Gamma, Cinegamma and Hypergamma

Before anyone complains that I have missed stuff out or that some technical detail is not quite right, one of the things I’m trying to do here is simplify the hows and why’s to try and make it easier for the less technical people out there. Lets face it this is an art form, not a science (well actually a bit of both really).

So what is a gamma curve anyway? Well the good old fashioned cathode ray tube television was a very non-linear device. You put 1 unit of power in and get one unit of light out. You put 2 units in and get 1.5 units out, put 3 in and get 2 out… and so on. So in order to get a natural picture the output of the camera also has to be modified to compensate for this. This compensation is the gamma curve, an artificial modification of the output signal from the camera to make it match TV’s and monitors around the world. See Wikipedia for a fuller explaination:   http://en.wikipedia.org/wiki/Gamma_correction

So, all video cameras will have a gamma curve, whether you can adjust it or not is another matter. Certainly most pro level cameras allow you some form of gamma adjustment.

The PMW-350 has 6 standard gamma curves, these are all pretty similar, they have to be otherwise the pictures wouldn’t look right, but small changes in the curve effect the relationship between dark and bright parts of the pictures. Todays modern cameras have a far greater dynamic range (range of dark to bright) than older cameras. This means that the full dynamic range of the sensor no longer fits within the gamma curves used for TV’s and monitors. In broadcast television any signal that goes over 100% gets clipped off and is discarded, so the cameras entire brightness range has to be squeezed into 0 to 100%. The PMW-350 sensors are capable of far more than this (at least 600%) so what can you do?

The older and simpler solution is called the “Knee”. The knee works because in most cases the brightest parts of a scene contains little detail and is generally ignored by our brains. We humans tend to focus on mid-tone faces, animals and plants rather than the bright sky. Because of this you can compress the highlights (bright) parts of the picture quite heavily without it looking hugely un-natural (most of the time at least). What the knee does is takes a standard gamma curve and up near it’s top, bends it over. This has the effect of compressing the brighter parts of the image, squashing a broad range of highlights (clouds for example) into a narrow range of brightness. While this works fairly well, it does tend to look rather “electronic” as the picture is either natural (below the knee) or compressed (above the knee).

The answer to this electronic video look is to replace the hard knee with gentle bend to the gamma curve. This bend starts some way down the gamma curve, very gentle at first but getting harder and harder as you go up the gamma curve. This has the effect of compressing the image gently at first with the compression getting stronger and stronger as you go up the curve. This looks a lot more natural than a hard knee and is far closer to the way film handles highlights. The downside is that because the compression starts earlier a wider tonal range is compressed. This makes the pictures look flat and uninteresting. You have to watch exposure on faces as these can creep into the compressed part of the curve. The plus point is that it’s possible to squeeze large amounts of latitude into the 100% video range. This video can then be worked on in post production by the editor or colorist who can pull out the tonal range that best suits the production.

These compressed gamma curves are given different names on different products. Panasonic call them “Film Rec”, on the EX1 they are “Cinegammas” on the PMW-350 they are “Hypergammas”. The 350 has four Hypergammas. The first is 3250. this takes a brightness range the equivalent to 325% and compresses it down to 100%. HG 4600 takes 460% and squeezes that down to 100%. Both of these Hypergammas are “broadcast safe” and the recordings made with them can be broadcast straight from the camera without any issues. The next Hypergamma is 3259. This takes a 325% range and squeezes this down to a 109% range, likewise 4609 takes 460% down to 109%. But why 109%? well the extra 9% gives you almost 10% more data to work with in post production compared to broadcast safe 100%. It also gives you the peak white level you need for display on the internet. Of course if you are doing a broadcast show you will need to ensure that the video levels in the finished programme don’t exceed 100%.

My preferred gamma is Hypergamma 4 (4609) as this gives the maximum dynamic range and gives a natural look, however the pictures can look a little flat so if I’m going direct from the camera to finished video without grading I use either a standard gamma or use the Black Gamma function to modify the curve. I’ll explain the Black Gamma in my next post.

There are 6 standard gammas to choose from. I like to stick with gamma 5 which is the ITU-709 HD standard gamma. To increase the dynamic range I use the Knee. The default knee point setting is 90, this is a reasonable setting, but if your shooting with clipping set to 100% you are not getting all the cameras latitude (the Knee at 90 works very well with clipping at 108%). Lowering the knee down to 83 gives you almost another stop of latitude, but you have to be careful as skin tones and faces can creep up towards 83%. It’s very noticeable if skin becomes compressed so you need to watch your exposure. This is also true of the Hypergammas and with them you may need to underexpose faces very slightly. The other option is to set the knee point to 88 and then also adjust the knee slope. The slope is the compression amount. A positive value is more compressed, negative less compressed. With the knee at 88 and slope set to +20 you get good latitude, albeit with quite highly compressed highlights.

