Having trouble with GRIP vision processing

Our team is trying to do vision processing using GRIP, and to do so, we are trying to calculate the centerX value of a rectangle around a contour, but the code for forming the rectangle keeps giving us an error:

ERROR 1 ERROR Unhandled exception: java.lang.IndexOutOfBoundsException: Index: 0, Size: 0 at [java.util.ArrayList.rangeCheck(Arraylist.java:638), java.util.ArrayList.get(ArrayList.java:414), org.usfirst.frc.team4013.robot.GripPipelinethree…process(GripPipelinethree.java:75), org.usfirst.frc.team4013.robot.Robot.teleopPeriodic(Robot.java:110), edu.wpi.first.wpilibj.IterativeRobot.startCompetition(IterativeRobot.java:130), edu.wpi.first.wpilibj.RobotBase.main(RobotBase.java:247)] edu.wpi.first.wpilibj.RobotBase.main(RobotBase.java:249)

I think the error is showing that the rectangle is being constructed with a width of zero, and we don’t know why.

If I’m wrong about the error, or if anyone sees anything in the code that could be messing up the rectangle, please let me know.

The code for the GRIP pipeline is below(that is the bit of code that keeps giving us the error, we think):

package org.usfirst.frc.team4013.robot;

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.HashMap;
import org.opencv.imgproc.Imgproc;

import edu.wpi.first.wpilibj.networktables.NetworkTable;
import edu.wpi.first.wpilibj.vision.VisionPipeline;
import edu.wpi.first.wpilibj.vision.VisionRunner;
import edu.wpi.first.wpilibj.vision.VisionThread;

import org.opencv.core.*;
import org.opencv.core.Core.*;
import org.opencv.features2d.FeatureDetector;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.*;
import org.opencv.objdetect.*;

* GripPipeline class.
* <p>An OpenCV pipeline generated by GRIP.
* @author GRIP
public class GripPipeline implements VisionPipeline {
	private VisionThread visionThread;
	private double centerX = 0.0;

	private Mat hsvThresholdOutput = new Mat();
	private ArrayList<MatOfPoint> findContoursOutput = new ArrayList<MatOfPoint>();
	public ArrayList<MatOfPoint> filterContoursOutput = new ArrayList<MatOfPoint>();
	static {

	 * This is the primary method that runs the entire pipeline and updates the outputs.
	@Override	public void process(Mat source0) {
		// Step HSV_Threshold0:
		Mat hsvThresholdInput = source0;
		double] hsvThresholdHue = {36.66666666666666, 46.66666666666666};
		double] hsvThresholdSaturation = {0.0, 5.0};
		double] hsvThresholdValue = {245.0, 255.0};
		hsvThreshold(hsvThresholdInput, hsvThresholdHue, hsvThresholdSaturation, hsvThresholdValue, hsvThresholdOutput);

		// Step Find_Contours0:
		Mat findContoursInput = hsvThresholdOutput;
		boolean findContoursExternalOnly = false;
		findContours(findContoursInput, findContoursExternalOnly, findContoursOutput);

		// Step Filter_Contours0:
		ArrayList<MatOfPoint> filterContoursContours = findContoursOutput;
		double filterContoursMinArea = 1000.0;
		double filterContoursMinPerimeter = 0;
		double filterContoursMinWidth = 0;
		double filterContoursMaxWidth = 1000;
		double filterContoursMinHeight = 0;
		double filterContoursMaxHeight = 100.0;
		double] filterContoursSolidity = {0.0, 100};
		double filterContoursMaxVertices = 1000000;
		double filterContoursMinVertices = 0;
		double filterContoursMinRatio = 0;
		double filterContoursMaxRatio = 1000;
		filterContours(filterContoursContours, filterContoursMinArea, filterContoursMinPerimeter, filterContoursMinWidth, filterContoursMaxWidth, filterContoursMinHeight, filterContoursMaxHeight, filterContoursSolidity, filterContoursMaxVertices, filterContoursMinVertices, filterContoursMinRatio, filterContoursMaxRatio, filterContoursOutput);
		Rect r = Imgproc.boundingRect(filterContoursOutput().get(0)); 
            centerX = r.x + (r.width / 2);
		NetworkTable.getTable("GRIP").putNumber("centerX", centerX);

	 * This method is a generated getter for the output of a HSV_Threshold.
	 * @return Mat output from HSV_Threshold.
	public Mat hsvThresholdOutput() {
		return hsvThresholdOutput;

	 * This method is a generated getter for the output of a Find_Contours.
	 * @return ArrayList<MatOfPoint> output from Find_Contours.
	public ArrayList<MatOfPoint> findContoursOutput() {
		return findContoursOutput;

