Thread: Compression Wedges

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  1. #21  
    Quote Originally Posted by Michael Tiemann View Post
    ...as will a second of video shot at 12 fps. I did my own testing and found that looking at still images did not give me nearly the information available from looking at a looped playback of video. In my tests, available via this dropbox link

    https://www.dropbox.com/sh/bhpx7hx6x...N0HaSIxja?dl=0

    the complex pattern of the floor is perfectly stable at lower REDCODE compression ratios, but becomes very distracting as compression rates go up. Pixel-peeping at a single image doesn't tell you how much motion noise you are going to see.
    Thanks! These are great. Yeah, it's hard to see chroma noise on a static image without tearing apart the HSV channels. This is a good illustration of how the grain characteristic changes.

    Quote Originally Posted by Tom Gleeson View Post
    Gavin,

    Thanks for spending the time to do some valuable testing. Can I ask at what crop or magnification are the images that you posted?
    Magnification was 1x for 6k, 1.2x for 5k, 1.5x for 4k and 2x for 3k. The framing was identical at every format (by adjusting focal length, moving the camera would be better if someone wants to improve on the test), all shots were rescaled to 6k so that the cropping would be consistent.
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  2. #22  
    REDuser Sponsor Gunleik Groven's Avatar
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    Gavin. This is really very valuable. Thanks a million!
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  3. #23  
    Some animated gifs to illustrate how noise gets more "chattery" at higher compression. 25MBs total. Might take a minute to load. (Note there is a good bit of atmospheric distortion in there too.

    EDIT: Changed to links. All of them couldn't play back smoothly at the same time.
    2:1
    6:1
    11:1
    15:1
    22:1


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  4. #24  
    I should have paid more attention to my motion tests and Michael's motion tests. I would strongly dissuade anyone from shooting above 13-14:1. I wonder how much Dragon grain complaints are related to compression issues. On the shoot I'm on currently we have 2 shots which aren't really usable because of compression at 17:1 (min compression at 96fps 6k WS on a 1.8" drive). As a result we've relegated 96fps to 5kWS.
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  5. #25  
    Quote Originally Posted by Gavin Greenwalt View Post
    I should have paid more attention to my motion tests and Michael's motion tests. I would strongly dissuade anyone from shooting above 13-14:1. I wonder how much Dragon grain complaints are related to compression issues. On the shoot I'm on currently we have 2 shots which aren't really usable because of compression at 17:1 (min compression at 96fps 6k WS on a 1.8" drive). As a result we've relegated 96fps to 5kWS.
    The money question in this case is whether 17:1 @ 6K scaled down to 4K is better or worse than 13:1 @ 5K scaled down. And also whether 17:1 @ 6K scaled down to 4K is better or worse than 8:1 @ 4K (no scaling). Of course I have my own data to look at, but I'm too lazy to do my own test right now.
    Michael Tiemann, Chapel Hill NC

    "Dream so big you can share!"
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  6. #26  
    Quote Originally Posted by Michael Tiemann View Post
    The money question in this case is whether 17:1 @ 6K scaled down to 4K is better or worse than 13:1 @ 5K scaled down. And also whether 17:1 @ 6K scaled down to 4K is better or worse than 8:1 @ 4K (no scaling). Of course I have my own data to look at, but I'm too lazy to do my own test right now.
    In my opinion less compression scaled up is less sharp but way less noisy which is a good tradeoff. The noise is also more than 15% larger so you're getting like 100% larger chroma flicker in exchange for 15% more luma resolution. That's a pretty big sacrifice. I think the HSV samples above illustrate that well. There is far more color detail in 11:1 compared to 17:1. And it makes sense, compression isn't magic, the data has to go somewhere. If you try to compress high frequency data to 1/17th of its original size then you are losing mathematically about 1/8th of the information no matter what. It's just gone.

    Excessive compression is a lot like aliasing. A soft image is better than image with irreversible artifacts. If you have a nearly static image with high frequency visual noise it's going to be way less distracting than the image being softer. It's also important to keep deliverables in mind. If you're delivering 6k for 1080p will the additional supersampling actually be that beneficial. On the other hand, like Moire, almost no amount of supersampling will defeat it. Best to just soften the image with an OLPF.
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  7. #27  
    Senior Member Matt Ryan's Avatar
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    Would you say the best tradeoff for quality and card space is still around 5:1?
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  8. #28  
    Senior Member Nick Morrison's Avatar
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    Gavin this is an incredibly informative and relevant thread!! Thanks so much for your time and energy in doing these tests (Michael you too!).
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  9. #29  
    I'm feeling less lazy today, so I took a page from Gavin's process. Using RCX, I made TIFF snapshots of my studio shots at 2:1, 6:1, 8:1, 10:1, 12:1, 14:1, and 22:1. I decomposed into HSV, then focused on the S layer. I used an edge-detector (Sobel) to help me see what the saturation layer looked like in that "light". Here are the images, and my take on what, if anything, they tell me.

    Using RC-S-Sobel-02.jpg (REDCODE 2:1 Saturation Sobel Edge Detection) as our baseline, we see some "edginess" throughout--an artifact of the edge-detection filter, but the grains are small, and really don't clump into lines and/or squares. If one were to blur the lower right quarter of the scene (studio floor), it would average a light shade of gray--indicated that what's being averaged are mostly the edges of the different saturation values.

    At REDCODE 6:1 this is largely preserved, and at REDCODE 8:1 this is still the predominant character, though lines and squares are beginning to emerge. The average of the floor remains light gray.

    At RECODE 10:1 and 12:1 the lines become more pronounced, and the average color of the floor begins to darken, indicating that edges are less frequent and saturation values are starting to clump together.

    At REDCODE 14:1 the lines become more pronounced, and there's a distinct darkening of the floor average.

    At REDCODE 22:1, even moreso, with the floor average distinctly dark.

    Since the forum will only let me attach 5 images per post, see the next posting for REDCODE 14:1 and 22:1.
    Attached Images
    Michael Tiemann, Chapel Hill NC

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  10. #30  
    Continuing above with REDCODE 14:1 and REDCODE 22:1...
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    Michael Tiemann, Chapel Hill NC

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