OpenCV distance transform outputting an image that looks exactly like the input image

Question

I am doing some detection work using OpenCV, and I need to use the distance transform. Except the distance transform function in opencv gives me an image that is exactly the sam

Answer 1

I believe the key here is that they look the same. Here is a small program I wrote to show the difference:

#include 
#include 
#include 
#include 

using namespace std;
using namespace cv;

int main(int argc, char** argv)
{
    Mat before = imread("qrcode.png", 0);

    Mat dist;
    distanceTransform(before, dist, CV_DIST_L2, 3);

    imshow("before", before);
    imshow("non-normalized", dist);

    normalize(dist, dist, 0.0, 1.0, NORM_MINMAX);
    imshow("normalized", dist);
    waitKey();
    return 0;
}

In the non-normalized image, you see this:

which doesn't really look like it changed anything, but the distance steps are very small compared to the overall range of values [0, 255] (due to imshow converting the image from 32-bit float to 8-bits for display), we can't see the differences, so let's normalize it...

Now we get this:

The values themselves should be correct, but when displayed you will need to normalize the image to see the difference.

EDIT : Here is a small 10x10 sample from the upper-left corner of the dist matrix show that the values are in fact different:

[10.954346, 10.540054, 10.125763, 9.7114716, 9.2971802, 8.8828888, 8.4685974, 8.054306, 7.6400146, 7.6400146;
  10.540054, 9.5850525, 9.1707611, 8.7564697, 8.3421783, 7.927887, 7.5135956, 7.0993042, 6.6850128, 6.6850128;
  10.125763, 9.1707611, 8.2157593, 7.8014679, 7.3871765, 6.9728851, 6.5585938, 6.1443024, 5.730011, 5.730011;
  9.7114716, 8.7564697, 7.8014679, 6.8464661, 6.4321747, 6.0178833, 5.6035919, 5.1893005, 4.7750092, 4.7750092;
  9.2971802, 8.3421783, 7.3871765, 6.4321747, 5.4771729, 5.0628815, 4.6485901, 4.2342987, 3.8200073, 3.8200073;
  8.8828888, 7.927887, 6.9728851, 6.0178833, 5.0628815, 4.1078796, 3.6935883, 3.2792969, 2.8650055, 2.8650055;
  8.4685974, 7.5135956, 6.5585938, 5.6035919, 4.6485901, 3.6935883, 2.7385864, 2.324295, 1.9100037, 1.9100037;
  8.054306, 7.0993042, 6.1443024, 5.1893005, 4.2342987, 3.2792969, 2.324295, 1.3692932, 0.95500183, 0.95500183;
  7.6400146, 6.6850128, 5.730011, 4.7750092, 3.8200073, 2.8650055, 1.9100037, 0.95500183, 0, 0;
  7.6400146, 6.6850128, 5.730011, 4.7750092, 3.8200073, 2.8650055, 1.9100037, 0.95500183, 0, 0]