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• Trouble concatenating with ffmpeg (threads)

  4 janvier 2014, par mistypants

  I'm writing a batch script to automate one of my video editing processes. I've gotten everything done up to the final step, when I concatenate the video intro (1080.avi) with the video itself, then output. Here's the concatenate part of the script :

  ffmpeg -i 1080.avi -i %input%-lossless.avi ^
  
  -filter_complex "[0:0] [0:1] [1:0] [1:1] concat=n=2:v=1:a=1 [v] [a]" ^
  
  -map "[v]" -map "[a]" ^
  
  -c:v libx264 -preset veryslow -pix_fmt yuv420p ^
  
  -b:a 160k ^
  
  -threads 2 %input%-final.mp4

  It seems to work fine, but my computer overheats (it's a laptop) while it runs. I had this problem with just simple conversions and ffmpeg, so I added the -threads 2 part which eliminated the problem. It's back now.

  Did I mess up something in the script, or does concatenating just use more of the CPU ? If the latter, could I just decrease it to 1 thread ?

• Debugging Video Frame Differences

  14 mai 2014, par jvenema

  I’m trying to decode an h264-encoded video frame using a decoder that has fairly limited capabilities (Broadway JS).

  Here’s the process I’m using for testing :

  1. Encode single image using h264 (output is 62KB) using OpenH264 from Cisco, write to disk
  2. Read file as binary in JS (loaded via XHR)
  3. Decode in JS using Broadway
  4. Display in canvas

  I actually have a functional example of this, but there’s a manual step at the moment between steps 1 and 2 - I have to call FFMPEG to force a small translation, and I can’t figure out why.

  I have successfully integrated Cisco’s Openh264 library into my project, and can encode the image as h264 and write it to disk. I’ve confirmed the encoding itself is valid by using FFMPEG to extract the frame back out as a JPEG (looks perfect).

  But, to make the frame decode properly in JS, I have to do a conversion using FFMPEG. It does some magic, the file size drops to 58KB, and voila, everything starts working.

  The Openh264 libraries only work with the baseline configuration, so I’m assuming I have some other random parameter set incorrectly. Problem is, if I use ffprobe to compare the frames - the settings are identical ! But the file sizes are different, so obviously something is different.

  So now the final question - what tool can I use to truly see what FFMPEG is doing behind the scenes here to make my "manually" encoded frame work ? Is there a tool that will let me compare all the details of two h264 frames ?

• VBV-CBR and ABR in X264

  11 décembre 2013, par xkfz007

  In x264 VBV-CBR RC mode is based on ABR and vbv-maxrate and vbv-bufsize are set. On the basis of frame QP decided by ABR, the MB row QP is decided by VBV-CBR for each row. I test these two modes and the command lines are listed :
  ABR : -o BasketballDrill_832x480_50_20131210102507.264 BasketballDrill_832x480_50.yuv 832x480 —fps 50 —frames 300 —bitrate 1120 —profile baseline —threads -1 -I 50 -v —psnr
  VBV-CBR : -o BasketballDrill_832x480_50_20131210102507_mb.264 BasketballDrill_832x480_50.yuv 832x480 —fps 50 —frames 300 —bitrate 1120 —vbv-maxrate 1120 —vbv-bufsize 1680 —profile baseline —threads 1 -I 50 -v —psnr

  The final results are :
  ABR : Y-PSNR 31.530dB, bitrate 1146.93kbps
  VBV-CBR : Y-PSNR 31.728dB, bitrate 1230.25kbps
  Compared to ABR, VBV-CBR achieved 4.9% bitrate promotion and 0.198dB PSNR promotion. But the PSNR promotion is lower than bitrate (1% bitrate=0.05dB).

  Having talked so much, I just want to ask what is the advantage of VBV-CBR compared ABR ? If ABR can get the reasonable results, why the VBV-cBR is needed ?
  In my opinion, VBV-CBR maybe can promote the subjective quality owing to deciding the QP for each MB row. But the bitrate also rises. Also I have compared some frames of the two modes, they are almost the same.
  Maybe there are some situations that are fit for VBV-CBR and I don't know. So guys, give me, the new to x264, some professional explanation.