Gamma Curves - An explanation by Alister Chapman

[Russell Heaton]

G'day all,

Having not come from a photography, videography, cinematography background, I have found the world of Gamma adjustment to be very mysterious. I did, however, read the linked articles (below) and found Alister Chapman's explanations to be very easy to understand.

So I thought I'd just publish the links for any others whom have been struggling a bit with the Gamma curve concepts. Apologies if this has already been done.

http://www.dvinfo.net/optical-scienc...and-s-log.html

Alister Chapman DoP, Stereographer XDCAM EX Gamma Curves and Knee

Cheers
Russ

[Serena Steuart]

Alister's articles are good. Maybe I should point to this too:
Serena Steuart's Blogs - Quantifying XDCAM PMWEX1 gamma curves

[Vincent Oliver]

Very interesting Serena, but I suspect these graphs will be lost on many people, A more useful exercise would be to see what effect each Gamma curve has on a "real life" situation. But thanks for sharing the information with us.

[Piotr Wozniacki]

Originally Posted by Serena Steuart

Alister's articles are good. Maybe I should point to this too:
Serena Steuart's Blogs - Quantifying XDCAM PMWEX1 gamma curves

Of course Vincent is right that some people might be unable to grasp on this, but for those who experimented long enough with their EX's "real life" picture, it's good to find some confirmation of their findings in this methodologically coherent article. If anything, I'd post higher resolution pictures :)

Thanks Serena!

[Serena Steuart]

Using images to show the effect of a changing a parameter is limited because one image is just one example. A set of images taken to show the effects of "twiddling the knobs" are most meaningful to the person who took them, which is why anyone who shoots video (and film) for a living always do their own tests. Yes, they start out with guidance from others, but rather than using that guidance as a recipe they run tests for the "look" they want for their project. Really understanding how an image interprets a scene requires knowledge of the original scene, which we can't get unless we were present. Describing the scene is inadequate (requires the old "1000 words"), which is why technical tests use standardised cards. People have tried to communicate the effects of different gamma curves by subjective terms such as "the brighter gamma", which really tell us very little.

Graphs show us how the camera translates subject brightness into image brightness, and importantly how camera adjustments affect that process. Alister used a graph to explain "knee", much more clearly than using just words and a photo.

The camera can record images of relative brightness that range from 0 to 109% -- black to "super" white --- I think everyone understands that. But how much light (subject brightness) is needed to get a specific image level depends on many things (aperture, filters, image processing algorithms). How the image is processed is influenced by our set picture profile, especially the selected gamma curve. In addition, the light coming into the camera is divided into RGB streams, each processed separately; best put that aside for the moment and think just of the total (white) light stream.

So, just as image brightness is given in relative terms ( 0 to 109%) so it is easiest to think of subject brightness in relative terms. I chose to make brightness (think: white card) relative to that which gave 109% when using the cinegamma curves. The curves are very flat at that point, and I might have chosen instead to plot subject brightness relative to that which gave 109% for a standard gamma (so then the curves would have shown subject relative brightness of 5.55 for 109% using cine 4). Or I could have chosen any other subject level --- the important thing is that all the curves are relative to the chosen "standard".

Looking at the curves you immediately see the way cinegamma handles highlight regions. Each halving of relative brightness (1.0 to 0.5, 0.5 to 0.25, etc) is a stop reduction in brightness (on our blank card), so we see that cine4 results in brighter mid-tone images than cine1 (for same settings otherwise). At the same time we see that STD1 will give a more fully modulated image (near saturated) at the same settings, and because the STD curve is steeper the image contrast will be greater; bigger change in image brightness for a given change in subject brightness.

Looking at these graphs we know, when we start shooting tests for our next project, that if we want to achieve contrast similar to the STD curves but need to use a cine curve because our subject has a wide brightness range (e.g. outdoors, white clouds, sunlit and shadow), then we will have to grade the image in post. If our subject is quite flat (small brightness range) then a STD curve might be preferable. In general, the cine gammas capture the most subject data (that is, within black level and white clipping) but require grading in post because otherwise the images look flat. If you want camera output direct to TV, then the STD gammas will be most pleasing (but may have white clipping and/or choked darks).

Going to the final graph, it shows that adjusting gamma level has a big influence on rendering highlights, but not much in mid-tones. And in other curves we see that log curves are more revealing of response at the darker (relative) range of the subject. Of course this set of curves on my blog is a limited one and there are many more parameters that can be plotted (black gamma, for example). And we haven't considered separately the RGB streams, where (for example) we might be concerned with how highlights are handled by each of them, or how skin tones can be distorted at higher zebra (image brightness) levels.

