Translate Python function to apply mask to image into Java

Question

I'm trying to translate the following Python function, that applies a mask to an image, into Java:

# Applies an image mask.
def region_of_interest(img, vertices):
    #defining a blank mask to start with
    mask = np.zeros_like(img)   

    #defining a 3 channel or 1 channel color to fill the mask with depending on the input image
    if len(img.shape) > 2:
        channel_count = img.shape[2]  # i.e. 3 or 4 depending on your image
        ignore_mask_color = (255,) * channel_count
    else:
        ignore_mask_color = 255

    #filling pixels inside the polygon defined by "vertices" with the fill color    
    cv2.fillPoly(mask, vertices, ignore_mask_color)

    #returning the image only where mask pixels are nonzero
    masked_image = cv2.bitwise_and(img, mask)
    return masked_image

So far, this is what I've got:

public static opencv_core.Mat applyMask(opencv_core.Mat image, opencv_core.MatVector vertices) {
  opencv_core.Mat mask = opencv_core.Mat.zeros(image.size(), opencv_core.CV_8U).asMat();

  opencv_core.Scalar color = new opencv_core.Scalar(image.channels()); // 3
  double[] colors = new double[] {
    255.0, 255.0, 255.0, 255.0,
    255.0, 255.0, 255.0, 255.0,
    255.0, 255.0, 255.0, 255.0};
  color.put(colors, 0, colors.length);

  opencv_imgproc.fillPoly(mask, vertices, color);

  opencv_core.Mat dst = new opencv_core.Mat();
  opencv_core.bitwise_and(image, mask, dst);
  return dst;
}

But, it isn't working. When I try invoking this method like in the following example:

opencv_core.MatVector points = new opencv_core.MatVector(
  new opencv_core.Mat(2, 3, opencv_core.CV_32F, new IntPointer(1, 2, 3, 4, 5, 6))
);

opencv_core.MatVector vertices = new opencv_core.MatVector(points);
opencv_core.Mat masked = LaneManager.applyMask(src, vertices);

(I'm assuming this is the right way to build a 2x3 matrix of three points with two coordinates each (1,2), (3, 4) and (5,6))

I get an exception:

java.lang.RuntimeException: std::bad_alloc
at org.bytedeco.javacpp.opencv_imgproc.fillPoly(Native Method)

I'm using OpenCV as provided by org.bytedeco.javacpp-presets:opencv-platform:3.2.0-1.3 via Maven Central.

I must admit that I'm at a loss here: What's the idiomatic Java way of doing the same thing as the Python function above?

Answer 1

Maybe some whiz has the complete answers. Here is food for thought:

1. The Java API is a direct copy of the CPP API: http://opencv.org/opencv-java-api.html

2. The error std::bad_alloc occurs when you fail to allocate required storage space. (http://answers.opencv.org/question/28959/cascade-training-killed-and-bad_alloc/)

3. There are two CPP methods:
   • C++: void fillPoly(Mat& img, const Point** pts, const int* npts, int ncontours, const Scalar& color, int lineType=LINE_8, int shift=0, Point offset=Point() ), and
   • C++: void fillPoly(InputOutputArray img, InputArrayOfArrays pts, const Scalar& color, int lineType=LINE_8, int shift=0, Point offset=Point() )

4. You don't need to convert from Mat to InputArray, but you can (and should) just pass a Mat object where an InputArray is requested (https://stackoverflow.com/a/32976883/1587329)

Answer 2

Alright, I finally figured it out. If you define your coordinates with:

int[] points = new int[] {x1, y1, x2, y2, ...};

Then you can simply apply a mask with the following code:

opencv_core.Mat mask = new opencv_core.Mat(image.size(), image.type());

// Array of polygons where each polygon is represented as an array of points
opencv_core.Point polygon = new opencv_core.Point();
polygon.put(points, 0, points.length);
opencv_imgproc.fillPoly(mask, polygon, new int[] {points.length / 2}, 1, new opencv_core.Scalar(255, 255, 255, 0));

opencv_core.Mat masked = new opencv_core.Mat(image.size(), image.type());
opencv_core.bitwise_and(image, mask, masked);

Where image is the original image, and masked is the masked result.

The problem was that the original list of points wasn't defined properly.