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• Surfaceview for subtitles alpha does not work

  27 mai 2018, par user654628

  Goal : trying to build video player with subtitles for android. Video can be low resolution but the subtitles should be resolution of phone (such that if video is 720p, the subtitles should render to screen size say 1080p).

  Issue : I am using FFMPEG to render a frame at say 720p but phone device is 1080p. I need to display subtitles that are different resolution than the subtitles resolution so pixel blending is difficult.

  I first tried to scale the frame (AVFrame) with sws_convert but each frame took 80ms so that is not an option (since it is running software).

  Then I tried two surface views, one for the video and one for subtitles where video would be 720p and subtitles SurfaceView is 1080p, then the video scales up to the phone size. The issue here is that the subtitles are not translucent. Black opacity 0 would be transparent but white with alpha 0 is still white. Why is this ?

  //Code from Java, the view that extends FrameLayout
  
  public VideoView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
  
      super(context, attrs, defStyleAttr);
  
      mVideoSurface = new SurfaceView(context);
  
      mSubtitlesSurface = new SurfaceView(context);
  
      addView(mVideoSurface);
  
      addView(mSubtitlesSurface);
  
      mVideoSurface.getHolder().addCallback(mSurfaceCallback);
  
      mSubtitlesSurface.getHolder().addCallback(mSurfaceCallback);
  
      mSubtitlesSurface.setZOrderMediaOverlay(true);
  
      mSubtitlesSurface.getHolder().setFormat(PixelFormat.TRANSLUCENT);
  
      //etc
  
  }

  Eventually I tried as a test to render a square to the subtitle surface view (C++)

  // Render the video frame, now render the subtitle frame
  
  ANativeWindow_Buffer buffer;
  
  ANativeWindow_setBuffersGeometry(subWindow, width, height, WINDOW_FORMAT_RGBA_8888);
  
  if ((ret = ANativeWindow_lock(subWindow, &buffer, NULL)) < 0) {
  
      return ret;
  
  }
  
  for (int j = height/2; j < height/2 + 100; j++) {
  
      for (int i = width/2; i < width/2 + 100; i++) {
  
          uint8_t * d =  (uint8_t*)buffer.bits + j * (buffer.stride * 4) + i * 4;
  
          d[0] = 0xff;
  
          d[1] = 0xff;
  
          d[2] = 0xFF;
  
          d[3] = 0;           /* alpha */
  
  
  
      }
  
  }
  
  ANativeWindow_unlockAndPost(subWindow);

  So above code should render a white square in the image with 0 alpha (so should be invisible), but it is shown. If I change it to yellow with alpha 0 it will be visible but not the correct color. If I change to white with 1 alpha, it is white and opaque. If I use black with alpha 0xCC, it is invisible, only if alpha is 0xFF then it is visible as black. Seems to have no translucency even though I added it to the SurfaceHolder. Why is it like this ? I can add more code if needed.

  Is my only option to do what I want to render frame as a texture in OpenGL and (GLSurfaceView), resize the image to phone resolution and blend the alpha subtitles onto the frame as a texture ?

  Thanks in advance.

• Is there a way for ffmpeg to overlay png subtitles ?

  19 mai 2019, par ib11

  I am working on a project to embed 24-bit png subtitles (they have custom letter and line-spacing as well as diffused dropshadow).

  I am trying to find a way to overlay them with ffmpeg.

  So far the only way I found was creating a DVDSUB file and overlay that, I simply used the following command from my earlier post :

  @echo off
  
  set srcVidFile1="myvideo.mov"
  
  set srcSubFile=mysub.sub
  
  set srcIdxFile=mysub.idx
  
  set destFile=D:\testsub.mp4
  
  
  
  ffmpeg.exe -i %srcVidFile1% -i %srcSubFile% -i %srcIdxFile% 
  
      -filter_complex "[0:v][1:s]overlay" -c:s dvdsub -c:v 
  
      libx264 -crf 24 -c:a aac -b:a 160K -ar 48K -strict -2 -y 
  
      %destFile%

  The DVDSUB pair is here.

  However I did not find yet a way to create a DVDSUB with the 24-bit png files I have.

  I also looked at using SSA format as it does have a picture input, but I was not able to overlay the PNGs with ffmpeg. It seems to me that ffmpeg supports only "Dialogue" events in the SSA file, no "Picture" events.

  Is there a way to overlay subtitles that are in PNG format ?

• Revision 16863 : when subtitles input comes from a srt file, always setup millisecond ...

  1er février 2010, par oggk — Log

  when subtitles input comes from a srt file, always setup millisecond
  timing resolution, as that’s what SRT timings may have.