FFMPEG scaling problem

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C++

Hello everyone, I'm working on a Project in FFMPEG, and right now i'm stuck on a Problem.

What i am trying to do is, convert a png pic in to a mpeg Video file. I've already managed to take informatin from the Picture, but somehow i cannot convert the Picture in YUV Format. It returns "0x0->0x0 is invalid scaling dimensions".

Here'S my code:

VFrame *pFrameYUV;
pFrame = av_frame_alloc();
pFrameYUV = av_frame_alloc();
int numBytes;//Groesse des Bildes
uint8_t *buffer= NULL;
numBytes=avpicture_get_size(AV_PIX_FMT_RGB24, pCodecCtx->width,pCodecCtx->height); 
buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t)); 

/*avpicture_fill((AVPicture *)pFrameYUV, buffer, AV_PIX_FMT_YUV420P,
                pCodecCtx->width, pCodecCtx->height);*/
av_image_fill_arrays(pFrameYUV->data,pFrameYUV->linesize,buffer,AV_PIX_FMT_YUV420P,pCodecCtx->width,pCodecCtx->height,32);


struct SwsContext *sws_ctx = NULL;

AVPacket packet;
// initialize SWS context for software scaling
sws_ctx=sws_getCachedContext(NULL,pFrame->width,pFrame->height,AV_PIX_FMT_RGB24,pFrameYUV->width,pFrameYUV->height,AV_PIX_FMT_YUV420P,0,0,0,0);
pFrameYUV->height= pFrame->height;
pFrameYUV->width= pFrame->width;

while (av_read_frame(pFormatCtx,&packet)>=0)
{
	if(packet.stream_index == videoStream)
	{
		avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet); 
		
		if(frameFinished)
		{

		sws_scale(sws_ctx, (uint8_t const * const *)pFrame->data,
		  pFrame->linesize, 0, pCodecCtx->height,
		  pFrameYUV->data, pFrameYUV->linesize);
		printf("%d",pFrameYUV->height);
		
	}
	}
	av_free_packet(&packet);
}
pre>

What I have tried:

I've tried the old functions, but it does not work either