Load, Modify, and Save an Image {#tutorial_load_save_image}

@note We assume that by now you know how to load an image using @ref cv::imread and to display it in a window (using @ref cv::imshow ). Read the @ref tutorial_display_image tutorial otherwise.

Goals

In this tutorial you will learn how to:

• Load an image using @ref cv::imread
• Transform an image from BGR to Grayscale format by using @ref cv::cvtColor
• Save your transformed image in a file on disk (using @ref cv::imwrite )

Code

Here it is: @code{.cpp} #include <opencv2/opencv.hpp>

using namespace cv;

int main( int argc, char** argv ) { char* imageName = argv[1];

Mat image; image = imread( imageName, IMREAD_COLOR );

if( argc != 2 || !image.data ) { printf( " No image data \n " ); return -1; }

Mat gray_image; cvtColor( image, gray_image, COLOR_BGR2GRAY );

imwrite( "../../images/Gray_Image.jpg", gray_image );

namedWindow( imageName, WINDOW_AUTOSIZE ); namedWindow( "Gray image", WINDOW_AUTOSIZE );

imshow( imageName, image ); imshow( "Gray image", gray_image );

waitKey(0);

return 0; } @endcode Explanation

-# We begin by loading an image using @ref cv::imread , located in the path given by imageName. For this example, assume you are loading a BGR image. -# Now we are going to convert our image from BGR to Grayscale format. OpenCV has a really nice function to do this kind of transformations: @code{.cpp} cvtColor( image, gray_image, COLOR_BGR2GRAY ); @endcode As you can see, @ref cv::cvtColor takes as arguments:

-   a source image (*image*)
-   a destination image (*gray_image*), in which we will save the converted image.
-   an additional parameter that indicates what kind of transformation will be performed. In
    this case we use **COLOR_BGR2GRAY** (because of @ref cv::imread has BGR default channel
    order in case of color images).

-# So now we have our new gray_image and want to save it on disk (otherwise it will get lost after the program ends). To save it, we will use a function analogous to @ref cv::imread : @ref cv::imwrite @code{.cpp} imwrite( "../../images/Gray_Image.jpg", gray_image ); @endcode Which will save our gray_image as Gray_Image.jpg in the folder images located two levels up of my current location.

-# Finally, let's check out the images. We create two windows and use them to show the original image as well as the new one: @code{.cpp} namedWindow( imageName, WINDOW_AUTOSIZE ); namedWindow( "Gray image", WINDOW_AUTOSIZE );

imshow( imageName, image );
imshow( "Gray image", gray_image );
@endcode

-# Add the waitKey(0) function call for the program to wait forever for an user key press.

Result

When you run your program you should get something like this:

And if you check in your folder (in my case images), you should have a newly .jpg file named Gray_Image.jpg:

Congratulations, you are done with this tutorial!