UPDATE
You can find all the images I have for testing on my GitHub here:
GitHub repository with sources
There are also 2 videos, where the detection should work on as well
ORIGINAL QUESTION
I tried to use OpenCV 4.x.x to find the edges of a blackboard (image following), but somehow I cannot succeed. My code at the moment looks like this: (Android with OpenCV and live camera feed), where imgMat is a Mat from the camera feed:
Mat gray = new Mat();
Imgproc.cvtColor(imgMat, gray, Imgproc.COLOR_RGB2BGR);
Mat blurred = new Mat();
Imgproc.blur(gray, blurred, new org.opencv.core.Size(3, 3));
Mat canny = new Mat();
Imgproc.Canny(blurred, canny, 80, 230);
Mat kernel = Imgproc.getStructuringElement(Imgproc.MORPH_RECT, new org.opencv.core.Size(2, 2));
Mat dilated = new Mat();
Imgproc.morphologyEx(canny, dilated, Imgproc.MORPH_DILATE, kernel, new Point(0, 0), 10);
Mat rectImage = new Mat();
Imgproc.morphologyEx(dilated, rectImage, Imgproc.MORPH_CLOSE, kernel, new Point(0, 0), 5);
Mat endproduct = new Mat();
Imgproc.Canny(rectImage, endproduct, 120, 230);
List<MatOfPoint> contours = new ArrayList<>();
Mat hierarchy = new Mat();
Imgproc.findContours(endproduct, contours, hierarchy, Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);
double maxArea = 0;
boolean hasContour = false;
MatOfPoint2f biggestContour = new MatOfPoint2f();
Iterator<MatOfPoint> each = contours.iterator();
while (each.hasNext()) {
MatOfPoint wrapper = each.next();
double area = Imgproc.contourArea(wrapper);
if (area > maxArea) {
maxArea = area;
biggestContour = new MatOfPoint2f(wrapper.toArray());
hasContour = true;
}
}
if (hasContour) {
Mat output = imgMat.clone();
MatOfPoint2f approx = new MatOfPoint2f();
MatOfPoint poly = new MatOfPoint();
Imgproc.approxPolyDP(biggestContour, approx, Imgproc.arcLength(biggestContour, true) * .02, true);
approx.convertTo(poly, CvType.CV_32S);
Rect rect = Imgproc.boundingRect(poly);
}
Somehow I am not able to get it working, although the same code(written in python) worked on my computer with a video. I take the output from the rectangle and display it on my mobile screen, where it flickers around a lot and does not work properly.
These are my images I tried the python program on, and they worked:
What am I doing wrong? I am not able to constantly detect the edges of the blackboard.
Additional information about the blackboard:
- always rectangular
- may have different lighting
- the text should be ignored, only the main board should be detected
- the outer blackboard should be ignored as well
- only the contour for the main board should be shown/returned
Thanks for any advice or code!
from OpenCV - find blackboard edges on video and images
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