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• Stop doing this in your encoder comparisons

  14 juin 2010, par Dark Shikari — Uncategorized

  I’ll do a more detailed post later on how to properly compare encoders, but lately I’ve seen a lot of people doing something in particular that demonstrates they have no idea what they’re doing.

  PSNR is not a very good metric. But it’s useful for one thing : if every encoder optimizes for it, you can effectively measure how good those encoders are at optimizing for PSNR. Certainly this doesn’t tell you everything you want to know, but it can give you a good approximation of “how good the encoder is at optimizing for SOMETHING“. The hope is that this is decently close to the visual results. This of course can fail to be the case if one encoder has psy optimizations and the other does not.

  But it only works to begin with if both encoders are optimized for PSNR. If one optimizes for, say, SSIM, and one optimizes for PSNR, comparing PSNR numbers is completely meaningless. If anything, it’s worse than meaningless — it will bias enormously towards the encoder that is tuned towards PSNR, for obvious reasons.

  And yet people keep doing this.

  They keep comparing x264 against other encoders which are tuned against PSNR. But they don’t tell x264 to also tune for PSNR (–tune psnr, it’s not hard !), and surprise surprise, x264 loses. Of course, these people never bother to actually look at the output ; if they did, they’d notice that x264 usually looks quite a bit better despite having lower PSNR.

  This happens so often that I suspect this is largely being done intentionally in order to cheat in encoder comparisons. Or perhaps it’s because tons of people who know absolutely nothing about video coding insist on doing comparisons without checking their methodology. Whatever it is, it clearly demonstrates that the person doing the test doesn’t understand what PSNR is or why it is used.

  Another victim of this is Theora Ptalarbvorm, which optimizes for SSIM at the expense of PSNR — an absolutely great decision for visual quality. And of course if you just blindly compare Ptalarbvorm (1.2) and Thusnelda (1.1), you’ll notice Ptalarbvorm has much lower PSNR ! Clearly, it must be a worse encoder, right ?

  Stop doing this. And call out the people who insist on cheating.

• batch ffmpeg conversion (aspect, crop, and concatenate) and naming based on subfolder

  3 avril 2015, par hordor

  TYIA for any help !

  Seems I have a somewhat complicated, multi-step process. If you can help any part of it that would be appreciated.

  What I hope to accomplish

  1. Example of folder/file is here : https://mega.co.nz/#F !RJRWRLTJ !c6b-z3Vl-Tlh0uGbCflx7Q
  2. I have many subfolders that each contain 7 files from a DVD (originally PAL-encoded I think). I only care about the VTS_01_1.VOB AND VTS_01_2.VOB files. The two files together are one match
  3. The videos are stretched (from 4:3 to 16:9) and letterboxed. I want to undo that
  4. The future title of the final MP4 video should be the name of the folder
  5. BONUS : title of the video should also include the folder before it (e.g. "Friday H2 G1 CAN - BEL.mp4")

  I’d like to do this via batch file because there are a lot of folders.

  What I have done :

  dir/b/s *.vob | findstr /v VIDEO | findstr /v 01_ >list.txt
  
  
  
  @echo off
  
  for %%i in list.txt 
  
  do   
  
    ffmpeg -i VTS_01_1.VOB -aspect 4:3 "%%~pi\v1step1.mp4"
  
    ffmpeg -i vob1.mp4 -vf "crop=704:432:10:72" "%%~pi\v1step2.mp4"
  
    ffmpeg -i vob11.mp4 -c copy "%%~pi\v1step3.ts"
  
    ffmpeg -i VTS_01_2.VOB -aspect 4:3 "%%~pi\v2step1.mp4"
  
    ffmpeg -i vob2.mp4 -vf "crop=704:432:10:72" "%%~pi\v2step2.mp4"
  
    ffmpeg -i vob22.mp4 -bsf h264_mp4toannexb -c copy "%%~pi\v2step3.ts"
  
    ffmpeg -i "concat:v1step3.ts|v2step3.ts" -c copy file.mp4 "%%~pi\Match.mp4"
  
  
  
  @echo on
  
  pause

  Problem with current code

  1. Doesn’t start converting (can’t see error message either)
  2. Doesn’t have the file naming that I want
  3. A lot of steps in conversation - can that be simplified ?
  4. Not sure why it wanted the -bsf command for the second conversion but not the first

  Apologies if I’d made basic mistakes. I’m a big newbie and just to get this far has taken me hours of research. Thanks again !

• Android.mk include ffmpeg and Opencv

  28 janvier 2013, par Mark

  this is my project situation : I compiled ffmpeg in windows using this tutorial and I imported the ffmpeg folder generated inside my android project (under jni folder).
  In my project I have a jni_part.cpp where I inserted my jni function to operate with opencv.
  In my Android.mk I'm using the code below to import opencv and compile the jni_part.cpp

  include $(CLEAR_VARS)
  
  include ../../sdk/native/jni/OpenCV.mk
  
  LOCAL_MODULE    := mixed_sample
  
  LOCAL_SRC_FILES := jni_part.cpp
  
  LOCAL_LDLIBS +=  -llog -ldl 
  
  LOCAL_SHARED_LIBRARY := ffmpeg-prebuilt
  
  include $(BUILD_SHARED_LIBRARY)

  But what code I should use to import also ffmpeg ? I tried to use this code :

  include $(CLEAR_VARS)
  
  LOCAL_MODULE := ffmpeg-prebuilt
  
  LOCAL_SRC_FILES := ffmpeg-0.8/android/armv7-a/libffmpeg.so
  
  LOCAL_EXPORT_C_INCLUDES := ffmpeg-0.8/android/armv7-a/include/
  
  LOCAL_EXPORT_LDLIBS := ffmpeg-0.8/android/armv7-a/libffmpeg.so
  
  LOCAL_PRELINK_MODULE := true
  
  include $(PREBUILT_SHARED_LIBRARY)

  but if from mu jni_part.cpp code I try to include the file "#include " I get this error :

  **** Build of configuration Default for project OpenCV Tutorial 4 - Mix Java+Native OpenCV ****
  
  
  
  C:\android-ndk-r8c\ndk-build.cmd 
  
  Prebuilt       : libffmpeg.so <= jni/ffmpeg-0.8/android/armv7-a/
  
  Install        : libffmpeg.so => libs/armeabi-v7a/libffmpeg.so
  
  "Compile++ thumb : mixed_sample <= jni_part.cpp
  
  jni/jni_part.cpp:24:32: fatal error: libavutil/avstring.h: No such file or directory
  
  compilation terminated.
  
  make: *** [obj/local/armeabi-v7a/objs/mixed_sample/jni_part.o] Error 1
  
  
  
  **** Build Finished ****

  Someone could help me please ? I need to do this to include ffmpeg codec with the hope to use the function cvCaptureFromAVi...
  Thanks in advance