Category Archives: HDR

What is HLG and what is it supposed to be used for?

While we wait for Sony to re-release the version 4 firmware for the FS5 I thought I would briefly take a look at what HLG is and what it’s designed to do as there seems to be a lot of confusion.

HLG stands for Hybrid Log Gamma. It is one of the gamma curves used for DISTRIBUTION of HDR content to HDR TV’s that support the HLG standard. It was never meant to be used for capture, it was specifically designed for delivery.

As the name suggests HLG is a hybrid gamma curve. It is a hybrid of Rec-709 and Log. But before you get all excited by the log part, the log used by HLG is only a small part of the curve and it is very agressive – it crams a very big dynamic range into a very small space – This means that if you take it into post production and start to fiddle around with it there is a very high probability of problems with banding and other similar artefacts becoming apparent.

The version of HLG in the FS5 firmware follows the BBC HLG standard (there is another NHK standard). From black to around 70% the curve is very similar to Rec 709, so from 0 to 70% you get quite reasonable contrast. Around 70% the curve transitions to a log type gamma allowing a dynamic range much greater than 709 to be squeezed into a conventional codec. The benefit this brings is that on a conventional Rec-709 TV the picture doesn’t look wrong. It looks like a very slightly darker than normal, only slightly flat mid range, but the highlights are quite flat and  washed out. For the average home TV viewer watching on a 709 TV the picture looks OK, maybe not the best image ever seen, but certainly acceptable.

However feed this same signal to an HDR TV that supports HLG and the magic starts to happen. IF the TV supports HLG (and currently only a fairly small proportion of HDR TV’s support HLG. Most use PQ/ST2084) then the HLG capable HDR TV will take the compressed log highlight range and stretch it out to give a greater dynamic range display. The fact that the signal gets stretched out means that the quality of the codec used is critical. HLG was designed for 10 bit distribution using HEVC, it was never meant to be used with 8 bit codecs, so be very, very careful if using it in UHD with the FS5 as this is only 8 bit.

So, HLG’s big party trick is that it produces an acceptable looking image on a Rec-709 TV, but also gives an HDR image on an HDR TV. So one signal can be used for both HDR and SDR giving what might be called backwards compatibility with regular SDR TV’s. But it is worth noting that on a 709 TV HLG images don’t look as good as images specifically shot or graded for 709. It is a bit of a compromise.

What about the dynamic range? High end HDR TV’s can currently show about 10 stops. Lower cost HDR TV’s may only be able to show 8 stops (compared to the 6 stops of a 709 TV). There is no point in feeding a 14 stop signal to a 10 stop TV, it won’t look the best. From what I’ve seen of the HLG curves in the FS5 they allow for a maximum of around 10 to 11 stops, about the same as the cinegammas. HLG can be used for much greater ranges, but as yet there are no TV’s that can take advantage of this and it will be a long tome before there are. So for now, the recorded range is a deliberately limited so you don’t see stuff in the viewfinder that will never be seen on todays HDR TV’s.  As a result the curves don’t use the full recording range of the camera. This means they are not using the recording data in a particularly efficient way, a lot of data is unused and wasted. But this is necessary to make the curves directly compatible with an HLG display.

What about grading them? My advice – don’t try to grade HLG footage. There are three problems. The first is that the gamma is very different in the low/mid range compared to the highlights. This means that in post the shadows and mid range will respond to corrections and adjustments very differently to the high range. That makes grading tricky as you need to apply separate correction to the midrange and highlights.

The second problem is that the is a very large highlight range squeezed into a very small recording range. It should look OK when viewed directly with no adjustment. But if you try stretching that out to make the highlights brighter (remember they never reach 100% as recorded) or to make them more contrasty, there is a higher probability of seeing banding artefacts than with any other gamma in the camera.

The third issue is simply that the limited recording range means you have fewer code values per stop than regular Rec-709, the cinegammas or S-Log2. HLG is the least best choice for grading in the FS5.

Next problem is color. Most HDR TV’s want Rec-2020 color. Most conventional monitors want Rec-709 color. Feed Rec-2020 into a 709 monitor and the colors look flat and the hues are all over the place, especially skin tones. Some highly saturated colors on the edge of the color gamut may pop out more than others and this looks odd.

Feed 709 into a 2020 TV and it will look super saturated and once again the color hues will be wrong. Also don’t fool yourself into thinking that by recording Rec2020 you are actually capturing more. The FS5 sensor is designed for 709. The color filters on the sensor do work a little beyond 709, but nowhere near what’s needed to actually “see” the full 2020 color space. So if you set the FS5 to 2020 what you are capturing is only marginally greater than 709. All you really have is the 709  with the hues shifted and saturation reduced so color looks right on a 2020 monitor or TV.