	 * This method is a generated getter for the output of a Filter_Contours.
	 * @return ArrayList<MatOfPoint> output from Filter_Contours.
	public ArrayList<MatOfPoint> filterContoursOutput() {
		return filterContoursOutput;

	 * Segment an image based on hue, saturation, and value ranges.
	 * @param input The image on which to perform the HSL threshold.
	 * @param hue The min and max hue
	 * @param sat The min and max saturation
	 * @param val The min and max value
	 * @param output The image in which to store the output.
	private void hsvThreshold(Mat input, double] hue, double] sat, double] val,
	    Mat out) {
		Imgproc.cvtColor(input, out, Imgproc.COLOR_BGR2HSV);
		Core.inRange(out, new Scalar(hue[0], sat[0], val[0]),
			new Scalar(hue[1], sat[1], val[1]), out);

	 * Sets the values of pixels in a binary image to their distance to the nearest black pixel.
	 * @param input The image on which to perform the Distance Transform.
	 * @param type The Transform.
	 * @param maskSize the size of the mask.
	 * @param output The image in which to store the output.
	private void findContours(Mat input, boolean externalOnly,
		List<MatOfPoint> contours) {
		Mat hierarchy = new Mat();
		int mode;
		if (externalOnly) {
			mode = Imgproc.RETR_EXTERNAL;
		else {
			mode = Imgproc.RETR_LIST;
		int method = Imgproc.CHAIN_APPROX_SIMPLE;
		Imgproc.findContours(input, contours, hierarchy, mode, method);

	 * Filters out contours that do not meet certain criteria.
	 * @param inputContours is the input list of contours
	 * @param output is the the output list of contours
	 * @param minArea is the minimum area of a contour that will be kept
	 * @param minPerimeter is the minimum perimeter of a contour that will be kept
	 * @param minWidth minimum width of a contour
	 * @param maxWidth maximum width
	 * @param minHeight minimum height
	 * @param maxHeight maximimum height
	 * @param Solidity the minimum and maximum solidity of a contour
	 * @param minVertexCount minimum vertex Count of the contours
	 * @param maxVertexCount maximum vertex Count
	 * @param minRatio minimum ratio of width to height
	 * @param maxRatio maximum ratio of width to height
	private void filterContours(List<MatOfPoint> inputContours, double minArea,
		double minPerimeter, double minWidth, double maxWidth, double minHeight, double
		maxHeight, double] solidity, double maxVertexCount, double minVertexCount, double
		minRatio, double maxRatio, List<MatOfPoint> output) {
		final MatOfInt hull = new MatOfInt();
		for (int i = 0; i < inputContours.size(); i++) {
			final MatOfPoint contour = inputContours.get(i);
			final Rect bb = Imgproc.boundingRect(contour);
			if (bb.width < minWidth || bb.width > maxWidth) continue;
			if (bb.height < minHeight || bb.height > maxHeight) continue;
			final double area = Imgproc.contourArea(contour);
			if (area < minArea) continue;
			if (Imgproc.arcLength(new MatOfPoint2f(contour.toArray()), true) < minPerimeter) continue;
			Imgproc.convexHull(contour, hull);
			MatOfPoint mopHull = new MatOfPoint();
			mopHull.create((int) hull.size().height, 1, CvType.CV_32SC2);
			for (int j = 0; j < hull.size().height; j++) {
				int index = (int)hull.get(j, 0)[0];
				double] point = new double] { contour.get(index, 0)[0], contour.get(index, 0)[1]};
				mopHull.put(j, 0, point);
			final double solid = 100 * area / Imgproc.contourArea(mopHull);
			if (solid < solidity[0] || solid > solidity[1]) continue;
			if (contour.rows() < minVertexCount || contour.rows() > maxVertexCount)	continue;
			final double ratio = bb.width / (double)bb.height;
			if (ratio < minRatio || ratio > maxRatio) continue;


I believe the error is indicating that your list is not actually populated, meaning that there are no contours being found and placed into that list. I’m not sure exactly what would be causing that, but make sure to check your parameters and tune them accordingly.

In GRIP, it’s finding the contours with the same parameters that we’ve set, and everything seems to be working properly aside from the GRIP pipeline in the Eclipse code, but we’ll look into it, thanks.

This line is absolutely the problem:

Rect r = Imgproc.boundingRect(filterContoursOutput().get(0)); 

You have to do bounds checking before getting data from lists

Judging by the stacktrace, it looks like you’re also calling the vision processing code directly from teleopPeriodic… do not do this. This kind of vision processing will take about 80ms, or four times longer than the refresh rate of the teleopPeriodic function. Your robot will very rarely respond to user input, or sensors, or update motors, or…

You need to read this ScreenSteps page and understand it.

Thanks, we actually just fixed the problem of not checking the pipeline first, and we aren’t running this vision processing code in teleop, just in autonomous.