Can someone explain "knee" & "slope"
 

Can someone explain to me in a not to complicated way what the "knee" and "slope" and "point" settings in the picture profile settings do, ie what effect they have.

Thanks.
Paul.

Paul,

I'm sure that when you take a look at a typical (best Cine) gamma curve, paying special attention to it's upper-right portion - you will figure it out by yourself :)

Thanks Piotr.
What is the "best cine" ?
And the upper right hand portion of what ?

Paul.

I don't have time now but don't feel like your an idiot. Its not totally obvious especially point and slope.

I'm just a knowledge addict !!

Paul.

Paul, I was typing in haste (the Euro Cup match Poland vs Austria is about to start now :)

I should have written "Best if it's a cine gamma, cause it shows the knee point more obviously).

I'm sure you can do it:)

"Point" controls the threshold where compression starts. "Slope" controls the amount of compression, so the flatness of the curve after the threshold-point.

To add to Dominick - The knee ( with its point and slope adjustments) is a circuit that compresses exposure areas near the 100% IRE range so you don't blow out highlights so fast. It looks like a rounded off knee on a graph of exposure - The term comes from film.

It has both positive and negative affects on the image and must be used judiciously but is a part of most video response curves.

Using the knee curve is akin to audio compression at the top end whereby you wish to shove 10 LBS of signal into a 5 LB bag.

But in order to get the excess to fit, you don't squeeze it at all points equally. You leave the bottom part intact and squeeze harder at the top.

-gb-

Gamma is the slope of the graph relating subject brightness (input) to output signal. A common video problem is that highlights (typically clouds) are too bright compared to mid tones to be recorded within the range of the sensor. Expose to hold the clouds and the foreground subjects are under exposed -- expose for the main subject and clouds are burnt out (appear as white blobs). Professional cameras provide facilities for modifying the gamma curve to increase the recordable range of subject brightness. You can change the slope of the gamma curve and the shape of the curve. The simplest adjustment for coping with highlights is to reduce the slope of the top of the curve, which you do by selecting where this begins (say 80%) -- the "knee point" --- and the slope of the curve above that knee point. Generally the curve above the knee point is linear. There are equivalent operations possible for low light. These adjustments approximate (very roughly) the S-shape of a film gamma curve. You will find brief illustrated explanations at http://www.tapestockonline.com/sovicase.html
In the EX1 the standard gammas (STD1,2,3,4) allow you to set a knee point. The cinegammas do not have a knee point, the curve being continuous. CINE4 provides the greatest dynamic range, but all the CINE gammas roll off the slope (compared to STD gammas) for better image latitude.

EDIT: The EX1 allows adjustments to gamma (within each set of curves): confining my comments to CINE4, plus gamma slightly (very slightly) decreases the slope of the curve in highlights with increase in slope for low lights, whereas negative increments have the opposite effect (with more noticeable change). You didn't ask about black gamma, but just in case: black gamma affects the lower portion of the gamma curve (that below 50% image intensity for CINE4). Positive increments in black gamma bulge the curve out (more "sensitive" to subtle black gradients) and negative values draw it in (less sensitive).

Cameras such as the Sony F23 permit the operator a lot of flexibility in setting up the image processing algorithms, such as loading in custom gamma curves, whereas the EX1 only allows us to make minor adjustments to the in-built curves. One of the sites selling custom gamma curves (not for the EX1) has some nice illustrations of what can be achieved. http://www.digitalpraxis.net/sonycurves.htm
Scroll down the page to the curves and photos. Note that the important issue is recording the data for post processing, not capturing images for direct display.

There is a very easy way to practice using the manual knee adjustments. If you're going to shoot in the STD gammas, you need to learn this because the Auto-knee sucks.

Raise your hand above your head. The height of your shoulder above the floor is the Knee Sat Level. The height of your elbow above the floor is the Point. The amount of bend at your elbow is the Slope.

1.) Practice shooting high contrast scenes into the sun. Put a tree or some foliage between the camera and the sun. You will be attempting to retain shadow detail and color of the foliage in strong backlight, and at the same time have a bright sun or sky with cloud details but no zebra stripes.

2.) Turn on Zebra 2 (100%).

3.) Expose for the mid tones. The backlit areas will be badly blown, zebra stripes everywhere.

4.) Start with the default manual settings, Point 90, Slope 0, Knee Sat Level 50.

5.) Bring down the Point, 90, 80, 70.... See how easy it was to make the zebras go away?

