[Libav-user] swscale, ARGB -> NV12, wrong NV12 image

[i.nixman at autistici.org]

hi,

I am getting ARGB frames and I need to convert them to NV12 format.
I am using the code below but the result seems to be the wrong picture 
because I cannot view it using ImageMagick's display tool and cannot 
convert to png using ffmpeg (ffmpeg -s 320x240 -pix_fmt nv12 -i 12-11- 
13.473.nv12 -f image2 -pix_fmt rgb24 captdump.png) because of error: 
12-11-13.473.nv12: Invalid data found when processing input.

to get swcontext:

```
m_scale_context = ::sws_getContext(
      m_width
     ,m_height
     ,AV_PIX_FMT_ARGB
     ,m_width
     ,m_height
     ,AV_PIX_FMT_NV12
     ,SWS_FAST_BILINEAR
     ,NULL
     ,NULL
     ,NULL
);
```

to convert:
```
QByteArray encode(QByteArray src) {
         QByteArray dst;

         int size_in_bytes = ::av_image_get_buffer_size(AV_PIX_FMT_NV12, 
m_width, m_height, 32);
         dst.resize(size_in_bytes);

         AVFrame *frame = ::av_frame_alloc();
         frame->width = m_width;
         frame->height = m_height;
         frame->format = AV_PIX_FMT_NV12;
         ::av_frame_get_buffer(frame, 32);

         int srcStride[AV_NUM_DATA_POINTERS] = { 
static_cast<int>(m_width) * 4, 0 };
         quint8 *srcPtr[AV_NUM_DATA_POINTERS] = { reinterpret_cast<quint8 
*>(src.data()), nullptr };
         ::sws_scale(
              m_scale_context
             ,srcPtr
             ,srcStride
             ,0
             ,m_height
             ,frame->data
             ,frame->linesize
         );

         int res = ::av_image_copy_to_buffer(
              reinterpret_cast<quint8 *>(dst.data())
             ,dst.size()
             ,(const uint8_t* const *)frame->data
             ,(const int*)frame->linesize
             ,AV_PIX_FMT_NV12
             ,frame->width
             ,frame->height
             ,32
         );

         qDebug().nospace().noquote()
             << "encoderNV12::encode(): width=" << m_width
             << ", height=" << m_height
             << ", bytes=" << size_in_bytes
             << ", writen=" << res
         ;

         ::av_frame_free(&frame);

#if 1
         auto dt = QDateTime::currentDateTime();
         QString fname = dt.toString("hh:mm:ss.zzz");
         fname.replace(":", "-");
         fname += ".nv12";
         QFile file(fname);
         file.open(QIODevice::WriteOnly);
         file.write(dst);
#endif

         return dst;
}

```

the output looks like this: encoderNV12::encode(): w=320, h=240, 
bytes=115200, written=115200


what am I doing wrong?


best!