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Posted on byOne of the problems I’ve seen with previous versions of Apple Compressor are variations in color saturation (chroma) and gray scale (gamma) when compressing files using the H.264 codec. These differences were easy to see when comparing the master file to the compressed image and I’ve written about them in the past.To fix this, I would increase color saturation and modify the gamma setting during compression. Hi Larry,Thanks for (yet another) great article. Always a great way to start Monday morning. Apple’s latest implementation of the H.264 compressor is, as you say, very good.
But with all due respect I’m not sure the method you used to test it is the most rigorous. In terms of still image quality simply comparing images on the scopes won’t give you enough data. What you really want to do is compare the original to the H.264 version by subtracting one from the other. Anything left will show the errors in the codec. Not likely to result in a perfect black frame.But even more important is measuring the inter frame performance. This is where H.264, like all inter frame compression schemes, can really fall apart. Anything that has a lot of motion between every frame, e.g., animation or images with lots of lovely film grain can get clobbered by H.264.
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I recently ran across this trying to use compressor to create H.264 versions of video art containing very (!) high inter frame motion. (Created at 29.97 progressive frames per second — they were never interlaced.) Comparing the ProRes masters to the H.264 versions was night and day. The solution was to force the H.264 compressor to make “all” frames a “keyframe” but that yielded very little reduction in file size vs.
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The ProRes 4:4:4:4 master. And they still did not match. You can view an example of the type of material I’m talking about here: (Created in FCP-X, btw.)For most projects will this mater? Probably not. And H.264 is certainly an excellent general distribution codec. But I don’t think it’s fair to say that even at identical frame sizes the “compressed images should virtually match the source.”Best Regards,Dean Winkler.
Dean:You are correct. The four determiners of image quality are: frame size, frame rate, bit rate and the amount of movement between frames. And you are also correct that the bit rate needs to grow as the amount of movement between frames increases. However, testing for that varies on a movie-by-movie basis.The problems I saw in the past were even more severe with significant losses in chroma and gray scale, which seem to be much improved with this version.Still, given enough movement, it will be impossible to compress some movies into a file size small enough to either stream or download.Larry. Hello Larry,In the history of MPEG video compression, the CCIR SMPTE Test Tape described here was the benchmark from which all high-end encoding solutions were judged:(Figure 17-N was especially difficult).We at Pacific Video Resources (PVR) saw this tape many times and were fortunate to win numerous encoding contracts during the 1990s based on our real-time video encoding solutions which we developed in partnership with Eidesign Technologies.
In our time, we encoded over 2000 feature films and television programs, primarily for those companies launching VOD systems.Please note that it is the differences between frames that makes video encoding most difficult, not the similarities. The capabilities found in Apple Compressor today far exceed those available to us in the 90s — even within our multi-million facility in which we pioneered Scene-by-Scene compression and mastered the art of converting film transfers containing 3:2 pulldown. Video compression has come a long way but the SMPTE Test Tape is still quite a viable test.
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