// registering all codecs...

AVCodec* codec;
AVCodecContext* context;
AVFrame* frame;
AVPacket packet;
SwsContext* transform;

codec = avcodec_find_decoder(CODEC_ID_MJPEG); // maybe using libav for jpeg is overoveroverkill, but I'm already using libav for better reasons, so I thought...

context = avcodec_alloc_context3(codec); // returns non-null

avcodec_open2(context, codec, nullptr) // returns 0

context->width = data.Width;
context->height = data.Height;

frame = av_frame_alloc();
av_init_packet(&packet);

packet.data = data.Ptr;
packet.size = data.Size;

int gotPicture = false;
avcodec_decode_video2(context, frame, &gotPicture, &packet); // returns >= 0, gotPicture is true

// We init the context the braindead way as using sws_setColorspaceDetails after initing the context doesn't seem to work... (and we can't otherwise set the ranges)
transform = sws_alloc_context ();
av_opt_set_int(transform, "srcw", frame->width, 0);
av_opt_set_int(transform, "srch", frame->height, 0);
av_opt_set_int(transform, "dstw", frame->width, 0);
av_opt_set_int(transform, "dsth", frame->height, 0);
av_opt_set_int(transform, "src_format", context->pix_fmt, 0); // this is AV_PIX_FMT_GRAY8
av_opt_set_int(transform, "dst_format", data.Format, 0); // this is AV_PIX_FMT_NV12
av_opt_set_int(transform, "src_range", 1, 0); // full-range
av_opt_set_int(transform, "dst_range", 1, 0); // full-range
sws_init_context(transform, nullptr, nullptr); // returns >= 0

// -1 for guaranteed crashes, can be 0 for potentially wrong behaviour (hard to say) and no error
byte* targetPtr[4] = {(byte*) -1, (byte*) -1, (byte*) -1, (byte*) -1};
int targetStride[4] = {-1, -1, -1, -1};

for (int i = 0; i < data.NumPlanes; i++) // this is 2
{
    targetPtr[i] = data.TargetPlane;
    targetStride[i] = data.TargetStride;
}

sws_scale(transform, frame->data, frame->linesize, 0, frame->height, targetPtr, targetStride); // returns frame->height

av_frame_free (&frame);
avcodec_close (context);
av_free (context);
sws_freeContext (transform);