> For the complete documentation index, see [llms.txt](https://docs.cb21829.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.cb21829.org/ftc-java-advanced-guide/computer-vision-apriltag-and-opencv.md).

# Computer vision: AprilTag and OpenCV

Vision lets the robot see the field, detect game elements, and localize from AprilTags.

### AprilTag with VisionPortal

The SDK includes **VisionPortal** and **AprilTagProcessor**.

```java
import org.firstinspires.ftc.vision.VisionPortal;
import org.firstinspires.ftc.vision.apriltag.AprilTagProcessor;
import org.firstinspires.ftc.vision.apriltag.AprilTagDetection;

AprilTagProcessor aprilTag;
VisionPortal visionPortal;

aprilTag = AprilTagProcessor.easyCreateWithDefaults();

visionPortal = new VisionPortal.Builder()
        .setCamera(hardwareMap.get(WebcamName.class, "Webcam 1"))
        .addProcessor(aprilTag)
        .build();

for (AprilTagDetection detection : aprilTag.getDetections()) {
    if (detection.metadata != null) {
        telemetry.addData("Tag ID", detection.id);
        telemetry.addData("Range (in)", detection.ftcPose.range);
        telemetry.addData("Bearing (deg)", detection.ftcPose.bearing);
        telemetry.addData("Yaw (deg)", detection.ftcPose.yaw);
    }
}
```

AprilTag alignment is one of the highest-value autonomous techniques available.

### Custom pipelines with EasyOpenCV

Use EasyOpenCV when you need to detect colors or shapes that are not AprilTags.

```java
import org.openftc.easyopencv.OpenCvPipeline;
import org.opencv.core.*;
import org.opencv.imgproc.Imgproc;

public class ColorDetectionPipeline extends OpenCvPipeline {
    private volatile String detectedSide = "UNKNOWN";

    @Override
    public Mat processFrame(Mat input) {
        Mat hsv = new Mat();
        Imgproc.cvtColor(input, hsv, Imgproc.COLOR_RGB2HSV);

        Mat mask = new Mat();
        Core.inRange(hsv, new Scalar(0, 100, 100), new Scalar(15, 255, 255), mask);

        Rect leftROI = new Rect(0, 0, input.width() / 3, input.height());
        double leftAmount = Core.sumElems(mask.submat(leftROI)).val[0];

        detectedSide = (leftAmount > 1_000_000) ? "LEFT" : "CENTER_OR_RIGHT";

        Imgproc.rectangle(input, leftROI, new Scalar(0, 255, 0), 4);

        hsv.release();
        mask.release();
        return input;
    }

    public String getDetectedSide() { return detectedSide; }
}
```

### Practical tips

* use HSV instead of RGB for color thresholding
* always release `Mat` objects
* tune on the real field under real lighting
* read the latest result each loop instead of blocking


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