Issues with Float Mats and warpAffine

[FromDarkHell]

When doing something like:

import 'package:opencv_dart/opencv_dart.dart' as cv;

void cvTest() {
  cv.Mat mask = cv.Mat.ones(512, 512, cv.MatType.CV_32FC1);

  List<cv.Point2f> faceTemplate = [
    cv.Point2f(192.98138, 239.94708),
    cv.Point2f(318.90277, 240.1936),
    cv.Point2f(256.63416, 314.01935),
    cv.Point2f(201.26117, 371.41043),
    cv.Point2f(314.08905, 371.15118)
  ];

  List<cv.Point2f> landmarks = [
    cv.Point2f(916.1744018554688, 436.8168579101563),
    cv.Point2f(1179.1181030273438, 448.0384765625),
    cv.Point2f(1039.171106147766, 604.8748825073242),
    cv.Point2f(908.7911743164062, 683.4760314941407),
    cv.Point2f(1167.2201416015625, 693.495068359375),
  ];

  var (affineMatrix, _) =
      cv.estimateAffinePartial2D(landmarks, faceTemplate, method: cv.LMEDS);

  var invMask = cv.Mat.empty();
  cv.warpAffine(mask, invMask, affineMatrix, (2048, 2048));

  for (int i = 0; i < 2047; i++) {
    for (int j = 0; j < 2047; j++) {
      var val = invMask.at<double>(i, j);
      assert(val.isFinite);
    }
  }
}

This immediately fails because 0,0 is Infinity instead of 0.

Here is equivalent C++ code which does return a Mat with finite values (in this case, just 1 iirc)

#include "opencv2/opencv.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <opencv2/core/types_c.h>
#include <iostream>
#include "opencv2/imgproc.hpp"
#include <vector>

using namespace cv;

int main(int argc, char **argv)
{
    Mat mask = Mat::ones(512, 512, CV_32FC1);

    std::vector<Point2f> faceTemplate = {
        Point2f(192.98138, 239.94708),
        Point2f(318.90277, 240.1936),
        Point2f(256.63416, 314.01935),
        Point2f(201.26117, 371.41043),
        Point2f(314.08905, 371.15118)};

    std::vector<Point2f> landmarks = {
        Point2f(916.1744018554688, 436.8168579101563),
        Point2f(1179.1181030273438, 448.0384765625),
        Point2f(1039.171106147766, 604.8748825073242),
        Point2f(908.7911743164062, 683.4760314941407),
        Point2f(1167.2201416015625, 693.495068359375),
    };

    Mat affineMatrix = cv::estimateAffinePartial2D(landmarks, faceTemplate, cv::noArray(), cv::LMEDS);

    Mat invMask = cv::Mat();
    cv::warpAffine(mask, invMask, affineMatrix, cv::Size(2048, 2048));

    for (int i = 0; i < 2047; i++)
    {
        for (int j = 0; j < 2047; j++)
        {
            float val = invMask.at<float>(i, j);
            assert(val == 0 || val == 1);
        }
    }

    return 0;
}

Edit: This also similarly happens when you use CV_64FC1; Except instead of +INF, it returns 1.7976931348623157e+308 or some similarly large number

[rainyl]

Got it, this is because the default value of Scalar.default_() is double.maxFinite, I forgot why I set maxfinite as the default value of scalar rather than zero...

For now, you can explicitly pass the borderValue: Scalar.black to fix,

opencv_dart/lib/src/imgproc/imgproc.dart

Line 1233 in 4ae19b7

Scalar? borderValue,

I will change it to zero in the next minor version.