You get a lot of information from response curves and they make it a lot easier to understand what the available controls do. Of course someone has to generate the curves and in the same way that Kodak and Fuji always publish data for their film emulsions (response curves, modulation transfer function, etc) I think it a pity that video camera manufacturers don't do similarly.

[Alister Chapman]

Originally Posted by Serena Steuart

Looking at these graphs we know, when we start shooting tests for our next project, that if we want to achieve contrast similar to the STD curves but need to use a cine curve because our subject has a wide brightness range (e.g. outdoors, white clouds, sunlit and shadow), then we will have to grade the image in post. If our subject is quite flat (small brightness range) then a STD curve might be preferable.

This is so important to understand (I realise those that have already posted to this thread know this). Sadly the very name "cinegamma" implies that simply switching to a cinegamma gives a "film look", this is not the case. What you do get is a shot that looks like it was filmed using film stock, but then you must remember that movies always undergo extensive post production before the final print is made and it's during the post production process that the "film look" is largely created. So while a cinegamma may to some degree emulate film stock, on it's own it will not give you a filmic look. If you are not going to grade your footage you really should be using a standard gamma in most cases, an ungraded cinegamma will not look good straight from the camera.

[Vincent Oliver]

Thank you for the clarification Serena and Alister. Colour Grading is a subject that fascinates me and I have spent many hours trying and acheiving the look I want from my footage.

Having studied and written about colour management for stills photography. I am quite surprised that none of the big colour houses haven't come up with a profiling device to ensure you display accurate colours on a studio monitor or LCD TV screen for video work. I know we have the SMPTE colour bars, but this has too many variables to be totally accurate, i.e. room lighting, warm up time before stable colours and reliant on personal eyesight etc.

The reason I mention this is because I spent hours if not days perfecting colours in post and then delivered the final product to the client. I was horrified to see their TV and monitor were not properly set up and I might just as well have set my camera to full auto and post production footage to Auto Color .

But thanks for your detailed post Serena.

[Marcus Durham]

Originally Posted by Vincent Oliver

The reason I mention this is because I spent hours if not days perfecting colours in post and then delivered the final product to the client. I was horrified to see their TV and monitor were not properly set up and I might just as well have set my camera to full auto and post production footage to Auto Color .

Most LCD TV's come with the colour turned up to max and people simply don't change the settings. When I visit peoples houses or clients offices and see how the displays are setup I wince. Orange faces so that everyone looks like David Dickenson, sharpness to maximum, and a display so bright that it X-Ray's passers by.

So in many ways those of us striving for a perfect picture are wasting our time. The only attitude you can take is "it was alright leaving me".

[Serena Steuart]

Originally Posted by Vincent Oliver

I know we have the SMPTE colour bars, but this has too many variables to be totally accurate, i.e. room lighting, warm up time before stable colours and reliant on personal eyesight etc..

You can't hope to compensate for the way the client has adjusted their TV, but of course you do have to set up your own grading monitor and the SMPTE colour bars are not sufficient for that (as I think you have said). Studio monitors have built-in calibration facilities, but after that it does rely on the grader's skill (i.e. eyesight).

[Larry Kelly]

Dealing with other people's display calibration can be very frustrating, we have problems with local theatres that host film fests. They don't generally use their primary digital projector and often rent something that is somewhat deficient and under powered. I have seen filmmakers work like dogs getting their films to where they are proud of the result, then see it on the big screen and are very disappointed. We are hoping to set up a system so that we can encode to jpeg2000 and convince the theatres to use their primary projectors for local fests.

When I build our own screening room in our studio I will be using a Barco 909 Reality CRT projector with a 12 foot screen and calibrating it with some of the tools found on this site. It will do about 3k.

Some of the calibration tools available on this site are invaluable for calibrating all types of displays and are not too difficult to master. The high end CRT projector is still second to none for critical viewing at 1080P and beyond and they are still being modded to accept the latest signals, deep colour, gamma control, even 3D

Larry

[Alister Chapman]

While it is frustrating trying to produce the best possible image only to see it degraded on a poorly set up TV, just consider how bad it would look if it was shot badly, then degraded!!

It's all very well having these wonderfull hypergammas, cinegammas and S-Log etc, but until the baseline gammas used for TV display are changed we are stuck trying to squash whatever we shoot into the current 7 stop range that this is tailored for. It's a shame that when the world started to transition from SD to HD that there was only a very small change from the 601 gamma used for SD to the 709 HD gamma curve. A more aggressive curve could have been chosen that would have allowed for a greater displayed dynamic range but instead the curves were kept very similar in the name of "backward compatibility" as opposed to forward thinking.