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I looked around for a Conversion of the YUV420 Image to a Grayscale Image and found this Issue in the DevZone

According to this, there is already a YUV to Grayscale Conversion enum for cvtColor(). But, as also mentioned in the issue, the enums are not good documented. My Question now is, which of these Enums do I have to use if I want to convert a packed YUV ByteBuffer into a Grayscale ByteBuffer or Image/cv::Mat?

Thanks for your attentiveness.

So I found the following ENUM CV_YUV2GRAY_420 is that the one I need?

I tried the enum above with this test:

I capture a Frame in Android from my Preview in YUV_420_888Format. I pass that to a function which packs the 3 Plane buffers into one ByteBuffer and put it into a byte[]:

 Image img = params[0];
 ByteBuffer dstBuffer = ByteBuffer.allocateDirect(img.getWidth() * img.getHeight() * ImageFormat.getBitsPerPixel(img.getFormat()) / 8);

  getDataFromImage(img, dstBuffer);

  byte[] data = new byte[dstBuffer.remaining()];
  dstBuffer.put(data);

Then I'll pass that byte[] to JNI and use cv::Mat(height + height/2, width, CV_8UC1, imageData) to create a Mat with the data (the byte[]was converted to a unsigned char*) Now I do this:

cv::Mat img(height + height/2, width, CV_8UC1, imageData)
cv::Mat gry;

cv::cvtColor(img, gry, CV_YUV2GRAY_420, 1)

But it seems, that the result is wrong, because I can't extract any Features out of this grayscale Image.