How to write a Float Mat to a file in OpenCV

In OpenCV, storing a floating-point Mat object to a file can be achieved through two primary methods: one involves directly storing it using the file storage functionality, while the other entails converting it to a visual format (such as an image) prior to storage. Below are the specific steps and examples.

Method 1: Using FileStorage

OpenCV provides a convenient class FileStorage for storing and reading data, supporting XML, YAML, and JSON formats. Here is an example code demonstrating how to save a floating-point Mat object to an XML file:

cpp
#include <opencv2/opencv.hpp>
using namespace cv;

int main() {
    // Create a floating-point Mat object
    Mat floatMat = (Mat_<float>(2, 2) << 1.0f, 2.0f, 3.0f, 4.0f);

    // Create FileStorage object for writing to file
    FileStorage fs("floatMat.xml", FileStorage::WRITE);

    // Write the Mat object to the file
    fs << "floatMatrix" << floatMat;

    // Release resources
    fs.release();

    return 0;
}

This code generates a file named floatMat.xml with the following content (which may vary slightly depending on the OpenCV version):

xml
<?xml version="1.0"?>
<opencv_storage>
<floatMatrix type_id="opencv-matrix">
  <rows>2</rows>
  <cols>2</cols>
  <dt>f</dt>
  <data>1. 2. 3. 4.</data></floatMatrix>
</opencv_storage>

Method 2: Convert to Image Format and Save

If you wish to convert a floating-point Mat to an image format (such as PNG or JPG) for storage, you first need to normalize the data range to 0-255 and convert the data type to unsigned integer (uchar). Then, use the imwrite() function to save the converted image. Here is an example:

cpp
#include <opencv2/opencv.hpp>
using namespace cv;

int main() {
    // Create a floating-point Mat object
    Mat floatMat = (Mat_<float>(2, 2) << 1.0f, 2.0f, 3.0f, 4.0f);

    // Normalize
    Mat normalizedMat;
    normalize(floatMat, normalizedMat, 0, 255, NORM_MINMAX);

    // Convert data type
    Mat ucharMat;
    normalizedMat.convertTo(ucharMat, CV_8UC1);

    // Save as image
    imwrite("floatMat.png", ucharMat);

    return 0;
}

This code creates a file named floatMat.png containing the normalized floating-point matrix.

These two methods are applied in different scenarios: when you require preserving the full data precision and structure, it is recommended to use FileStorage; when you need visualization or compatibility with other software, you can convert it to an image format.