How to Extract Pixel Channel Values from a Mat Image in OpenCV?-C++-php.cn

How to Extract Pixel Channel Values from a Mat Image in OpenCV?

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Release： 2024-11-15 22:47:02

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How to Extract Pixel Channel Values from a Mat Image in OpenCV?

OpenCV: Extracting Pixel Channel Values from Mat Images

Question: How can you retrieve the channel value of a specific pixel in a Mat image?

Answer:

For a Mat object where the pixel type is CV_8UC3 (an 8-bit unsigned 3-channel image), the following method can be used:

for(int i = 0; i < foo.rows; i++)
{
    for(int j = 0; j < foo.cols; j++)
    {
        Vec3b bgrPixel = foo.at<Vec3b>(i, j);

        // Process the BGR values...
    }
}

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In this code, Vec3b represents a 3-channel pixel with 8-bit unsigned integers for each channel. at is a member function that allows you to access the pixel value at the specified row and column.

Performance Optimization:

For improved performance, consider accessing the data buffer directly:

uint8_t* pixelPtr = (uint8_t*)foo.data;
int cn = foo.channels();
Scalar_<uint8_t> bgrPixel;

for(int i = 0; i < foo.rows; i++)
{
    for(int j = 0; j < foo.cols; j++)
    {
        bgrPixel.val[0] = pixelPtr[i*foo.cols*cn + j*cn + 0]; // B
        bgrPixel.val[1] = pixelPtr[i*foo.cols*cn + j*cn + 1]; // G
        bgrPixel.val[2] = pixelPtr[i*foo.cols*cn + j*cn + 2]; // R

        // Process the BGR values...
    }
}

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Alternatively, you can use the row method:

int cn = foo.channels();
Scalar_<uint8_t> bgrPixel;

for(int i = 0; i < foo.rows; i++)
{
    uint8_t* rowPtr = foo.row(i);
    for(int j = 0; j < foo.cols; j++)
    {
        bgrPixel.val[0] = rowPtr[j*cn + 0]; // B
        bgrPixel.val[1] = rowPtr[j*cn + 1]; // G
        bgrPixel.val[2] = rowPtr[j*cn + 2]; // R

        // Process the BGR values...
    }
}

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Note: OpenCV internally stores images in BGR format, not RGB.

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