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What to use ProRes or ProRes HQ
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I use YUV because Y’Cr’Cb is really hard to type on an iPhone. I know that YUV is not the best way to write it, but to me, it’s shorthand. 🙂
Also, please note the original poster is talking about 8bit 4:2:0 RGB source material, not 4:2:2 YUV.
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[Jeremy Garchow] “Why wouldn’t it?”
The question was whether ProRes might do RGB-to-YUV conversion differently from ProResHQ. I assumed simplicity. A YUV codec crunching some RGB stuff first converts it to YUV and then compresses it. The color space conversion is multiplication by a 3×3 matrix. Why should a codec aiming at a greater compression use a different matrix than a codec aiming at lesser compression uses? What’s gained by changing the numbers in the matrix?I can imagine a more agressive codec chiselling more on the color subsampling over large parts of the image and even on the bit-depth over large parts of the image but what could be accomplished by deviating from the RGB-to-YUV conversion parameters in the unbusy parts of the image where color and tonality are most visible? Could a hyper-agressive codec clip the range of Y, U, or V? That would require a messy coding for decoding and would be a visual disaster for very little gain.
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Jeremy, sorry for insinuating that you dislike the Y’CrCb notation as I do. I find the symbols Cr and Cb inapt. Cr is no more about red than about green. Cb is no more about blue than about yellow. So even in a knowledgeable discussion at xlinx.com there comes this terribly sloppy statement: “Engineers found that 60 to 70 percent of luminance or brightness is found in the ‘green color’. In the chrominance part Cb and Cr, the brightness information can be removed from the blue and red colors.” Doug Kerr explains how the Cb,Cr notation came about: “The calculation of the analog quantities U and V underlying Cb and Cr involve B and R, respectively, thus the notation Cb and Cr.” This doesn’t mean, for example, that the quantity V involves the quantity R to the exclusion of the quantity G. It only means that the simplest writing of the formula for V uses the letter “R” without using the letter “G”. But the quantity G is contained in the quantity Y’ whose letter is used in that formula. Bone-headed engineers. Bad notation sows nonsense, including nonsense justifying the notation.
Sorry I forgot that the original poster was talking about RGB source material, not YUV. Is the RGB really subsampled 4:2:0? Does this mean that there is Green data for every pixel but Blue and Red data for alternating 2×2 blocks of pixels? That’s a horrible subsampling from the visual standpoint and another example of the identification of Green with luminance even though Red is a significant contributor to luminance and therefore to visual sharpness.
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Dennis Couzin: “Transcodings involve the interactions of two different codecs.”
That’s not really true. If you recompress an H.264 to ProRes, the H.264 has to be decompressed before it’s compressed to ProRes. So you’re essentially working with uncompressed every time you compress/recompress.
Obviously, the legacy of previous compression remains; but the judgement call you make when choosing a codec should not be about matching bit rates (and compensating for different compression techniques) but about how much there is/you want to preserve.
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Dennis Couzin: “Is the RGB really subsampled 4:2:0? Does this mean that there is Green data for every pixel but Blue and Red data for alternating 2×2 blocks of pixels?”
I’m not certain but I think it’s actually full range (0-255) YCbCr Rec.601. Anyone confirm or correct that?
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[Keith Pratt] “I’m not certain but I think it’s actually full range (0-255) YCbCr Rec.601”
For Canon HDSLR footage? I wouldn’t think so. You have a couple of contradictions there.
601 is SD, full range is usually reserved for RGB as YUV is usually 16-235 (SMPTE).
I’d imagine it’s sRGB, or Adobe RGB if anything, and not 709 (which is traditional HD video color space).
Jeremy
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[Dennis Couzin] “Is the RGB really subsampled 4:2:0? “
How else would it be? It’s h264 video made for the web. These are not video cameras. They are still camera with a video capability thrown in. It was made for photojournalists to take a movie and post it on an electronic newspaper website.
[Dennis Couzin] “Does this mean that there is Green data for every pixel but Blue and Red data for alternating 2×2 blocks of pixels?”
Something exactly like that, yep.
[Dennis Couzin] “That’s a horrible subsampling from the visual standpoint and another example of the identification of Green with luminance even though Red is a significant contributor to luminance and therefore to visual sharpness.”
Now you’re getting it. Why go to HQ? There’s nothing to be gained. As I said many posts ago, the damage has been done.
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sRGB would’ve been my guess seen as it’s full range and coming from a stills camera, but in this post Gary says it’s “REC 601 encoded LGOP, RGB video files compressed as h.264.”
I wasn’t aware something could be both RGB and Rec.601 at once but assumed this was some sort of bastardisation of that standard (perhaps just Rec.601 gamut or something).
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[Keith Pratt] “That’s not really true. If you recompress an H.264 to ProRes, the H.264 has to be decompressed before it’s compressed to ProRes. So you’re essentially working with uncompressed every time you compress/recompress.”
Sorry Keith
Thats incorrect, when converting from H.264 to ProRes the system works in a decompressed version, not an uncompressed version.The QT engine is designed to handle the conversion without going thru the legacy UC codec structure, thats why the conversion to ProRes does not suffer from the QT gamma issues
gary adcock
Studio37Post and Production Workflow Consultant
Production and Post Stereographer
Chicago, ILhttps://blogs.creativecow.net/24640
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[Dennis Couzin] “Is the RGB really subsampled 4:2:0? “
There is no such thing as subsampled RGB. You can reduce the bit depth but not the sampling.
[Dennis Couzin] “That’s a horrible subsampling from the visual standpoint and another example of the identification of Green with luminance even though Red is a significant contributor to luminance and therefore to visual sharpness.”
Sorry no, not on this planet.
Red contributes to the contrast but not the luma in video. Much like the human eye, the contrasty nature of a red light and its rather short wavelength have the effect allowing you to “see” more, but in reality you are seeing is the edge contrast rather and actually luminance. The longer wavelength in the Green spectrum actually allows that color to hold both luma and chroma, whereas red and blue with shorter spectral gamuts being on either end of the spectrum have less effect do to the smaller wavelengths.
gary adcock
Studio37Post and Production Workflow Consultant
Production and Post Stereographer
Chicago, ILhttps://blogs.creativecow.net/24640
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