CODEWITHSUNDEEP
opencv
Shulz
Shulz

Reputation: 625

What is the purpose of CvType.CV_8UC4

I have a snippet code from colorblobdetection but I don't know what is the purpose of CvType.CV_8UC4. The Link here does not really explain how does it works.

public void onCameraViewStarted(int width, int height) {
    mRgba = new Mat(height, width, CvType.CV_8UC4); 
    mDetector = new ColorBlobDetector();
    mSpectrum = new Mat();
    mBlobColorRgba = new Scalar(255);
    mBlobColorHsv = new Scalar(255);
    SPECTRUM_SIZE = new Size(200, 64);
    CONTOUR_COLOR = new Scalar(255,0,0,255);
}

Upvotes: 14

Views: 23820

Answers (3)

Patrick Z
Patrick Z

Reputation: 303

All Answers are correct, additionally: the 4th channel is not in every situation an alpha/transparency information. Sometimes it is used if the incomming buffer was an 24bit color image with a padding byte (most at the front or end) to fill it up so that addressing a specific pixel can be done atomically (just by the adress) without finding the specific byte offset. E.g. QImage loads jpegs as 32bit images while only 3 of the 4 bytes are relevant.

So the CV_8UC4 can be used directly for that. BTW, without the use of Transforming RGB to somewhat like HSV all channels are treated separately, and therefore:

It might be a good idea to reformat it to a 3bpp image if there are a lot of heavy cv-operations comming in the next processing steps. This can reduce the processing-time on roughly 25% because of the missing useless 4th channel.

Upvotes: 1

Caspar Wylie
Caspar Wylie

Reputation: 2843

This makes a specific form of Matrix. Where 8u means each element will be unsigned (only positive) integers, 8-bit.

The reason there are 4 channels (like 4 slices that layer together to make an image of different colours) is to make up the image. The first 3 in this are R, G, B, and the last is Alpha, which is a value between 0 and 1 representing transparency. When these slices combine you get the correct combination of colours.

enter image description here

Upvotes: 13

Unapiedra
Unapiedra

Reputation: 16248

OpenCV types can be read the following:

  • CV_
  • 8U: Unsigned int 8-bit
  • C4: Four channels.

Thus mRgba = new Mat(height, width, CvType.CV_8UC4); creates a Matrix with four color channels and values in the range 0 to 255.

Upvotes: 36

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