So really, unless you are actually feeding an Rec 2100 (HLG + 2020) TV, there is no point in using 2020 color as this require you to grade the footage to get the colors to look right on most normal TV’s and monitors. As already discussed, HLG is far from ideal for grading, so better to shot 709 if that’s what your audience will be using.

Don’t let the hype and fanfares that have surrounded this update cloud your vision. HLG is certainly very useful if you plan to directly feed HDR to a TV that supports HLG. But if you plan on creating HDR content that will be viewed on both HLG TV’s and the more common PQ/ST2084 TV’s then HLG is NOT what you want. You would be far – far better off shooting with S-Log and then grading your footage to these two very different HDR standards. If you try to convert HLG to PQ it is not going to look nearly as good as if you start with S-Log.

Exposure levels: If you want to get footage that works both with an HLG HDR TV and a SDR 709 TV then you need to expose carefully. A small bit of over exposure wont hurt the image when you view it on a 709 TV or monitor, so it will look OK in the viewfinder. But on an HDR TV any over exposure could result in skin tones that look much too bright and an image that is unpleasantly bright. As a guide you should expose diffuse 90% white (a white card or white piece of paper) at no more than 75%. Skin tones should be around 55 to 60%. You should not expose HLG as brightly as you do Rec-709.

Sure you can shoot with HLG for non HDR applications. You will get some slightly flat looking footage with rolled off highlights. If that’s the image you want then I’m not going to stop you shooting that way. If that’s what you want I suggest you consider the Cinegamma as these capture a similar DR also have a nice highlight roll off (when exposed correctly) and do use the full recording range.

Whatever you do make sure you understand what HLG was designed for. Make sure you understand the post production limitations and above all else understand that it absolutely is not a substitute for S-log.

Want to shoot direct to HDR with the PXW-FS7, PMW-F5 and F55?

Sony will be releasing an update for the firmware in the Sony PXW-FS5 in the next few days. This update amongst other things will allow users of the FS5 to shoot to HDR directly using the Hybrid Log Gamma HDR gamma curve and Rec2020 color. By doing this you  eliminate the need to grade your footage and could plug the camera directly in to a compatible HDR TV (the TV must support HLG) and see an HDR image directly on the screen.

But what about FS7 and F5/F55 owners? Well, for most HDR productions I still believe the best workflow is to shoot in S-Log3 and then to grade the footage to HDR. However there may be times when you need that direct HDR output. So for the FS7, F5 and F55 I have created a set of Hybrid Log Gamma LUT’s that you can use to bake in HLG and Rec2020 while you shoot. This gives you the same capabilities as the FS5 (with the exception of the ability to add HLG metadata to the HDMI).

For a video explanation of the process please follow the link to my new Patreon page where you will find the video and the downloadable LUT’s.

PXW-FS5 and PXW-Z150 to get Hybrid Log Gamma for direct HDR production.

For the full details please see the official Sony announcement: https://www.sony.co.uk/pro/press/pr-sony-expands-hdr-production-capabilities

The PXW-FS5 and PXW-Z150 will both get a free firmware update some time around June that will add the ability to shoot using a special gamma curve called “Hybrid Log Gamma” or HLG.

In the case of the FS5 this will be added through an additional picture profile, PP10. As well as HLG the camera will also have the ability to record using Rec2020 color. As a result the camera will become compatible with the new Rec2100 standard for HDR television.

In addition the FS5 will get the ability to change the base ISO for S-Log2 and S-Log3 from 3200 ISO to 2000 ISO. This will help produce cleaner images that are easier to grade. On top of that via a paid firmware update you will be able to shoot continuously at up to 120fps in full HD, no need to use the Super Slow Motion memory cache function.

These are all great upgrades for this little highly versatile camera.

By selecting Picture Profile 10  the camera will shooting using Hybrid Log Gamma. If you were to plug the camera into an HDR TV that supports HLG then what you would see on the TV would be a HDR image with an extended dynamic range. This should give brighter more realistic highlights and a quite noticeable increase in overall contrast compared to SDR (Standard Dynamic Range). There will be no need to grade the footage to get a perfectly watchable vibrant HDR image. The real beauty of HLG (developed by the BBC and NHK) is that it is backwards compatible with normal SDR (Standard Dynamic Range) TV’s. So feed the very same signal into a conventional SDR TV and it will look just fine. Skin tones will be a touch darker than with Rec709 and  it won’t be HDR, but it will be perfectly watchable picture and most people won’t realise it’s anything different to normal SDR TV.