6.) Now raise the point back up to 90, zebras galore again. Raise the Knee Sat Level from 50 to 75 and increase the slope (+) until the zebras go away. See how easy that was?

What was different between 5 and 6 above? In 5 you just lowered the clipping point. In 6, you changed the rate (delta y/delta x) at which the clipping point is approached. Either can be a correct course to take depending on the situation. You are almost always safe just doing 5. Doing step 6 can additionally make things possible like retaining cloud details, contrast and texture. Or you can take flat lighting and go the other way (negative) on the Slope and brighten up a flat sky to add punch to a dull scene, without affecting the mid-tones.

Just remember the default manual settings are very good for typical situations, so commit the numbers to memory, 90, 0, 50 so you can always return to them. And even just switching off the Auto-Knee function (but keeping the Knee circuit enabled) with the default manual settings will cure or greatly reduce a lot of overblown highlight situations that occur in Auto-Knee mode. And when you do make adjustments to Point, Knee Sat level and slope, you can go pretty bold without too much worry. The Zebra stripes are your friend.

This may come as a surprise, you can shoot straight into the sun, and create beautiful effects, while retaining shadow detail, color and sharpness, even in STD-3!!! You just have to be willing to try new things!

And don't limit your line of thinking to just sun and sky. Using a combination of Knee Sat Level, Slope and Point, you can for example, tone down a blown out sky, put back detail in the clouds, and add or take away highlight details like the glinting reflections on water, or the reflective highlights of wind blowing through tree leaves and foliage. Totally cool. Try it and have fun with it.

Serena & Tom,

We all thank you for these excellent explanations - I couldn't have done it better.

And Paul, once again sorry I wasn't very helpful last night - was very preoccupied and excited about the "last chance" match of the Polish team at Euro-Cup in Vienna. Unfortunately we didn't win, after all - in spite of being so damn close!

Thanks everyone for all your answers.
I think i get it now.

My dilema is i usually shoot with Bills PP, however, recently i've been working with another operator who uses a Z1 so i'm using the EX1 standard PP, for matching footage purposes.
I've been shooting weddings with this guy and find it hard to control the highlighs using std PP. A shoot i did recently was an outside ceremony on a very sunny day, so bright white dress, a few white clouds in the sky etc, you get the picture.
So i just wanted to find a safe way to remove the 100% zebras. So i'll play around with Tom's suggestion, Numbers 5 & 6.

Thanks very much guys.

Paul.

One information to add is that compression-algorithms lower the maximal output. To compensate this audio-compressors have a parameter often called "makeup-gain". The ex1 also has such a gain, but you can't adjust it - it's fixed. Turning knee off doesn't disable that gain because it's "hard-coded" into the std-gammas, so turning knee off leads to earlier clipping and not using the whole dynamic range of the sensors respectively.
So if you don't need the knee-compression but want the whole dynamic range of the sensors for getting lowest noise I recommend to use cine1, because it's most similar to no-knee-std-gammas, but of course uses the whole dynamic range because cine-gammas don't have/need makup-gain because they don't have that knee-feature.

Ok this is what i did.
My willing participant (the missus) stands with her back to the window, i expose correclty for her with 65% zebras on the skin, window behind has 100% all over it, so i lower the knee point till the 100% zebras disappear, which is about 76 (originally 90).
Did i do this correctly ?

When i flick between std and this new PP i can definately see the difference when the zebras are off/out of the way.
With the std PP,from my same original position, looking out of the window is almost white,nothing there, however with the modified std PP i can see detail such as house roofs,hills in the distance etc.

Thanks guys.

Paul.

The thing to be aware of is that optimal knee, point & slope settings vary for every shot so don't just dial in an extreme setting because it saved your overexposed window in one shot. it may screw up another one. An overzealous knee looks awful.

The problem with the auto knee in the EX-1 is that it reacts too much, and can create changing highlight exposure when panning past bright objects. Most auto knees are OK but the EX-1's sucks.

Lenny

Just want to mention that Stu Maschwitz is doing a multi part post On Clipping, Part 1 that may be relevant.

George/

Beautiful Paul. Don't be concerned about getting it wrong. If you don't try, you won't advance your skills. You did just fantastic. And your numbers are consistent with my observations for that situation.

