How can I write float image in OpenCV

Question

Someone gave me this function:

Mat tan_triggs_preprocessing(InputArray src, float alpha = 1, float gamma = 10.0,
                 float tau = 1000.0, int sigma1 = 2) {
  Mat X = src.getMat();
  Mat I, tmp, tmp2;
  double meanI;

  X.convertTo(X, CV_32FC1);
  pow(X, gamma, I);

  meanI = 0.0;
  pow(abs(I), alpha, tmp);
  meanI = mean(tmp).val[0];
  I = I / pow(meanI, 1.0/alpha);

  meanI = 0.0;
  pow(min(abs(I), tau), alpha, tmp2);
  meanI = mean(tmp2).val[0];
  I = I / pow(meanI, 1.0/alpha);

  for(int r = 0; r < I.rows; r++) {
    for(int c = 0; c < I.cols; c++) {
      I.at<float>(r,c) = tanh(I.at<float>(r,c) / tau);
    }
  }
  I = tau * I;

  return I;
}

The function takes an input as a gray scale image or CV_8UC1 type, and it outputs a matrix of CV_32FC1 type. All I know is the function makes the input image lighter, increases its contrast. When I show the image using imshow function, I can see the output of tan_triggs_preprocessing very clearly, and actually the output lighter, more contrast compares to the source image. But the problem is when I save it as image format (JPG for example) using imwrite function, it's totally black. I can't see anything.
I checked the value of elements in the output, and I saw that their values are between [0.06.., 2.3...]. Here are my questions, hopefully you can help me, thank you so much.

Can I write an CV_32FC1 as image file format?
Why is the file written by imwrite above totally black?
I also looked for min and max value in the output, so I can normalize it in to 256 bins for CV_8UC1, but it doesn't work, even when I use imshow or imwrite.
How can I convert it to CV_8UC1 or write it as image file format? I used convertTo but it doesn't work as well.

Thank a lot.

You can save the image as an HDR image, .hdr files are floating point. To do this you'll need to use OpenCV 3.0, which is currently in beta. — Maggick
Hi maggick. Thank you so much for your attention. I will think about it. — user2812045

berak berak · Accepted Answer · 2014-05-14T19:16:42

imwrite/imread can only handle 8/16/24/32bit integral data, not floats (if you don't count Ilm/exr)

you probably want :

Mat gray_in = ...
Mat gray_out;
cv::normalize( tan_triggs_preprocessing(gray_in), gray_out, 0, 255, NORM_MINMAX, CV_8UC1);

(admittedly hard to spot, but it's even in the small print of bytefish's code ;)

also, please look at alternatives to that, like equalizehist and CLAHE

How can I write float image in OpenCV

1 Answers