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• yet another screenshot encoding exercise with ffmpeg - stuck at getting AVFrame from ALT::CImage - VC++

  11 septembre 2013, par sith

  Total AV newbee here - trying to learn the ropes on using FFMpeg functions to encode movies. On searching for tutorials I found a few similar questions that I have linked here for reference :

  Encoding a screenshot into a video using FFMPEG

  [Libav-user] Encoding a screenshot into a video using FFMPEG

  Save bitmap to video (libavcodec ffmpeg)

  When converting from RGB to YUV using ffmpeg the video file the color is spread why ?

  How to convert RGB from YUV420p for ffmpeg encoder ?

  Encode bmp sequence with libavcodec...Help !

  Not able to encode image with ffmpeg

  For my setup FFMPEG is on VS12 - VC++ with MFC on win7.

  With the help of above samples, I am able to get "some" output from the encoder, but I am not sure in what format or state the output has been encoded. Neither VLC nor WMP can play this file. It does not even seem to recognize the metadata in the file to display the FPS or video length. What would normally cause that ? Also any pointers on what could be going wrong and how to approach fixing the problems would be great. [1]

  Here is the flow of my code :

  Step1 : capture desktop on to a CImg :

  int W=GetSystemMetrics(SM_CXSCREEN), H=GetSystemMetrics(SM_CYSCREEN), bpp=24; 
  
  CImage cImg; cImg.Create(W,H,bpp) 
  
  HDC hDC = cImg.GetDC(); 
  
  CWindowDC winDC(GetDesktopWindow()); 
  
  
  
  BitBlt(hDC, 0,0, rez.W(), rez.H(), winDC.m_hDC, 0, 0, SRCCOPY);

  At this point I am able to dump a screen shot into a bmp file -
  using cImg.Save( _T("test.bmp"), Gdiplus::ImageFormatBMP) ;

  Step2 : Extract the BMP bits from the CImg.

  HBITMAP hBitmap = (HBITMAP)cImg;
  
  HDC memDC = CreateCompatibleDC(NULL);
  
  SelectObject( memDC, hBitmap );
  
  
  
  BITMAPINFO bmi; // initialized bmi with {W,-H, plane=1, bitCount=24, comp=BI_RGB, size=W*H*3 }
  
  << removed bmi init code for conciseness. >>>
  
  
  
  BYTE *rgb24Data = new BYTE[W*H*3]; // 3 for 24bpp. 4 for 32...
  
  int ret = GetDIBits(memDC, hBitmap, 0, H, rgb24Data, &bmi, DIB_RGB_COLORS);

  At this point I faithfully believe rgb24Data points to pixel data :) - copied out of the cImg bitmap

  Step 3 : next I try to create an AV frame with the rgb24Data got from this CImg. Also this is where I have a massive knowledge gap. I am going to try and recover

  // setup the codecs and contexts here as per mohM's post
  
  
  
  AVCodec *currCodec = avcodec_find_encoder(CODEC_ID_MPEG4);
  
  
  
  AVCodecContext *codeCtxt = avcodec_alloc_context();  // init this with bate=400k, W, H,
  
  << removed codeCtxt init code for conciseness. >>>   //  time base 1/25, gop=10, max_b=1, fmt=YUV420
  
  
  
  avcodec_open(codeCtxt, currCodec);
  
  
  
  SwsContext *currSWSCtxt = sws_getContext( W, H, AV_PIX_FMT_RGB24, // FROM
  
                                            W, H, AV_PIX_FMT_YUV420P, // TO
  
                                            SWS_FAST_BILINEAR,
  
                                            NULL, NULL, NULL);
  
  
  
  // allocate and fill AVFrame
  
  int numBytes = avpicture_get_size(PIX_FMT_YUV420P, W, H);
  
  uint8_t *buffer=new uint8_t[numBytes];
  
  AVFrame *avFrame = avcodec_alloc_frame();
  
  avpicture_fill( (AVPicture*)avFrame, buffer, PIX_FMT_YUV420P, W, H );

  Step 4 : transform the data frame into YUV420P as we fill the frame.

  uint8_t * inData[1] = { rgb24Data };
  
  int inLinesize[1] = { 3*W }; // RGB stride
  
  sws_scale( currSWSCtxt, inData, inLinesize, 0, H,
  
             avFrame->data, avFrame->linesize);

  step 5 encode the frame and write out the output buffer into a file.

  int out_size = avcodec_encode_video( codeCtxt,
  
                                       outBuf,
  
                                       outBufSize,
  
                                       avFrame );
  
  
  
  fwrite(outBuf, 1, outBufSize, outFile );

  finally I close the file off with [0x00 0x00 0x01 0xb7]

  The first hint of things gone haywire is that for a 50 screens of 1920X1080 at 24bpp encoded at 25fps gives me a 507MB unplayable-mpeg file.

  As mentioned earlier, neither VLC nor WMP can play this file nor they even recognize the metadata in the file to display the FPS or video length. What would normally cause that ? Also any pointers on what could be going wrong and how to approach fixing the problems would be great. [2]

  Any guidance is much appreciated.

• FFmpeg flags for second input / audio source

  10 juillet 2015, par LouisK

  Reading FFmpeg docs it seems it’s totally possible to use two streams of buffer data to transcode. Is this wrong ?

  I’m trying to use 2 streamed inputs via a child process in node.js, but only have 1 working.

  Only appending 1 -i, 1 source piped in works for video only or an already combined stream :
  ffmpeg -i - -f mp4 -movflags frag_keyframe+faststart pipe:1

  two inputs, 2 piped sources fails :
  ffmpeg -i - -map 0:v -i - -map 0:a -ar 44100 -ac 2 -ab 96000 -f mp4 -movflags frag_keyframe+faststart pipe:1 pipe:2

  gives : Error opening input files : Invalid argument — what argument is invalid ? Is the data I’m sending it an issue ?

• Creating debug build FFMpeg static libraries with Windows compiler

  6 juillet 2015, par LostBoy

  We are using the FFMpeg static libraries compiled via the —toolchain=msvc switch with the VS2010 compiler linked to the static runtime (-MT). These libraries work fine but they are linked to libcmt.lib.

  We would like to compile a debug build of these libraries (i.e. linking against libcmtd.lib). In the configure call I added :

  ./configure --toolchain=msvc --enable-debug --arch=x86 --extra-cflags=-MTd

  This will result in some calls to cl.exe with the -MTd switch but also some with the -MT switch and cause conflicts in the linker stage.

  What am I missing ?