To build on what you just obvserved, you can try repeating the experiment with a few variations. You were already successful recovering detail that was buried in the white of the window. Now, will you attempt to enhance the contrast, the outlines within that detail? I would start where you left off with the knee point at 76. Next bring up the Knee Sat level to about 65-70 or so. You may not immediately see much change. Next, start dropping the Slope into the negative numbers, and watch for enhancement of outlines within the recovered detail. Don't worry if the 100% zebras start coming back, just work on definition of the outlines in the recovered detail for now. If you don't get any enhancement, go the other way with it. Whether you are able to enhance the outlines is about 50-50 on the probability. Don't worry, it just depends on what's there to recover. When you think you have the outline definition within the recovered details as good as it can be, move the knee point and slope together, or in combinations to get the 100% zebras back under control. Going up (into the positive numbers for the slope) makes the 100% zebras go away, but along the slope as opposed to the knee point which moves the clipping point. You also may find that when the slope and sat level are right, you can increase the knee point back up into the 90+ numbers, increasing the overall punch, while not losing the highlight detail.

I agree with Leonard that the optimal settings vary for every shot, however the sun moves predictably, and as long as you are shooting in the same direction in the same general period, the characterization that you can get really overzealous and awful with it (on the EX1 anyway) seems actually fairly hard to do.

Cine1 is a versatile, safe gamma but you can fine tune a custom look using Std3 and manual knee adjustments.

Tom,

This has been one of the best posts ever on this forum :)

Using a very precise language, in just a few sentences you have confirmed and defined what I (not a professional videographer) managed to comprehend myself through a quite lengthy process of trial & error.

Thanks!

Thank you Piotr, I'm blushing...but no one has done more than you to create awareness about the unique characteristics of the EX1.

About the trial and error, I applaud your diligence. In my first days with the cam, I really messed up what should have been gorgous footage at sunset. To prevent the sky from being blown out white, I reduced exposure using iris and shutter speed, but the landscape went black. Using trial and error, I adjusted gamma, black gamma and black in various combinations to brighten the landscape. What a mistake!

Since that time, some beautiful sunset footage shot straight into the sun with full tonality of the landscape, by using manual knee adjustments.

Tom.
Have you got a Picture Profile which you use ?
If so could you share it with us.

Thanks.
Paul.

You may find this sony info helpful as well..

http://pro.sony.com/bbsccms/assets/f...r-exposure.pdf

Not too much to "share" Paul. The emperor has no clothes. I have some experience with the manual knee settings, because the need was urgent. All the other settings are a work in progress subject to change. So with a few exceptions, they are mostly the defaults. (I have learned more from your backfocus video.)


***************************************

Profile Name: Outdor 24

Matrix
Setting: On
Select: Standard
Level: +4
Phase: 0
R-G: 0
R-B: 0
G-R: 0
G-B: 0
B-R: 0
B-G: 0

Color Correction
Setting: Off

White
Offset White: Off

Detail
Setting: On
Level: -13
Frequency: 0
Crispening: 0
H/V Ratio: 0
White Limiter: 0
Black Limiter: 0
V DTL Creation: Y
Knee APT Level: 0

Skin Tone Detail
Setting: Off

Knee
Setting: On
Auto Knee: On (by default if no problems)
Point: 90
Slope: 0
Knee SAT Level: 50

Note: Manual adjust if situation dictates it, backlighting, shooting toward the sun etc.

Gamma
Level: 0
Select: STD3

Black: -5

Black Gamma: 0

Low Key SAT: 0

Thanks Ethan.

That's the best explanation for the adjustments. It's very simple after reading.

My suggestions:

Try to set Matrix to OFF: less CAs!
Or/and lower knee saturation to 0-20: less CAs!
Try to set Detail to OFF: much more organic feeling, less artifacts, more film-like
Try using a Cine Gamma: eliminates white halos around high contrasty objects
Try to use a higher Black Gamma (and black saturation): much more shadow information


Dennis

Great thread. I've read all of it and hope I've understood all of it. Now where I'm still a little confused is there's 8 gamma curves available. What's the difference between say starting with STD1 and tweaking all the adjustments to produce a desired curve and starting with say Cine4 and tweaking away.

As far as tweaking the KNEE portion, the CINE curves are fixed so this leaves you with the 4 STD ones :)

BTW, if you go up one level from the page Ethan gave us the link to, you will find a lot of other useful info, too.

