Log Vs. Linear

K

kampfpudel

Active member
Any thoughts about it?

One of the more interesting features of the Thomson´s Viper Filmstream camera is the ability to record the data logarithmic instead of linear.

Could it be possible to add that feature in the RED?

Regards
 
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Good question! I think since the cam has 66dB signal-to-noise, 11bit linear would be sufficient to grab all the sensor info. And the red saves 12bit, if I'm correctly informed. Logarithmic would only be needed if you tried to save that data with only 8bits to cut down on memory usage, or if your workflow is logarithmic.

But I would be glad to know more info about this one from of the red crew people.
 
Logarithmic files is to prefer when it comes to color correcting. It is possible to extract more information from the same amount of data and is non-destructive (but taxes harder on processing power).
Both Shake and Combustion has workflow for working with logarithmic files. There is one drawback, you need to use a LUT (Look Up Table) to view the material or it will look greenish on the monitor.
But the advantage for using it is huge when it comes to looking, CCR and FX.
As I understad RAW it is a direct dump of sensor information without processing the information, right? But what kind of file system the data is stored in is a open issue, it could both be log or lin-files.
(There is also no standard to the RAW? Is that correct?)
It should be quite easy to provide the user with the option to choose to record in either log or linear.
 
Hans, I don't agree. Colour correction works best on linear data, and the lin to log conversion is generally destructive, and the processing power is negligibly different for processing log or lin data, given they'd both be bumped to floating point anyway for colour correction.

Log is basically perceptual based compression, giving more data to the shadows than the highlights. Log is not "that" different from video gamma (which people often mistakingly call "linear" which it is not), only that it's more extreme.

Generally 12bit lin would be turned to 10bit log, like cineon. We will provide for this option as although it's compression, it does work to get the maximum amount of perceptually useful data off the camera while saving two bits on the process.

SO, really for recording, there's probably 3 options depending on what modes you're using, 12bit linear, 10bit log, or 10bit video gamma. There may be more gamma options based upon perceptual nuances that are not precisely log nor video gamma, but the architecture would provide, internally, for any "lin to log" conversion LUT, so it's pretty easy to accommodate.

Hope that explains things.

Graeme
 
Hello Graeme.
Thank You for taking time to answer. :)

I agree, log--->lin or lin--->log would degrade the material.

"Generally 12bit lin would be turned to 10bit log, like cineon."

The point would be to stay in log-space all the way. From sensor all the way to post.
The advantage of it is that you can shoot a bit "underexposed" and save information in the highlights and still get maximum of information from shadows.
Found a good link about it: http://digitalpraxis.net/dplinearvslog.html

About video gamma. Applying a gamma curve on a linear file "changes" the information (I do not say it degenerates it) to achieve a special "look".
(Panasonic´s "Cinegamma" for example)
Nothing wrong with that. But if you shooting a scene against green/blue screen you want to stay away from applying gamma curves in the camera and take the shoot as "clean" as possible.

BUT we are speaking of a camera that has 66dB and shoots in 4:4:4 uncompressed...so I think keying will be a breeze! :)

I didn´t mean to sound like I was nagging. If I did I am sorry and I do apologize.
(But agree it would be cool to say: "Hey, we give you a Viper for 17.500! C´mon give us your wallet!" ...eh...something like that anyway. ;-)

Best regards
Hans
 
I always correct in Lin. In shake I always place a log/lin on incomming Log material, and if going log out put a lin/log note at the end.
Color correcting in Log space is very unpredicteble.
Isn't Lin/log log/lin almost lossless when going to/from float, with a non truncating translation?
 
BTV. Isn't most sensors linear anyways?
I seem to remember the viper has a 12 bit A/D converter from the analog CCD signal which is translated into 10 log in filmstream mode.
 
Sensors are, for the very most part, linear.

Hans, you're confusing video gamma with a gamma to make the video look funky. By video gamma, I mean a gamma of ~2.5 which is the gamma of all normal looking video, and approximates the inverse gamma characteristic of CRT monitors. A lot of people refer to video gamma as linear, which it is not. Log is similar to video gamma but a more extreme curve is used.

Once you're into your computer, you can put the video into floating point and then you can change between log, lin, video gamma or something funky without real loss. It's in camera, where your recording to a specific bit depth that you've got to be careful.

Anders - you're spot on. Colour correcting in a non-linear gamma is troublesome, and really should be avoided.

