8 Replies Latest reply on Nov 29, 2011 6:43 PM by supersmartdude

    Transcoding Multiple Times. Loss of visual quality?

    supersmartdude

      Hey guys,

       

      I know this may come across as quite a stupid question n is not a direction reflection of my screen name. However, I have been searching around for the answer n it seems a challenge to find. Basically what I want to know is, as h.264 as an encoder is visually losless and the fact that a majority of my work is purely YouTube based, I use it as a means of transcoding when the project gets too laggy on after effects. That way I can do a render in h.264 n then continue at a much faster speed.

       

      The question is, does multiple transcoding of the project cause the loss in quality to be visually noticeable? For example if I have encoded some footage and some compositing in h.264 n then I combine it with some other stuff n then do it again? Will it start to look worse each time I do it ?

       

      Thanks for your help in advance.

        • 1. Re: Transcoding Multiple Times. Loss of visual quality?
          marian.ene Level 2

          H.264 is far from lossless, and it's not the fastest codec neither. For a good compromise between speed and size, I'd recommend Avid DNxHD or Apple ProRes for codec choice if you use standardised resolutions, or QT animation with quality at 100 for odd frame sizes, and only the final output as H.264

          • 2. Re: Transcoding Multiple Times. Loss of visual quality?
            supersmartdude Level 1

            i've got the Avid DNxHD codec and I tried to use it as I heard a lot of good things about it....but it only gives you a massive losless file....so the problem becomes a continuous storage issue with massive files everywhere.....that's why I used H.264....

            • 3. Re: Transcoding Multiple Times. Loss of visual quality?
              marian.ene Level 2

              You've probably chose 1:1x instead of DNxHD codec. If you did choose DNxHD you could choose a lower bitrate from the "resolutions" dropdown

              dnxhdresolutions.jpg

              just make sure you choose a setting that matches your project for frame size, interlacing and FPS.

              The bitrate is the 2nd to last number (36Mbps for the attached screengrab).

              • 4. Re: Transcoding Multiple Times. Loss of visual quality?
                Mylenium Most Valuable Participant

                A Marian said, H.264 is a lossy Codec like any MPEG. After only 3 geeraions artfacts appear evn on the most pristine sources... And you do not transcode when running though AE but a full nw encode fom scratch on top of it. Since AE always fully uncompresses, it loses all realtion to the original compression. And even in other tools this is true for regions that change or have bee treated with effects. They need to be re-rendered and reencoded. And simply get another drive for storage, if you run out. Despite thee floods in Thailand making them mor scarce and a bit more expensive, even 2TB drives ar still affordable, if not ridiculously cheap...

                 

                Mylenium

                • 5. Re: Transcoding Multiple Times. Loss of visual quality?
                  supersmartdude Level 1

                  "And you do not transcode when running though AE but a full nw encode fom scratch on top of it."

                   

                  what do you mean by this? sorry i don't understand...

                   

                  thanks for your help though, looks like I need to start using a losless method of transcoding

                  • 6. Re: Transcoding Multiple Times. Loss of visual quality?
                    Dave LaRonde Level 6

                    supersmartdude wrote:

                    ... looks like I need to start using a losless method of transcoding

                     

                    Yup.  Remember the old computer programmer's mantra from Days Of Yore: "Garbage In, Garbage Out".  To preserve image quality on the trip through AE, you need to use lossless codecs, and they have huge file sizes.  You get used to it.

                    • 7. Re: Transcoding Multiple Times. Loss of visual quality?
                      Mylenium Most Valuable Participant

                      AE expands all data to a full uncompressed buffer. Thus any compression artifacts that already exist in footage become valid pixels. Since AE does not retain any memory of the original compression, it will on successive passes simply treat e.g. edges of blocks as if you intentionally put them there, which is of course not what you want. Therefore people go to quite some lengths to reduce and remove these artifacts by stacking layers and using various effects to break up these unwanted edges. I suggest you look up "MPEG artifact removal" or "DV artifact removal" on a web search to get some idea about techniques. For intermediates simply use uncompressed formats or formats that only do binary compression, but no visual compression. Anything from Quicktime animation to PNG or TIFF sequences. Only apply the compression in the final output.

                       

                      Mylenium

                      • 8. Re: Transcoding Multiple Times. Loss of visual quality?
                        supersmartdude Level 1

                        thanks heaps guys!!! this has been a very insightful thread....appreciate your help and hopefully the quality of work will start to increase!