If you want to play with the gammas and knee and see how they work one method you can use is to use a paint package on your PC (such as photoshop) to create a full screen left to right graduated image going from Black to white. Then shoot this with the camera (slightly out of focus) while making adjustments to the curves or knee and record the results along with a vocal description of each setting. Import the clips into your favorite editing package and use the waveform monitor or scopes you should be able to see a reasonable representation of the shape of the gamma curve and knee.

So my Gamma Choices are:

For material that will be post produced: Hypergamma 4609 (HG4)

For material that will be used straight from the camera: Standard Gamma 5 Knee at 90 with clip at 108% for non broadcast or Knee at 88 with slope +20 with white clip at 100% for direct to broadcast.

12 Responses to Gamma, Cinegamma and Hypergamma

  1. Pingback: Hypergamma « DENNY MEDIA PRODUCTIONS

  2. Bob Docherty says:

    Thanks Guys
    great reading and I’m getting to know something about the more tech. aspects.
    Good practical advice . I am going to try out the excercise.
    One thing though. Why should the camera be slightly out of focus?
    I’ll be reading on
    Bob

    • alisterchapman says:

      If you shoot a computer screen in sharp focus you will see the pixel structure which will distort the waveform on the waveform monitor. By defocussing the pixel structure is no longer visible. All you want to see is the gradation from light to dark.

  3. HarryLlama says:

    Alister, I think you answered this previously, but I can’t seem to find it… the “specs” for the cinegammas in the EX1, EX3, and now the F3 (the percentage captured and the 100 or 109%) and how they compare to the hypergammas of the 350. Since we usually shoot with the 350 and at least one of the others, it would be helpful to match them!

    Harry

  4. singledollarbill says:

    Alister, do you ever use negative gamma knee slopes? I hypothesize that a negative slope would exaggerate highlights (for example moonlit edges in a dark scene), but I am not sure I’m seeing that.
    -Bill

    • alisterchapman says:

      The slope gain control on most camcorders only increases or decreases the gain of the slope, so it just alters the ratio of light to dark. A positive number will increase the gain, so brighter parts of the image will appear brighter on screen and a negative number will make brighter parts darker. It won’t have an effect on overall dynamic range unless you are deliberately shooting underexposed in which case +ve gain will bring you back up in level.

      It’s a little bit like changing the cameras over all gain control.

  5. AlexD says:

    Thanks for this. Very useful info. I’m working with the 350 and getting to know it in and out as best I can.

  6. Eric says:

    Hi Alister, Just found your website and youtube tutorials…thanks so much for the info!

    I’m using a Z7U to record promos for a local playhouse; which gamma settings would you recommend for this? Obviously at a playhouse the dynamic range is huge, from deep blacks to the bright whites of the theater lighting. I’ve been using cinegamma 1, I’m thinking next time I have to boost the black gamma in order to get some details in the shadows.

    Thanks, Eric

    • alisterchapman says:

      I didn’t think the Z7U had Cinegammas. It has Cinematone which is very different to a Cinegamma. Sadly the Cinematone gammas do not increase dynamic range, they are purely there to give a film like look. Raising the black gamma will bring out more shadow detail but at the expense of more noise and with a small sensor noise can get objectionable very quickly.

  7. Eric says:

    Sorry, you’re right, I meant Cinematone. I had watched Juan Martinez’s video on gamma on Sony’s HDV microsite and thought they increased the dynamic range on the HDV cameras as well. I see now. Thanks for your help.

  8. Andi says:

    Alister, thanks for your explanations and for the insight of the site in general. One question I have from an absolute beginners point of view is; you say the various gamma curves are compressing the image gently at first and then harder as you near the top of the curve (to avoid the hard clip of the knee), which is what gives these various curves their flat look. If these curves are compression, happening in camera, how is it we are able to pull so much information back from them in post? Why doesn’t expanding the tonal range (in a grade for example) introduce compression artefacts like you might find if you were pushing, say, a DSLR image heavily in a grade where the highly compressed H.264 codec would break down? Pardon my ignorance, just trying to fully understand the concept. Thanks!

    • alisterchapman says:

      Compression can take on many forms. In the case of highlight compression what is happening is this:

      In the normal part of the gamma curve one stop of exposure may occupy a recording range of 0.1 volts, so perhaps 0.2 to 0.3 volts or 0.3 to 0.4 volts.
      This is then encoded by the encoder and turned from volts into data code values.
      In the highlights to squeeze more range in, one stop might only be recorded using a range of only 0.05 volts, a much smaller range. So perhaps 0.6 to 0.65 volts.
      This is then encoded by the encoder and turned from volts to data code values, only now because the voltage range is smaller, the number of resulting code values is also smaller, so it takes less space to record. This is compression.

      Expanding that range in post production will introduce artefacts such as banding or stair stepping. But as this is in bright highlights very often it goes un noticed by the viewer.

Add Comment Register



Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>