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Des sites réalisés avec MediaSPIP
2 mai 2011, parCette page présente quelques-uns des sites fonctionnant sous MediaSPIP.
Vous pouvez bien entendu ajouter le votre grâce au formulaire en bas de page. -
Publier sur MédiaSpip
13 juin 2013Puis-je poster des contenus à partir d’une tablette Ipad ?
Oui, si votre Médiaspip installé est à la version 0.2 ou supérieure. Contacter au besoin l’administrateur de votre MédiaSpip pour le savoir -
Changer son thème graphique
22 février 2011, parLe thème graphique ne touche pas à la disposition à proprement dite des éléments dans la page. Il ne fait que modifier l’apparence des éléments.
Le placement peut être modifié effectivement, mais cette modification n’est que visuelle et non pas au niveau de la représentation sémantique de la page.
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Il suffit ensuite de se rendre dans l’espace de configuration du (...)
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Why is ffmpeg's conversion to YUV420 so poor ?
8 novembre 2020, par HuguesI have been using
ffmpegand other compression tools to compare rate-distortion curves for YUV420-resampled video.
In these comparisons, results fromffmpegare consistently worse, with PSNR values that are 0.5-1.0 dB lower.

I tracked the problem to
ffmpeg's conversion between RGB and YUV420.
To simplify, let us assume "lossless compression" and therefore consider only RGB -> YUV420 -> RGB.
Also, we operate on a single PNG image frame.

# Use some default options.
ffmpeg="ffmpeg -nostdin -hide_banner -v error"

# Obtain a source image.
wget -nv -O original.png https://i.stack.imgur.com/8J1qY.png
size="256x256"

# Compare it with itself to verify that we get an infinite average PSNR.
$ffmpeg -v info -i original.png -i original.png -lavfi psnr -f null - |& grep PSNR
# average:inf

# Convert the image to YUV420, and convert back to RGB.
$ffmpeg -i original.png -pix_fmt yuv420p -f rawvideo -y temp1.yuv420
$ffmpeg -f rawvideo -s $size -pix_fmt yuv420p -i temp1.yuv420 -y result1.png

# Compare it with the original image to measure the PSNR (in dB).
$ffmpeg -v info -i result1.png -i original.png -lavfi psnr -f null - |& grep PSNR
# average:36.894551


Now, as an alternative, we perform the RGB <-> YUV420 chroma resampling manually :


yuv444_to_yuv420="extractplanes=y+u+v[y][u][v];\
 [u]scale=w=iw/2:h=ih/2:flags=area[u];\
 [v]scale=w=iw/2:h=ih/2:flags=area[v];\
 [y][u][v]mergeplanes=0x001020:yuv420p"
yuv420_to_rgb="extractplanes=y+u+v[y][u][v];\
 [u]scale=w=iw*2:h=ih*2:flags=neighbor[u];\
 [v]scale=w=iw*2:h=ih*2:flags=neighbor[v];\
 [y][u][v]mergeplanes=0x001020:yuv444p,format=rgb24"

$ffmpeg -i original.png -pix_fmt yuv444p -f rawvideo - | \
 $ffmpeg -f rawvideo -pix_fmt yuv444p -s $size -i - \
 -lavfi "$yuv444_to_yuv420" -f rawvideo -y temp2.yuv420
$ffmpeg -f rawvideo -pix_fmt yuv420p -s $size -i temp2.yuv420 \
 -lavfi "$yuv420_to_rgb" -y result2.png

# Measure PSNR by comparing with the original image.
$ffmpeg -v info -i result2.png -i original.png -lavfi psnr -f null - |& grep PSNR
# average:37.536444
# This is an improvement of 0.64 dB!


This brings up two questions :


- 

- Why doesn't
ffmpegimplement a better conversion to/fromyuv420pby default ? - Is there any simpler way to obtain or express this improved conversion ?






- Why doesn't
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Why is ffmpeg cut setting the wrong duration on my output file
9 mai 2023, par danielson317Setup


I am trying to break a video up into parts using some basic commands I have seen available.


I have tried :


ffmpeg -ss 00:00:00 -t 00:11:42 -i 'original.m4v' -acodec copy -vcodec copy 'output.m4v'

I also tried more generic (and much slower) :


ffmpeg -ss 00:00:00 -t 00:11:42 -i 'original.m4v' 'output.m4v'

I am on Ubuntu 20 with ffmpeg 4.2.7-0. I know it is a little outdated but this computer lost it's network card so I cannot update it at this time.


The problem


In both cases the video cuts correctly to only have the first 11:42. However the video properties state the video is 31 minutes long (the length of the original). This file size also reflects the reduced size I would expect from the cut. When I play the video in VLC it reports the video is 31 minutes long but scanning or playing past 11:42 "crashes" the playback.


Question


Why is the new video reporting the wrong length and how do I correct this ? Preferably in the conversion process not a secondary command.


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How to convert YUV422P image to JPEG using ffmpeg's libraries ?
21 juillet 2017, par user3743908I’m trying to convert a YUV422 image (YUV422_8_UYVY, unsigned ,unpacked, 16bpp) in to jpeg using ffmpeg’s ,this is Code which i am following
How to convert YUV420P image to JPEG using ffmpeg’s libraries ?
Image size : 2448x2050
Original YUV Image : not able to upload as the format is YUV(Original Image Decodec by ffmpeg command prompt)
Image :This is original ImageImage size : 2448x2050
reconstruct Image :Reconstruct Image through above Codeso the reconstruct image is not as the original image
my format is UYVY whereas supported format is AV_PIX_FMT_YUVJ420P
so what should be the correct format for UYVY input image...?
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