So HLG provides a simple very fast, direct HDR workflow that is backwards compatible with SDR TV’s. As a result you don’t need any special monitors to shoot with it, you can just monitor with existing SDR monitors, although it would be beneficial to have an HDR monitor to check the HDR aspect of the signal. HLG isn’t designed to be graded, although a little bit of post production tweaking can be applied, just as with Rec709.  Bottom line is it’s quick and easy, no special monitors or skills needed – simples.

If you want the very best possible HDR then you should shoot with S-Log2/S-Log3 or raw and then grade the material in post using an HDR capable monitor.  But that takes time and large HDR monitors are not cheap (for a small monitor you could use an Atomos Flame or Inferno).  The FS5 will give you the ability to work either way. HLG for simple and quick, S-Log for the best possible image quality.

Hopefully we will see HLG rolled out to other cameras in the near future.

What does Rec-2020 on the PXW-FS7 II really mean?

So, as you should have seen from my earlier post Sony has included Rec-2020 as a colorspace in custom mode on the new FS7 II. But what does this mean and how important is it? When would you use it and why?

Recommendation ITU BT.2020 is a set of standards created by the International Telecommunications Union for the latest and next generation of televisions. Within the standard there are many sub-standards that define things such as bit depth, frame size, frame rates, contrast, dynamic range and color.

Sony-Colorspaces-1024x815 What does Rec-2020 on the PXW-FS7 II really mean?
The colorspaces that Sony’s cameras can capture.

The Rec-2020 addition in the the FS7 II specifically refers to the color space that is recorded, determining the range of colors that can be recorded and the code values used to represent specific tones/hues.

First of all though it is important to remember that the FS7 II shares the same sensor as the original FS7, the FS5 and F5. Sony has always stated that this sensor is essentially a “709” sensor. The sensor in Sony’s PMW-F55 can capture a much greater color range (gamut) than the F5, FS5 and FS7, only the F55 can actually capture the full Rec-2020 color space, the FS7 II sensor cannot. It’s very difficult to measure the full color gamut of a sensor, but from the tests that I have done with the F5 and FS7 I estimate that this sensor can capture a color gamut close to that of the DCI-P3 standard, so larger than Rec-709 but not nearly as large as Rec-2020 (I’d love someone to provide the actual color gamut of this sensor).

So given that the FS7 II’s sensor can’t actually see colors all that far beyond Rec-709 what is the point of adding Rec-2020 recording gamut as the camera can’t actually fill the recording Gamut? Similarly the F5/FS5/FS7 cannot fill S-Gamut or S-Gamut3.

The answer is – To record the colors that are captured with the correct values. If you capture using Rec-709 and then play back the Rec-709 footage on a Rec-2020 monitor the colors will look wrong. The picture will be over saturated and the hues slightly off. In order for the picture to look right on a Rec-2020 monitor you need to record the colors at the right values. By adding Rec-2020 to the FS7 II Sony have given users the ability to shoot Rec-2020 and then play back that content on a Rec-2020 display and have it look right. You are not capturing anything extra (well, maybe a tiny bit extra), just capturing it at the right levels so it at least looks correct.

As well as color, Rec-2020 defines the transfer functions, or gamma curves to you and me, that should be used. The basic transfer function is the same as used for Rec-709, so you can use Rec-709 gamma with Rec-2020 color to get a valid Rec-2020 signal. For full compatibility this should be 3840×2160 progressive and 10bit (the Rec-2020 standard is a minimum of 10bit and as well as 3840×2160 also includes 7680×4320).

But, one of the hot topics right now in the high quality video world is the ability to display images with a much greater dynamic range than the basic Rec-709 or Rec-2020 standards allow. There is in fact a new standard called Rec-2100 specifically for HDR television. Rec-2100 uses the same colorspace as Rec-2020 but then pairs that bigger colorspace with either Hybrid Log Gamma or ST2084 gamma, also know as PQ (Perceptual Quantiser). As the FS7 II does not have PQ or HLG as gamma curves you cannot shoot material that is directly compatible with Rec-2100. But what you can do is shoot using S-Log2/S-Log3 with S-Gamut/S-Gamut3/SGamut3.cine which will give you the sensors full colorspace with the sensors full 14 stop dynamic range. Then in post production you can grade this to produce material that is compatible with the Rec-2100 standard or the Rec-2020 standard. But of course you can do this with an original FS7 (or F5) too.

