OpenCV double mat shows up as all white

Question

I have a type 6 (double-valued, single channel) mat with data ranging from 0 to 255. I can print out the data using the following code:

    double* data  =  result.ptr<double>();

    for(int i = 0; i < rows; i++)
        for(int j = 0; j < cols; j++)
    std::cout<<data[i*step+j]<<"\t";

And this appears perfectly normal--in the range from 0 to 255 and the size that I'd expect. However, when I try to show the image:

imshow(window_name, result);
waitKey();

I just get a white image. Just white pixels. Nothing else.

Loading other images from files and displaying in the window works fine.

Using Windows 7, OpenCV 233

Answer 1

cv::imshow works in following ways -

1. If the image is 8-bit unsigned, it is displayed as is.
2. If the image is 16-bit unsigned or 32-bit integer, the pixels are divided by 256. That is, the value range [0,255*256] is mapped to [0,255].
3. If the image is 32-bit floating-point, the pixel values are multiplied by 255. That is, the value range [0,1] is mapped to [0,255].

Your matrix lies in the 3rd category where imshow is expecting the values to be between 0 and 1 and so it multiplies it by 255. Since your values are already between 0 and 255, you are getting unwanted result. So normalizing the pixels between 0 and 1 will work.