For instance, explanation of the KNEE APERTURE setting (to stick to this thread main subject), can be found here:

http://pro.sony.com/bbsccms/assets/f...ture_Edges.pdf

Also, those who hate edge enhancement but don't like to loose any bit of the EX1's great resolution by turning Detail off, should consider leaving DETAIL on but compensate for it only where needed, such as noise, with the CRISPening function:

http://pro.sony.com/bbsccms/assets/f..._Sharpness.pdf

Last but not least, when you want punchier image and decrease BLACK GAMMA down to the negative territory, remember to de-saturate the colours in areas affected by also decreasing LOW KEY SATURATION:

http://pro.sony.com/bbsccms/assets/f...urturation.pdf

All in all, the creative picture composing capabilities of the EX1 are anormous, but very disappointed will be somebody who believes the 6 PP's that can be saved are enough for every occasion! This is why I have my 6 PP's ready, but only use them as good starting point for further tweaking in some typical - yet not identical - lighting conditions...

Only part of the information on the web pages linked to above, applies directly to the EX1 (they are addressed mainly to the full-pro, highest-end, Sony cameras' users). On the other hand, in the EX1's PP setting menu there are options not mentioned directly in these articles; since I cannot imagine too much of discrepancy between the tweaking capabilities of the EX1 and those of other Sony models, I GUESS this might be just a matter of the nomenclature used.

I believe I'll be expressing the needs of all EX1 users if I ask those having access to the pro models to help us identify which is which. For example:

The Y BLACK GAMMA, or LEVEL DEPEND discussed on the site in question, may have their counterparts in the EX1's BLACK LIMITER...

But wee need some firm confirmation, TIA!

Did you manage to find the directory page to those articles?
I'm left taking stabs in the dark trying to find an index of them on that Sony site.

Yes, no Crisping in the EX1 that I can find.

Yes. Piotr started a new thread that links to the 9 Sony Shooting Tips.

http://dvinfo.net/conf/showthread.php?t=123870

One of the lessons is how to use the crispening feature. It's quite like the Canon XH-A1/G1 CORING setting.

Me messing with my knee !!
 

Had a play around today with knee and slope settings today, very interesting.
I put some PP's with knee alterations on Vimeo if anyone's interested in having a look.

www.vimeo.com/paulkellett

Paul.

Tom,

Amazing post, great info - time to play and practice!

Paul, thanks for the video samples - that was great

No problem. I had some time on my hands today so thought i'd do something useful.
I hope it helps someone out.
I can see that PP helping me a lot at weddings, you know, outside,sunlight, white dresses everywhere.
Hopefully it'll be ok in darker scenes, i believe that the cine's aren't so good in low light, as is detail on, ie creating noise.

Paul.

paul, in the video a bunch of the profiles you say have no 100% zebras definitely still look like they are still blown out in spots. I think those profiles just have the knee set in such a way that it is throwing off the white level. this doesnt prevent blowing out, it just makes blown out areas greyish which looks a bit unnatural as does the gamma curve in the highlights. if this is the case, you also arent using all 8bits you have available, which isnt a ton to begin with, so id recommend using profiles that use all 8bits. do those picture profiles show 100% zebras even when you point a bright light right at the camera and everything is definitely blown out? if your white level is like 95% then your 100% zebras will never show no matter what you shoot.

edit: sorry just read this thread a bit more and it sounds like this was covered to some extent, sorry if it was redundant at all.

Be conservative in setting your knees parameters. An aggressive knee that looks great on one shot and saves your highlights can totally screw up another. Most techs like to paint their knee on a per shot basis and if they set something for general use it is not that intense.

The auto knee on this camera is the worst circuit in the Ex-1 because it will change the picture as you pan past something bright. Unacceptable in my book. Worst auto knee I've seen since the BVP550 years ago which was even worse. usually they are pretty seamless.

I can't remember what I set mine at but I think it was around point of 93 with a slope of 50. If its different I'll post that. it works pretty well as a general setting.

Lenny Levy

Lenny is right - only modify you knee's point and slope on the "per scene" basis, or you will ruin other scenes.

Also, switching the auto knee off (with other parameters at default values, or close) is a good idea. This alone helped me tremendously in avoiding the "abrupt highlights clipping" phenomenon (well, in most cases).

Best PP and knee settings for unusual lighting situation
 

I occasionally shoot in an outdoor amphitheater that has a huge awning diffusing the sun. The main speaker in on the stage with a hot forest background with a lake. The talks are about 90 minutes long. I have to shot a whole week of them in a few weeks and I have yet to get satisfactory video using PP setting alone. I have to color correct to get some contrast in the speakers face or anything on stage. I hate having to color correct since it takes so long to render and compress for the web. Any PP and knee settings that you pro's have that will work in a situation like this?

Thanks.

Answer my own question
 

I watched Paul's Vimeo video and tried his settings. Much better image. Has a lot of punch and is not flat looking at all. Now over saturated to the point of being almost cartoonish. Will continue tweaking tomorrow.