Graeme
 
"Color correcting in Log space is very unpredicteble."

Why? Apply a LUT and then tweak the parameters for the LUT. It´s quite straight forward I think. No harder than linear imho.
(see http://www.creativecow.net/show.php?forumid=14&page=/articles/bell_alan/bit_depth/index.html for more info on log/LUT and Combustion)

"Isn't Lin/log log/lin almost lossless when going to/from float, with a non truncating translation?"

Yes.
The most beneficial way of using log is when shooting, perhaps not in post. At least not when we have float.

"BTV. Isn't most sensors linear anyways?
I seem to remember the viper has a 12 bit A/D converter from the analog CCD signal which is translated into 10 log in filmstream mode."

Yes, all sensors are analog. The analog information (electric current) from the sensor(s) can be described as a sinus curve(s). That curve is "translated" by the A/D converter(s) to a digital form either as a linear file within a bit space (8, 10, 12, 14, 16bit, etc) or a logarithmic file within a bit space ( 8, 10, 12, 14, 16bit etc).
I belive the Viper´s A/D converter "translate" the information as a log-file
directly.
But I could be wrong.

There is no difference for a A/D converter to describe 10^3=1000 or Log(10)=3
But in binary log the base is 2, not 10 of-course.

Just to clarify why I am being tedious about log-this and log-that, 4:4:4, uncompressed and so on...
I dream of a 35mm film camera.
That shoots digitally.
In a sense I do not want a ´video camera´. I want maximum information denseness so I can work with the material as you can with a 35mm negative.
No "auto focus", no "AWB", no "auto iris", no gamma that makes the picture look "pretty". Most of these stuff that makes filming "easy" is the reason that video, for the most part, looks...bad.
If the set is properly lit and the camera is used in a correct manner and you have the latitude of the negative for post, it will look good. Thats the main reason 35mm looks good.
The closes thing has been the Viper (and in 2:nd place F950).
Enter the RED...
Perhaps my dream will come true. :)
 
"Hans, you're confusing video gamma with a gamma to make the video look funky. By video gamma, I mean a gamma of ~2.5 which is the gamma of all normal looking video, and approximates the inverse gamma characteristic of CRT monitors. A lot of people refer to video gamma as linear, which it is not. Log is similar to video gamma but a more extreme curve is used."

Aha! I see. Then I am with You. :)

"It's in camera, where your recording to a specific bit depth that you've got to be careful."

Absolutely. I agree.
 
Hans Nystroem said:
"Color correcting in Log space is very unpredicteble."

Why? Apply a LUT and then tweak the parameters for the LUT. It´s quite straight forward I think. No harder than linear imho.
Click to expand...

The problem is how normal image operators affect the image when in log space. That's what's unpredictable, not the monitoring, which can as you state it be very accurate using a viewing LUT.
 
"The problem is how normal image operators affect the image when in log space. That's what's unpredictable, not the monitoring, which can as you state it be very accurate using a viewing LUT."

That is true.
And as You said. There is really no need to work in log when You have the precision of float.
It is when shooting log interests me for the extra latitude in highlights and shadows. :)
 
Hans Nystroem said:
No "auto focus", no "AWB", no "auto iris", no gamma that makes the picture look "pretty". Most of these stuff that makes filming "easy" is the reason that video, for the most part, looks...bad.
Click to expand...
i can even follow your viewpoint.. but you are not correct.
 
"i can even follow your viewpoint.. but you are not correct."

Auto focus, auto iris, AWB that kind of stuff is good for holiday films at the beach or ENG.
But if you want to do a feature film or a commercial you want manuel controls and the latitude of negative film.
But I get your point. Poor lenses, 1/3 CCD´s and low resolution doesn´t help either...
 
Auto white balance has as much appeal to me as Celine Deon at Ceasers. Thats what they make gels and color match tubes for.

Auto Focus has its place in certain enviroments and Auto iris is useless as long as you have gears on your lens.
 
Jarred Land said:
Auto white balance has as much appeal to me as Celine Deon at Ceasers.

LOL!!! :)

The "lack" of help from the camera will make you a better cinematographer. It will force you to think ahead and plan your shoot.
Click to expand...
 
ha ha.. and please dont anyone correct the spelling of Celine's last name.. i dont want DVXuser to come up when someone googles her.
 
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