Creating a Mat object from a YV12 image buffer - opencv

Question

I have a buffer which contains an image in YV12 format. Now I want to either convert this buffer to RGB format or create a Mat object from it directly! Can someone help me? I tried this code :
cv::Mat input(widthOfImg, heightOfImg, CV_8UC1, vy12Buffer);
cv::Mat converted;
cv::cvtColor(input, converted, CV_YUV2RGB_YV12);

Answer 1

That's possible.
cv::Mat picYV12 = cv::Mat(nHeight * 3/2, nWidth, CV_8UC1, yv12DataBuffer);
cv::Mat picBGR;
cv::cvtColor(picYV12, picBGR, CV_YUV2BGR_YV12);
cv::imwrite("test.bmp", picBGR); //only for test
Opencv color conversion flags
The height is multiplied by 3/2 because there are 4 Y samples, and 1 U and 1 V sample stored for every 2x2 square of pixels. This results in a byte sample to pixel ratio of 3/2
4*1+1+1 samples per 2*2 pixels = 6/4 = 3/2
YV12 Format
Correction: In the last version of OpenCV (i use oldest 2.4.13 version) is color conversion code changed to
COLOR_YUV2BGR_YV12
cv::cvtColor(picYV12, picBGR, COLOR_YUV2BGR_YV12);

Answer 2

here is the corresponding version in java (Android)...
This method was faster than other techniques like renderscript or opengl(glReadPixels) for getting bitmap from yuv12/i420 data stream (tested with webrtc i420 ).
long startTimei = SystemClock.uptimeMillis();
Mat picyv12 = new Mat(768,512,CV_8UC1); //(im_height*3/2,im_width), should be even no...
picyv12.put(0,0,return_buff); // buffer - byte array with i420 data
Imgproc.cvtColor(picyv12,picyv12,COLOR_YUV2RGB_YV12);// or use COLOR_YUV2BGR_YV12 depending on output result
long endTimei = SystemClock.uptimeMillis();
Log.d("i420_time", Long.toString(endTimei - startTimei));
Log.d("picyv12_size", picyv12.size().toString()); // Check size
Log.d("picyv12_type", String.valueOf(picyv12.type())); // Check type
Utils.matToBitmap(picyv12,tbmp2); // Convert mat to bitmap (height, width) i.e (512,512) - ARGB_888
save(tbmp2,"itest"); // Save bitmap

Answer 3

That's impossible.
Y'UV420p is a planar format, meaning that the Y', U, and V values are
grouped together instead of interspersed. The reason for this is that
by grouping the U and V values together, the image becomes much more
compressible. When given an array of an image in the Y'UV420p format,
all the Y' values come first, followed by all the U values, followed
finally by all the V values.
but cv::Mat is a RGB color model, and arranged like B0 G0 R0 B1 G1 R1... So,we can't create a Mat object from a YV12 buffer directly.
Here is an example:
cv::Mat Yv12ToRgb( uchar *pBuffer,long bufferSize, int width,int height )
{
cv::Mat result(height,width,CV_8UC3);
uchar y,cb,cr;
long ySize=width*height;
long uSize;
uSize=ySize>>2;
assert(bufferSize==ySize+uSize*2);
uchar *output=result.data;
uchar *pY=pBuffer;
uchar *pU=pY+ySize;
uchar *pV=pU+uSize;
uchar r,g,b;
for (int i=0;i<uSize;++i)
{
for(int j=0;j<4;++j)
{
y=pY[i*4+j];
cb=ucharpU[i];
cr=ucharpV[i];
//ITU-R standard
b=saturate_cast<uchar>(y+1.772*(cb-128));
g=saturate_cast<uchar>(y-0.344*(cb-128)-0.714*(cr-128));
r=saturate_cast<uchar>(y+1.402*(cr-128));
*output++=b;
*output++=g;
*output++=r;
}
}
return result;
}

Answer 4

You can try as YUV_I420 array
char filePath[3000];
int width, height;
cout << "file path = ";
cin >> filePath;
cout << "width = ";
cin >> width;
cout << "height = ";
cin >> height;
FILE *pFile = fopen(filePath, "rb");
unsigned char* buff = new unsigned char[width * height *3 / 2];
fread(buff, 1, width * height* 3 / 2, pFile);
fclose(pFile);
cv::Mat imageRGB;
cv::Mat picI420 = cv::Mat(height * 3 / 2, width, CV_8UC1, buff);
cv::cvtColor(picI420, imageRGB, CV_YUV2BGRA_I420);
imshow("imageRGB", imageRGB);
waitKey(0);