23 Apr 2015

Why everyone should be shooting 4K - even for HD delivery

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Why everyone should be shooting 4K - even for HD delivery Red Shark


Overcoming Subsampling

There are other advantages in terms of colour precision. Many 8-bit images will, as a purely practical matter, find themselves represented as some variant on what's generally referred to as YUV data, wherein the Y channel represents a black-and-white image, and the U and V channels represent the colour difference between that greyscale and the red and blue RGB channels, respectively. The usefulness of this, for those who haven't encountered it before, is that the human eye sees detail most acutely in terms of brightness, perceiving colour information at lower effective resolution. The ratios often used in relation to this approach to storing images, 4:2:2, 4:1:1 etc, refer to a reduction in resolution of the U and V channels, a technique referred to as subsampling. Higher-end broadcast formats generally use 4:2:2 subsampling, with the U and V channels stored at half the horizontal resolution of the greyscale Y channel, reducing the space requirements to store the image by a third without obvious penalties to image quality.

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This approach works well for distribution and for acquisition of material which will not be heavily processed in post production. In fact, with low compression ratios, it tends to work reasonably well for many applications. It can begin to fail when heavy grading or techniques such as chromakey are used, where the image is processed by something that isn't a human eye and which can see fine detail in colour information. Downscaling a 4K or QHD frame to HD, however, can clearly yield U and V channels without subsampling, permitting a more complete conversion to an un-subsampled result - perhaps an RGB image with identical colour and brightness resolution. There are caveats: converting between YUV and RGB data is intrinsically slighty lossy, since there are colours that either system can represent that the other can't, and it's difficult, mathematically, to claim both the un-subsampled benefit and the 10-bit-accuracy benefit simultaneously; the benefit differs for the luminance and chrominance information in this case. But, again, in practice, at least some benefit will exist.

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Wait, there's more...

There are other advantages to oversampling. Noise is reduced (on linear data) ideally by an amount representing more than a stop of either additional sensitivity or additional highlight range. On set, a focus puller can refer to a 4K display in the confidence that the display is better than the final result will be. Reframing or stabilisation can be performed in postproduction without compromising image quality. And of course, a production finished in HD now might conceivably be remastered in 4K in the future. So, even if there is only a limited market for 4K images at the moment, it can still make a lot of sense to acquire them.

Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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