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• sws_scale YUV —> RGB distorted image

  3 janvier 2015, par Sami susu

  I want to convert YUV420P image (received from H.264 stream) to RGB, while also resizing it, using sws_scale.
  The size of the original image is 480 × 800. Just converting with same dimensions works fine.
  But when I try to change the dimensions, I get a distorted image, with the following pattern :

  • changing to 481 × 800 will yield a distorted B&W image which looks like it’s cut in the middle
  • 482 × 800 will be even more distorted
  • 483 × 800 is distorted but in color
  • 484 × 800 is ok (scaled correctly).

  Now this pattern follows - scaling will only work fine if the difference between divides by 4.

  Here’s a sample code of the way that I decode and convert the image. All methods show "success".

  int srcX = 480;
  
  int srcY = 800;
  
  int dstX = 481; // or 482, 483 etc
  
  int dstY = 800;
  
  
  
  AVFrame* avFrameYUV = avcodec_alloc_frame();
  
  avpicture_fill((AVPicture *)avFrameYUV, decoded_yuv_frame, PIX_FMT_YUV420P, srcX , srcY);
  
  
  
  AVFrame *avFrameRGB = avcodec_alloc_frame();
  
  
  
  AVPacket avPacket;
  
  av_init_packet(&avPacket);
  
  avPacket.size = read; // size of raw data
  
  avPacket.data = raw_data; // raw data before decoding to YUV
  
  
  
  int frame_decoded = 0;
  
  int decoded_length = avcodec_decode_video2(g_avCodecContext, avFrameYUV, &frame_decoded, &avPacket);
  
  int size = dstX * dstY * 3;
  
  
  
  struct SwsContext *img_convert_ctx = sws_getContext(srcX, srcY, SOURCE_FORMAT, dstX, dstY, PIX_FMT_BGR24, SWS_BICUBIC, NULL, NULL, NULL);
  
  
  
  avpicture_fill((AVPicture *)avFrameRGB, rgb_frame, PIX_FMT_RGB24, dstX, dstY);
  
  sws_scale(img_convert_ctx, avFrameYUV->data, avFrameYUV->linesize, 0, srcY, avFrameRGB->data, avFrameRGB->linesize);
  
  
  
  // draws the resulting frame with windows BitBlt
  
  DrawBitmap(hdc, dstX, dstY, rgb_frame, size);
  
  
  
  sws_freeContext(img_convert_ctx); 

• FFmpeg - C - Encoding video - Set aspect ratio

  22 août 2013, par Knossos

  I am decoding a video from mp2 and encoding to mp4.

  The original file :

  Video: mpeg2video (Main) ([2][0][0][0] / 0x0002), yuv420p, 720x576 [SAR 64:45 DAR 16:9]

  The resulting file :

  Video: mpeg4 (Simple Profile) (mp4v / 0x7634706D), yuv420p, 720x576 [SAR 1:1 DAR 5:4]

  As you can see, the resolution has not changed, but the aspect ratio has.

  My question, is how can I set these values (SAR and/or DAR) ?

• Can you splice a 1 min clip out of a larger file, without transcoding it ?

  9 novembre 2016, par user15063

  I have a site that allows people to upload large video files in various formats (avi, mp4, mkv and flv). I need to generate a 1 minute "sample" from the larger file that has been uploaded, and the sample needs to be in the same format, have the same frame dimensions and bit-rate as the original file. Is there a way to simply cut out a section of the file into a new file ? Preferably in ffmpeg (or any other tool if ffmpeg is impossible).