it is perfect for most use-cases. The same JSON dump may be retrieved using the REST/HTTP, Websocket, and NetworkTables APIs. Parsing this JSON output is incredibly easy with our FRC libraries.

Limelightlib has built-in functionality that parses the JSON Dump into a LimelightResults object.

https://github.com/LimelightVision/limelightlib-wpijava

https://github.com/LimelightVision/limelightlib-wpicpp

Every JSON result contains the following entries:

pID	Current pipeline index
tl	Targeting latency (milliseconds consumed by tracking loop this frame)
cl	Capture latency (milliseconds between the end of the exposure of the middle row to the beginning of the tracking loop)
ts	Timestamp in milliseconds from boot.
v	Validity indicator. 1 = valid targets, 0 = no valid targets
botpose	Botpose (MegaTag): x,y,z, roll, pitch, yaw (meters, degrees)
botpose_wpired	Botpose (MegaTag, WPI Red driverstation): x,y,z, roll, pitch, yaw (meters, degrees)
botpose_wpiblue	Botpose (MegaTag, WPI Blue driverstation): x,y,z, roll, pitch, yaw (meters, degrees)

Retroreflective Results

The “Retro” array contains entries with the following structure:

pts	Individual corner points as an array of {x,y} in pixels. Center-zero, positive right and down. Must be enabled.
t6c_ts	Camera Pose in target space as computed by solvepnp (x,y,z,rx,ry,rz)
t6r_fs	Robot Pose in field space as computed by solvepnp (x,y,z,rx,ry,rz)
t6r_ts	Robot Pose in target space as computed by solvepnp (x,y,z,rx,ry,rz)
t6t_cs	Target Pose in camera space as computed by solvepnp (x,y,z,rx,ry,rz)
t6t_rs	Target Pose in robot space as computed by solvepnp (x,y,z,rx,ry,rz)
ta	The size of the target as a percentage of the image (0-1)
tx	X-coordinate of the center of the target in degrees. Positive-right, center-zero
txp	X-coordinate of the center of the target in pixels. Positive-right, center-zero
ty	Y-coordinate of the center of the target in degrees. Positive-down, center-zero
typ	Y-coordinate of the center of the target in pixels. Positive-down, center-zero

{
    "Results": {
        "Classifier": [],
        "Detector": [],
        "Fiducial": [],
        "Retro": [
        {
            "pts": [],
            "t6c_ts": [],
            "t6r_fs": [],
            "t6r_ts": [],
            "t6t_cs": [],
            "t6t_rs": [],
            "ta": 0.028124958276748657,
            "tx": -6.858895301818848,
            "txp": 125.89512634277344,
            "ty": 8.712546348571777,
            "typ": 79.79258728027344
        }
        ],
        "pID": 0,
        "tl": 4.795886039733887,
        "ts": 4524421.206346,
        "v": 1
    }
}

AprilTag/Fiducial Results

The “Fiducial” array contains entries with the following structure:

fid	Fiducial tag ID
fam	Fiducial Family (16H5C, 25H9C, 36H11C, etc)
pts	Return individual corner points. Must be enabled.
skew	Currently unused
t6c_ts	Camera Pose in target space as computed by this fiducial (x,y,z,rx,ry,rz)
t6r_fs	Robot Pose in field space as computed by this fiducial (x,y,z,rx,ry,rz)
t6r_ts	Robot Pose in target space as computed by this fiducial (x,y,z,rx,ry,rz)
t6t_cs	Target Pose in camera space as computed by this fiducial (x,y,z,rx,ry,rz)
t6t_rs	Target Pose in robot space as computed by this fiducial (x,y,z,rx,ry,rz)
ta	The size of the target as a percentage of the image (0-1)
tx	X-coordinate of the center of the target in degrees. Positive-right, center-zero
txp	X-coordinate of the center of the target in pixels. Positive-right, center-zero
ty	Y-coordinate of the center of the target in degrees. Positive-down, center-zero
typ	Y-coordinate of the center of the target in pixels. Positive-down, center-zero

{
    "Results": {
        "Classifier": [],
        "Detector": [],
        "Fiducial": [
        {
            "fID": 2,
            "fam": "16H5C",
            "pts": [],
            "skew": [],
            "t6c_ts": [
            0.33247368976801916,
            -0.05672695778305914,
            -2.5042031405987144,
            -4.680849607956358,
            -5.171154989721864,
            4.528697946312339
            ],
            "t6r_fs": [
            4.738896418276903,
            -1.5926603672041666,
            0.5194469577830592,
            4.522658587661256,
            4.258580454853879,
            5.5236539893713275
            ],
            "t6r_ts": [
            0.33247368976801916,
            -0.05672695778305914,
            -2.5042031405987144,
            -4.680849607956358,
            -5.171154989721864,
            4.528697946312339
            ],
            "t6t_cs": [
            -0.09991902572799474,
            -0.1234042720218289,
            2.5218203039582496,
            4.278368708252767,
            5.508508005282244,
            -4.1112864453027775
            ],
            "t6t_rs": [
            -0.09991902572799474,
            -0.1234042720218289,
            2.5218203039582496,
            4.278368708252767,
            5.508508005282244,
            -4.1112864453027775
            ],
            "ta": 0.005711808800697327,
            "tx": -2.0525293350219727,
            "txp": 149.4874725341797,
            "ty": 2.7294836044311523,
            "typ": 107.14710235595703
        }
        ],
        "Retro": [],
        "pID": 0,
        "tl": 19.78130340576172,
        "ts": 3284447.910569,
        "v": 1
    }
}

Neural Detector Results

The “Detector” array contains entries with the following structure:

class	Human-readable class name string
classID	ClassID integer
conf	Confidence of the predicition
pts	Individual corner points as an array of {x,y} in pixels. Center-zero, positive right and down. Must be enabled.
ta	The size of the target as a percentage of the image (0-1)
tx	X-coordinate of the center of the target in degrees. Positive-right, center-zero
txp	X-coordinate of the center of the target in pixels. Positive-right, center-zero
ty	Y-coordinate of the center of the target in degrees. Positive-down, center-zero
typ	Y-coordinate of the center of the target in pixels. Positive-down, center-zero

{
    "Results": {
        "Classifier": [],
        "Detector": [
        {
            "class": "person",
            "classID": 0,
            "conf": 0.83984375,
            "pts": [],
            "ta": 0.2608712911605835,
            "tx": -2.45949649810791,
            "txp": 147.5,
            "ty": -10.066887855529785,
            "typ": 165.5
        }
        ],
        "Fiducial": [],
        "Retro": [],
        "pID": 0,
        "tl": 63.50614547729492,
        "ts": 4932985.266867,
        "v": 1
    }
}

Neural Classifier Results

The “Classifier” array contains entries with the following structure:

class	Human-readable class name string
classID	ClassID integer
conf	Confidence of the predicition

{
    "Results": {
        "Classifier": [
        {
            "class": "digital clock",
            "classID": 531,
            "conf": 0.16796875
        }
        ],
        "Detector": [],
        "Fiducial": [],
        "Retro": [],
        "pID": 0,
        "tl": 16.704740524291992,
        "ts": 4751332.7542280005,
        "v": 1
    }
}