So, when would you actually  use the FS7 II’s Rec-2020 colorspace rather than S-Log/S-Gamut?

First of all you don’t want to use it unless you are producing content to be shown on Rec-2020 displays. Recording using Rec-2020 color gamut and then showing the footage on a Rec-709 display will result in washed out colors that don’t look right.

You would probably only ever use it if you were going to output directly from the camera to a monitor that only supports Rec-2020 color or for a project that will be specifically shown on a standard dynamic range Rec-2020 display. So, IMHO this extra colorspace is of very limited benefit. For most productions regular Rec-709 or S-Log/S-Gamut will still be the way forward unless Sony add Hybrid Log Gamma or PQ gamma to the camera as well. Adding HLG or PQ however has problems of it’s own as the existing viewfinders can only show standard dynamic range images, so an external HDR capable monitor would be needed.

Rec-2020 recording gamut is a nice thing to have and for some users it may be important. But overall it’s not going to be a deal breaker if you only have a standard FS7 as the S-Log workflow will allow you to produce Rec-2020 compatible material.

 

 

Webinar Recordings.

In case you missed the webinars I presented yesterday here are recordings of the 2 afternoon sessions. The first one on HDR, what is it and what does it mean for you. The second is a question and answers session on Sony’s large sensor cameras, from the FS5 to the F55. There were quite a few a6300 and A7s questions thrown in there too!

Hopefully I will be able to find a sponsor that will be able to make these a regular event.

 

What does HDR mean in the world of video production?

I’m really, really excited by HDR.

4K is cool, higher resolution, sharper pictures, but sometimes it’s tough to tell the difference between good HD and 4K, especially on a small screen. But HDR….. well HDR really does have that wow factor. Finally we will be able to see images on the screen that have contrast and brightness that is true to life. Vibrant, vivid high resolution images, no more muddy low contrast, dull images.

First of all, lets get this straight, this is not the same thing as the HDR photography that’s been around for an age where you take multiple images at different exposures to capture a massive dynamic range that you then process with photoshop or similar to create some kind of artistic but otherwise un-natural looking picture with un real contrast. Video HDR (or at least what’s being talk about now) is about displaying images on screens that have a dynamic range that closely matches the real world.

And you know what the best bit of this is? A lot of us already have cameras that are shooting HDR or that can be made at least partially HDR capable at no additional cost!

So what is HDR?

Historically television and cinema standards have been limited by display technology. Today a high quality LCD TV will only show a dynamic range of around 6 stops. If you have a light meter check it out. Measure the brightest whites and the deepest blacks and you should find a 5 to 6 stop range. Because cinema screens depend on reflecting light there are limits as to the contrast range that you will see in the cinema too. This has been the case since the very beginning of film making. Yet today we have cameras that can capture dynamic ranges well in excess of this, even an older camera such as a Sony EX1 can manage to record 11 stops, but with normal TV’s and monitors we have no way of showing this 11 stop (or more) range 1:1. So we massage the captured image to fit within the current conventional 6 stop display range using fancy things like the “knee” or special gamma curves like cinegamma, hypergamma or by grading and fine tuning the image in post production.

It’s all about the display.

The key technology that’s changing this is OLED displays. Remember dynamic range is not just about highlights but also about deep, dark shadows. We can make LCD screens brighter, much brighter. An LCD panel works by having a large light source behind an array of tiny electronically controlled ND filters. But these ND filters are never 100% dark, some light always leaks through. This means that when you make the back light brighter the blacks become less black as more light leaks through, so you don’t increase the contrast, only the brightness. The other issue is that current TV broadcasting standards (rec-709) are designed to work within the limitations of current display tech, so we don’t actually broadcast more than a 6 stop range. If we did the pictures wouldn’t look right on a 6 stop screen.

An OLED display on the other hand uses an array of individual LED emitters that are totally dark when they are off and can be very bright when all the way on. As a result they are capable of displaying a far greater dynamic range. TV and monitor manufacturers are now producing displays that are capable of showing dynamic ranges well in excess of the normal 6 stop range. If we then change the standard of the signals that we send to these displays to include a greater dynamic range that’s when the magic starts to happen. But it’s not just about contrast, it’s also about color. OLED displays can also show much more highly saturated colors.

