Problem Understanding OpenCV Convert Mat to BufferedImage - opencv

Question

I am a new user of JAVA OpenCV, and I am just learning through the official tutorial today about how to convert a Mat object to BufferedImage.
From the demo code, I can understand that the input image source is a Matrix form, and then sourcePixels seems going to be an array of bytes representation of the image, so we need to get the values from the original matrix to the sourcePixels. Here the sourcePixels has the length of the whole image bytes length (with size: w * h * channels), so it would take the whole image byte values at once.
Then it comes this which is not intuitive to me. The System.arraycopy() seems copying the values from the sourcePixels to the targetPixels, but what actaully returns is image. I can guess from the code that targetPixels has relationship with image, but I don't see how we copy values from sourcePixels to targetPixels, but it actually affects values of image?
Here's the demo code. Thanks!
private static BufferedImage matToBufferedImage(Mat original)
{
BufferedImage image = null;
int width = original.width(), height = original.height(), channels = original.channels();
byte[] sourcePixels = new byte[width * height * channels];
original.get(0, 0, sourcePixels);
if (original.channels() > 1)
{
image = new BufferedImage(width, height, BufferedImage.TYPE_3BYTE_BGR);
}
else
{
image = new BufferedImage(width, height, BufferedImage.TYPE_BYTE_GRAY);
}
final byte[] targetPixels = ((DataBufferByte) image.getRaster().getDataBuffer()).getData();
System.arraycopy(sourcePixels, 0, targetPixels, 0, sourcePixels.length);
return image;
}

Answer 1

Each BufferedImage is backed by a byte array just like the Mat class from OpenCV, the call to ((DataBufferByte) image.getRaster().getDataBuffer()).getData(); returns this underlying byte array and assigns it to targetPixels, in other words, targetPixels points to this underlying byte array that the BufferedImage image is currently wrapping around, so when you call System.arraycopy you are actually copying from the source byte array into the byte array of the BufferedImage, that's why the image is being returned, because at that point, the underlying byte array that image encapsulates contains the pixel data from original, it's like this smal example, where after making b points to a, modifications to b will also reflect in a, just like tagetPixels, because it points to the byte array image is encapsulating, copying from sourcePixels into targetPixels will also change the image
int[] a = new int[1];
int[] b = a;
// Because b references the same array that a does
// Modifying b will actually change the array a is pointing to
b[0] = 1;
System.out.println(a[0] == 1);