Recherche avancée

Sur d’autres sites (4291)

• Making Sure The PNG Gets There

  14 juin 2013, par Multimedia Mike — General

  Rewind to 1999. I was developing an HTTP-based remote management interface for an embedded device. The device sat on an ethernet LAN and you could point a web browser at it. The pitch was to transmit an image of the device’s touch screen and the user could click on the picture to interact with the device. So we needed an image format. If you were computing at the time, you know that the web was insufferably limited back then. Our choice basically came down to GIF and JPEG. Being the office’s annoying free software zealot, I was championing a little known up and coming format named PNG.

  So the challenge was to create our own PNG encoder (incorporating a library like libpng wasn’t an option for this platform). I seem to remember being annoyed at having to implement an integrity check (CRC) for the PNG encoder. It’s part of the PNG spec, after all. It just seemed so redundant. At the time, I reasoned that there were 5 layers of integrity validation in play.

  I don’t know why, but I was reflecting on this episode recently and decided to revisit it. Here are all the encapsulation layers of a PNG file when flung over an ethernet network :

  
  
  

  So there are up to 5 encapsulations for the data in this situation. At the innermost level is the image data which is compressed with the zlib DEFLATE method. At first, I thought that this also had a CRC or checksum. However, in researching this post, I couldn’t find any evidence of such an integrity check. Further, I don’t think we bothered to compress the PNG data in this project long ago. It was a small image, monochrome, and transferring via LAN, so the encoder could get away with signaling uncompressed data.

  The graphical data gets wrapped up in a PNG chunk and all PNG chunks have a CRC. To transmit via the network, it goes into a TCP frame, which also has a checksum. That goes into an IP packet. I previously believed that this represented another integrity check. While an IP frame does have a checksum, the checksum only covers the IP header and not the payload. So that doesn’t really count towards this goal.

  Finally, the data gets encapsulated into an ethernet frame which has — you guessed it — a CRC.

  I see that other link layer protocols like PPP and wireless ethernet (802.11) also feature frame CRCs. So I guess what I’m saying is that, if you transfer a PNG file over the network, you can be confident that the data will be free of any errors.

• AWS Lambda and Fluent FFMPEG error "cannot read property "isStream" of undefined"

  29 mai 2021, par Travis Lee

  so here's the goal : convert a .webm file hosted in an S3 into a gif and upload that to a new bucket. This all works fine when run locally, but when trying to translate it into a lambda, fluent-ffmpeg throws errors when it runs the command.

  


  Here's the code snippet :

  


  ffmpeg(new URL(vid))
  .outputOptions("-vf", "scale=320:-1:flags=lanczos,fps=14")
  .on('progress', () => {
      console.log('progress');
  })
  .on('end', () => {
     //Do stuff with the result when it is done
  })
  .output(newKey)
  .run(newKey);


  


  in this snippet, "vid" is a presigned GET url for an S3 bucket containing the .webm video file, and "newKey" is the name of the new bucket (and a temporary writeStream/File that is created in the lambda to store the new .gif file until we upload it to S3 - not super relevant to this issue).

  


  What should happen (and does locally) is that a new output is created containing the converted .gif file

  


  What happens when it is deployed in a lambda is that it reaches the .outputOptions call and throws a type error saying that it cannot read property isStream of undefined.

  


  At first glance, this seems like I simply don't have FFMPEG installed in the lambda, but I do. I have tried with the prebuilt layer using NodeJS 10 found here : https://serverlessrepo.aws.amazon.com/applications/us-east-1/145266761615/ffmpeg-lambda-layer ,
with a NodeJS 12 layer that was built by some engineers here previously, and tried building a NodeJS 14 FFMPEG layer myself and using that. I tried for all three using no configuration and letting it call the PATH ffmpeg, using the FFMPEG_PATH and FFPROBE_PATH environment variables set to either what was specified in the previous layers, or what I made it in the newly built one, and even manually setting the path to the executables using the setFfmpegPath and setFfprobePath functions found on the fluent-ffmpeg object.

  


  Lastly, I even tried bundling the executables in with the actual lambda code itself and uploading it through an S3, trying all three above methods of getting it to point to the correct paths once again to no avail.

  


  I'm seriously in need of help if anyone else has encountered something similar or just might know what is going on. I'm at wit's end here trying to figure this out.

  


• FFMPEG Layering two .mov files color issue

  21 février 2023, par JDC

  I have two folders containing numbered .mov files all the same length, one folder for front and one for back. I'm using ffmpeg to combine the front and back videos with matching numbers into one output video (the front video has an alpha channel). The below command works, but in the output video the front layer has slightly incorrect colors compared to the input (reds are too yellow for example). The back layer in the output looks correct. Both input files use the bt709 color space (checked with ffprobe). Is there something in the overlay options which is causing the front layer to look incorrect, or something I'm doing wrong with the colorspace options ?

  


  


  for f in Video_Back/Video_Back_*.mov ; do
num=$(echo "$f" | sed -n 's/.*Video_Back_([0-9]5).mov$/\1/p')
if [ -f "Video_Front/Video_Front_$num.mov" ] ; then
ffmpeg -color_primaries bt709 -color_trc gamma28 -colorspace bt709 -i "$f" -color_primaries bt709 -color_trc gamma28 -colorspace bt709 -i "Video_Front/Video_Front_$num.mov" -filter_complex "[1:v]format=rgba[fg] ;[0:v][fg]overlay=shortest=1:alpha=straight,eq=gamma=1.0[mixed]" -map "[mixed]" -map 0:a:0 -c:v libx264 -crf 18 -preset veryfast -c:a copy "Video_Merged/Video_Merged_$num.mp4"
fi
done