Basic image editing using OpenCV

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Basic image editing using OpenCV

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#1 by (8 votes)

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Hello, I'm learning how to use the features that OpenCV offers for image processing, and the following question came up:

How do I edit only a predetermined area of an image?

To make it easier for me to understand, I'm going to use the cv::cvtColor() function and change a color image (RGB) to Gray Scale, but I'd like the proposed solution to work with any other function. What I've already been able to do:

Upload and View a disk image:

cv::Mat img = cv::imread("/home/anderson/Pictures/Outras/house.jpeg");
if (img.empty()) {
    std::cout << "Falha ao carregar o arquivo do disco!" << std::endl;
}

cv::namedWindow("RGB");
cv::imshow("RGB", img);

Selectanareaandturntograyscale:

cv::MatimgMod(img,cv::Rect(100,100,150,150));cv::MatimgModOut;cv::cvtColor(imgMod,imgModOut,CV_BGR2GRAY);cv::namedWindow("Gray");
cv::imshow("Gray", imgModOut);

So long, but how do I combine the gray cutout with the original image and create a new partially gray (trimmed area) image?

c++ opencv

asked by anonymous 27.05.2014 / 23:43

1 answer

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score 8 · Accepted Answer

What you are looking for can be achieved by setting a ROI region of interest in the original image.

An ROI specifies an area of interest within an image and allows you to extract this sub-region to a new cv::Mat , or insert another image within it. These are exactly the procedures that the following code demonstrates to accomplish the effect you are looking for:

// Carregar a imagem de teste a partir do disco.
cv::Mat input = cv::imread("house.png");
if (input.empty())
{
    std::cout << "!!! imread(): arquivo não encontrado" << std::endl;
    return -1;
}

// Especificar a localização da ROI, assumindo que: 
// x,y = (190,230)   
// largura, altura = (260, 60)
cv::Rect roi = cv::Rect(190, 230, 260, 60);

// Criar uma nova imagem a partir da ROI setada na imagem original:
cv::Mat sub_img = input(roi);
//cv::imwrite("house_roi.png", sub_img);

At this point, sub_img stores:

// Converter a subárea para tons de cinza.
cv::Mat gray;
cv::cvtColor(sub_img, gray, CV_BGR2GRAY);
//cv::imwrite("house_gray.png", gray);

gray stores:

It is important to note that this color image conversion to grayscale creates a new image with only 1 color channel. Therefore, it is essential to create a new cv::Mat with 3 color channels before copying the sub-image into the original image (which has 3 channels); otherwise OpenCV will complain that the channel numbers of the images are different.

// Converter subárea de 1 canal de cor para 3 canais. Esta operação não afeta os pixels da imagem.
cv::Mat tmp;
cv::cvtColor(gray, tmp, CV_GRAY2BGR);
//cv::imwrite("house_tmp.png", tmp);

// Copiar a subimagem para dentro da imagem original, na localização especificada pela ROI.
tmp.copyTo(input(roi));
//cv::imwrite("house_result.png", input);

When copyTo() is executed, the tmp pixels are inserted into the original image, obeying the subarea specified by roi .

At the end of this process, input stores: