2022-A bit confused: what’s a HEIF and why do we need 10-bit stills and video?: Digital Photography Review

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Canon EOS R5 R6
The most recent cameras from most manufacturers are including 10-bit video (and sometimes stills) seize choices. The Canon EOS R5 and R6, as an illustration, achieve each.

10-bit video, 10-bit stills modes and the power to shoot ‘HEIF’ recordsdata are all options more and more being added to cameras. However what is the profit and when do you have to use these modes? We’ll take a look at how information is captured, the way it’s saved and therefore what advantages it is best to (and should not) count on from 10-bit seize.

Linear encoding

The inefficiency of linear encoding

Linear encoding devotes half the values to the brightest cease, 1 / 4 to the subsequent cease, and so forth.

Now contemplate photon shot noise; the randomness of the sunshine you captured. Shot noise is actually the sq. root of the sign. So the very shiny elements of the picture (that you simply’re devoting the majority of your uncooked values to) has the very best quantity of noise, as a result of the sq. root of a bigger quantity is greater than that of a smaller quantity. It would not look as noisy, as a result of noisiness relates extra carefully to the signal-to-noise ratio, not absolutely the noise stage. But it surely implies that you are capturing a number of very effective element about one thing with very excessive variance.

Worse nonetheless, the human visible system is much less good at discerning element and colour in shiny areas than it’s in darkish: you are over-encoding a loud sign that is not particularly visually significant. Briefly: linear encoding is vastly inefficient. (Some Uncooked compression takes benefit of this: compressing the over-encoded highlights in such a means that has no significant impact on the picture visually or when it comes to enhancing flexibility).

In contrast to the human visible system, cameras report mild in a linear method: twice as many photons hitting the sensor ends in a sign that is twice as giant, and is recorded utilizing a digital quantity that is twice as giant.

This implies half of your obtainable uncooked values are all the time consumed by the brightest cease of sunshine that you simply captured. That is simply fundamental logic: regardless of the brightest worth you captured was, half as a lot mild (ie: one cease much less mild) might be captured with a price half as giant.

The upshot of that is that in Uncooked, you may solely retailer roughly the identical variety of stops of dynamic vary as your digicam’s bit depth. Or, to get trigger and impact the suitable means spherical: the analog-to-digital converter could have been chosen as a result of it has adequate bit-depth to encode the indicators coming off the sensor. It is primarily a query of making certain you may seize and retain all the knowledge coming off the sensor. Upping the bit-depth past what’s required to completely encode the sign is not going to offer you ‘extra delicate gradation’ or ‘X million extra colours,’ it will simply imply producing a lot greater recordsdata which have recorded the noise within the shadows in additional element.

So, why can we report linear Uncooked recordsdata? As a result of it is the best factor to do from a processing perspective, retains all* of the knowledge you first captured and is not unmanageably large, because you’re usually solely capturing a single uncooked worth at every pixel, not separate crimson, inexperienced and blue values.

Gamma encoding

Gamma encoding is the method of making use of a non-linear transformation to the linear information or, extra merely: redistributing the Uncooked information in a extra space-efficient method. Sometimes the other gamma curve is utilized while you open the picture to view it, so that you simply get again to one thing that appears like the unique factor you have been attempting to seize.

Gamma encoding + tone curve

As a result of this encoding is non-linear, you may squeeze some or the entire information of a linear Uncooked file right into a a lot decrease bit-depth file. Virtually all fashionable cameras output 8-bit JPEGs, which usually embody round 9 stops of DR (extra when DR growth modes and adaptive tone curves are used). In precept you may in all probability match nonetheless extra, however along with the gamma encoding**, an ‘S’ curve is often utilized, which supplies a pleasant punchy picture.

S Curve
After gamma encoding, an ‘S’-shaped curve is utilized to most JPEG recordsdata to provide a picture with a very good stage of distinction. This assigns practically 70% of the obtainable 256 values to the 4 stops round center gray. This reduces the scope for making modifications.

With intelligent compression, a JPEG can simply be 1/sixth the dimensions of a Uncooked file however nonetheless do a very good job of conveying all the pieces you noticed while you took the picture. Or, no less than, all the pieces your 8-bit, Customary Dynamic Vary show can present. Nevertheless, as a result of lots of information has been disposed of, and since the ‘S’ curve has crushed the highlights and shadows, a JPEG presents a lot much less flexibility if you wish to make main modifications to it.

An 8-bit file has 256 information values for every color channel, and by the point you’ve got shared these values out between 9 stops, there’s not a lot scope for adjusting the values with out gaps beginning to seem, and posterization creeping in, as an alternative of easy tonal transitions. It is an amazing endpoint, although, particularly for SDR shows.

The choice of whether or not to shoot Uncooked or JPEG is, broadly talking, a query of whether or not you intend to edit the outcomes or current them kind of as-shot. Uncooked information is often 12 or 14-bit and though its linear encoding is actually inefficient, the actual fact it hasn’t been demosaiced makes it manageable. However for many makes use of, the ultimate picture might be properly expressed in an 8-bit JPEG. So why do we’d like 10-bit choices?

Log encoding – a center floor

Log encoding shares its obtainable values out extra evenly: most stops are given the identical variety of information values, slightly than being spectacularly weighted in direction of the highlights, as they’re with linear encoding or targeted on the mid-tones like most traditional tone curves.

SLog3 Curve
Log curves (Sony’s S-Log3 on this occasion), share the obtainable values far more evenly between the captured information, retaining a very good stage of flexibility for enhancing however with out the inefficiency of linear encoding. Word that the connection within the shadows shouldn’t be logarithmic (the place that method would commit extra information values than the unique linear seize).

Basically, it is a intelligent means of retaining a very good diploma of editability in a way more environment friendly file. You’ll be able to see why it will grow to be a well-liked means of working with video, the place you may retain the pliability to edit, however nonetheless profit from the very environment friendly, well-optimized codecs and filetypes which were developed for video.

A transfer from 8 to 10-bit means you’ve 1024 values to share out, so you may retain 4 instances extra details about every cease you captured. In flip, this implies tons extra flexibility when you attempt to make important changes to paint and distinction, with a lot much less threat of posterization.

Sometimes a producer will take a look at the efficiency of its cameras then develop a log curve that may encode many of the digicam’s usable dynamic vary. For this reason most producers have ended up with a number of Log curves: you do not wish to share your 1024 values out throughout 14 stops in case your digicam’s output is unusably noisy past cease 11.

For many functions, although, 10-bit encoding of Log offers an enormous increase in editabilty with out the file sizes changing into too unwieldy.

Why else would I want 10-bit?

So, 10-bit seize lets cameras supply far more editable video, with out the added dimension, and potential compatibility points (and authorized problems) of capturing Uncooked video. However there are different makes use of, which promise profit for each video and stills shooters.

A brand new technology of TVs are actually extensively obtainable that may present a wider dynamic vary than older, SDR shows. An rising variety of films and TV reveals are being shot in HDR and streaming companies can ship this HDR footage to peoples’ properties.

Hybrid Log Gamma (HLG) and Perceptual Quantizer (PQ) are the 2 commonest methods of encoding HDR information. Each require 10-bits of information as a result of they’re attempting to protect a wider tonal vary than a typical 8-bit footage. Do not get fooled by the phrase ‘Log’ within the identify HLG: solely a part of the curve is logarithmic. Each HLG and PQ are, like JPEGs, designed to be end-points, slightly than intermediates.

An rising variety of cameras can shoot HLG or PQ video for playback on HDR TVs. But it surely’s additionally more and more widespread for them to supply 10-bit stills primarily based round these requirements, for playback on HDR shows.

So 10-bit stills are for lifelike HDR?

What the HEIC? What are HEIF recordsdata?

The ten-bit stills modes on most up-to-date digicam shoot the single-image ‘HEIC’ type of HEIF recordsdata, that are basically single frames of H.265 video. Regardless of being 10-bit, the extra environment friendly compression means they’ll retain the identical picture high quality in a a lot smaller file. Panasonic as an alternative makes use of the much less widespread ‘HSP’ format for its 10-bit stills, which is part of the HLG customary. That is even much less extensively supported than HEIF, however you are prone to must plug your digicam right into a show to view the recordsdata correctly, both means.

It is truthful to say that there is not a lot consistency in what the totally different digicam makers are providing. Some digicam makers solely mean you can shoot 10-bit recordsdata while you’re capturing true HDR pictures, whereas others solely supply SDR profiles in HEIF mode, and a few allow you to shoot no matter mixture you would like.

From our perspective, there’s not lots of level capturing 10-bit stills with standard SDR curves: the info is not saved in a way that is designed for enhancing and Uncooked stays a extra highly effective choice anyway, so all you are doing is capturing one thing that’ll find yourself trying lots like a JPEG however is not as extensively supported.

Capturing true HDR pictures (HLG or PQ) in 10-bit makes much more sense. These can look spectacular when proven on an HDR show, with highlights that glisten in a means that is exhausting to convey in standard images. For the time being, although, you usually must plug your digicam right into a TV utilizing an HDMI result in view the pictures, which is not very sensible. However for us, that is the place the worth of 10-bit stills lies, photographically.

There’s lots of work to be carried out throughout the imaging business to spice up help for true HDR stills. We’d like enhancing instruments to allow us to fine-tune Raws into HDR stills, simply as we’re used to doing when produce our personal JPEGs. However above all, we’d like wider help and cross-compatibility in order that we will share and look at 10-bit recordsdata with out having to attach our digicam to the show. Till that is resolved, the power to shoot 10-bit stills is of disappointingly restricted use.


*Or practically all in case your digicam presents lossy Uncooked compression

**Strictly talking the ‘Electro Optical Switch Perform’ (EOTF) of sRGB is not a easy 2.2 gamma transformation, however it’s related sufficient that it is sometimes called such.

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