You have to see it to believe it.

sony-hdr1 What does HDR mean in the world of video production?
Sony HDR demo at NAB

Unfortunately there is no way I can show you HDR here. Your computer monitor probably only has a 6 stop range. So to really see and appreciate HDR you are going to have to go and see a demo. There will be many at IBC. Imagine a picture of a sunset where the clouds and sky really are VIVID orange, an orange that positively glows from the screen illuminating the faces of the viewers. Meanwhile in the same shot there is a deep dark canyon in the foreground and you can see every detail in the deepest shadows of that canyon. Or how about a night time cityscape where the unlit buildings are dark but still clear to see, meanwhile the lights on the buildings are like tiny bright diamonds of brilliant light. Maybe the easiest to imagine is a shot across water where the ripples of the water catching the sun really, really do sparkle and catch your eye. But remember it’s not just about a brighter display, but ones with much, much greater contrast and much richer colors.

HDR Limits.

Are there limits to HDR? Well yes there are, but these limits are becoming more to do with what is comfortable to view rather than what the display technology can do. Sony’s X300 HDR monitor can show I believe around an 11 stop range. Some colorists have told me that working on HDR material in a dark grading suite for a few hours can be quite tiring due to the increased eyestrain. Let’s face it, we wouldn’t want a shot of the setting sun that is so bright that it makes your eye’s hurt. So in practice I think 10 to 11 stops is about the natural limit of what we will be comfortable watching on a screen at home. But this is an enormous 10 fold increase over what we have now.

It’s FREE! Well, at least shooting and distribution is.

One key thing about HDR is that it doesn’t need any more bandwidth to broadcast or distribute than any other image of the same size. All you need to do is change the gamma curve and color space used. This means that OTT (Over The Top – web and internet) delivery services such as Netflix or Amazon can stream in HDR without needing to make any changes to their infrastructure, all they need to do is grade the programme to suit an HDR display. Right now you can already find a number of movies to stream in HDR and many, many more programmes will be available from Netflix and Amazon Prime in HDR this year.

The next thing to consider is that if you have been shooting material with a camera dynamic range greater than 10 or 11 stops then you may already have content that is going to look great in HDR. S-Log material is perfect for HDR, material shot by the F55 using S-Gamut or S-Gamut3 is excellent for HDR as not only does it have a high dynamic range but it’s sensor  also has a wide color gamut that will capture those highly saturated vibrant colors that only an HDR display can show. The F5 and FS7 will also shoot great material ready for HDR, although without some of the extended color range that the F55 is capable of. Even material shot with a Cinegamma or Hypergamma can be graded for HDR and in most cases will look better in HDR than it does on a conventional display.

BVM-X300-300x180 What does HDR mean in the world of video production?
Sony BVM-X300 HDR monitor.

So for us shooters, many of us already have equipment that can produce HDR content, in fact HDR will be the first time many of us will actually truly be able to see what we are shooting! To grade and produce HDR content you are going to need to invest in an HDR display. I’d love to get one of the new HDR capable Sony BVM-X300 monitors, but at £25k it’s too steep for me, so I will have to wait for a good quality HDR TV. The biggest issue with HDR will be that you will need to produce a different grade for HDR distribution compared to conventional Rec-709 distribution. But it must be remembered that many high end productions will already have different grades depending on the distribution method. After all a DVD or Bluray Rec-709 release of a film will need a different grade to the DCI-P3 cinema release.

Like 4K, HDR is already here. It’s not mainstream just yet, but it really isn’t far away. This isn’t a technology for the future, it’s a technology for today. Give it another 18 months and HDR TV’s will be common place in most TV stores, just as 4K TV’s are readily available now. Movie studios are sitting on huge archives of films that with a re-grade will look amazing in HDR. With the new 4K Bluray standard able to carry HDR content we really are looking at a tangible revolution in the quality of home viewing. Not just higher resolution but also higher contrast and dynamic range. If you are starting a big project now or want your material to have a long shelf life, you really should be shooting in 4K and using log or raw right now.

Better at home than in the cinema?

It’s interesting to consider that HDR is something that’s going to work much better in the home or at least via a display panel rather than via projection, so the home viewing experience may well exceed the cinema viewing experience (assuming that you have  decent sized screen and good sound system). The next generation broadcasting standard Rec-2020 allows for HDR. These are exciting times times and in the coming weeks I’m hoping to spend some time over at Sony’s Digital Motion Picture Center at Pinewood to learn more about grading and producing content for HDR, the details of which I’ll share with you here.

Bureaucracy in the way in Europe 🙁

In Europe we may have a problem. For HDR to work TV’s need to be brighter. Brighter TV’s consume more energy. At the moment it’s hard to sell an HDR TV in Europe as most exceed the power consumption limits laid down by the EU for televisions which are based on typical LCD technology 🙁 So you may need to personally import an HDR TV from elsewhere.