Howto get luminance pictrue from AVFrame?

Question

I am working with mpeg-4 and h264 streams

First I tryed converting to rgb24 and use imagemagic to make grayscale,it doesnot work

while(av_read_frame(pFormatCtx, &packet)>=0)
    {
        // Is this a packet from the video stream?
        if(packet.stream_index==videoStream)
        {
            // Decode video frame
            avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished,
                                  &packet);

            // Did we get a video frame?
            if(frameFinished)
            {
                f++;
                //this->fram->Clear();
               // if (pFrame->pict_type == AV_PICTURE_TYPE_I) wxMessageBox("I cadr");
               // if (pFrame->pict_type != AV_PICTURE_TYPE_I)
               // printMVMatrix(f, pFrame, pCodecCtx);
                pFrameRGB->linesize[0]= pCodecCtx->width*3; // in case of rgb4  one plane

                sws_scale(swsContext, pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize);


                Magick::Blob* m_blob = new Magick::Blob(pFrameRGB->data,pCodecCtx->width*pCodecCtx->height*3);
                Magick::Image* image =new Magick::Image(*m_blob); // this doesnotwork

                image->quantizeColorSpace( Magick::GRAYColorspace );
                image->quantizeColors( 256 );
                image->quantize( );

But ffmpeg gives me YUV picture?! so need only Y component, how to get it? to get Ypicture[x][y]

Answer 1

I assume you have configured swscale to YUV420p color space. 420P means 4:2:0 planar. Planar means the color channels are separate.

The luminance data (Y) is stored in the buffer point to pFrame->data[0] (Cb and Cr are in pFrame->data[1] and pFrame->data[2] respectively). in YUV420 the Y plane is 1 byte per pixel.

hence:

uint8_t getY(int x, int y, AVFrame *f)
{
    return f->data[0][(y*f->linesize)+x];
}