Let's talk LUTs

S

sonofaresiii

Well-known member
Because they mystify me.

Having worked with DSLRs, I've never really been exposed to LUTs much, but it's becoming more and more apparent I need to know what I'm dealing with here.

I get the basic idea... but can someone help me figure out what these things do, how they're used, practically?

It's my understanding (and please, please correct me if I'm wrong) that...
LUTs are image settings used to alter an image to give a specific "look," more for reference than actual viewing. E.g., if one needs to grade for a film print, they'd apply the image characteristics of the print as a LUT onto the ungraded image, then grade it accordingly. With the new grading settings applied, what you see on the monitor (or projection) is what you'd get when you print to film. If one is aiming for digital distribution, a different LUT mimicking those viewing settings would be used instead.
Also, LUTs can be applied on-set if someone's shooting in, say a log format (Redlog and the like) and want to know approximately how it would look when color corrected, to monitor the footage and be able to say "Yes, this is the right look." Now, at this point, no grading is done on the LUT, just a general sense of what the footage would look like on a computer screen (or possibly film print?).

So if I have all that right, can you guys help me out in understanding... how is a LUT created, what IS it, physically (is it like some type of file?) and HOW does it get applied to what you're viewing?
I saw a demonstration once of someone grading using a LUT... to me, it looked like he just pressed some buttons, opened a menu with a dozen or so different LUTs, clicked on one, and voila. That helped me understand a bit, but not as much as I needed.
Also, is the LUT actually a part of the footage metadata, thus needing to be removed when you're done with it? Or is it somehow just a temporary setting which isn't saved with the footage at all?
And finally... when used for viewing purposes on set, what criteria would one look for when creating/choosing a LUT?

Thanks for the help, guys. Hard to find a lot of this information out there...
 
sonofaresiii said:
Because they mystify me.

Having worked with DSLRs, I've never really been exposed to LUTs much, but it's becoming more and more apparent I need to know what I'm dealing with here.

I get the basic idea... but can someone help me figure out what these things do, how they're used, practically?

It's my understanding (and please, please correct me if I'm wrong) that...
LUTs are image settings used to alter an image to give a specific "look," more for reference than actual viewing. E.g., if one needs to grade for a film print, they'd apply the image characteristics of the print as a LUT onto the ungraded image, then grade it accordingly. With the new grading settings applied, what you see on the monitor (or projection) is what you'd get when you print to film. If one is aiming for digital distribution, a different LUT mimicking those viewing settings would be used instead.
Click to expand...

LUT is short for 'look up table'... The use of a LUT is to allow for 'fast' translations between input to output, especially when a formula is either awkward, or effectively non-existent...

In the olden days this was a necessity to produce a device that could perform image corrections or transformations, without very expensive hardware. For example the 'false color' images of satellite images was done via 'LUTs' for many applications. Turning an image in to a 'high contrast' image was 'simple' with a LUT... just place 0 in the LUT table for intensity values that would be seen on the screen as 'black' and place 255 for areas that would be seen as white... say any image value below 128 -> 0, any image value equal to or above 128 -> 255... for an image that had values 0-255...

One can correct for non-linearities in the sensor output via a LUT... or in applications with cameras, induce 'non-linearity' when it seems useful.

In Photoshop, and in Premiere, the 'curves' adjustment filter is in essence a LUT filtering mechanism.

So, anything you would do with Curves, is what you would do with a LUT.

Since I don't have access to an Alexa or other 'big time' camera... probably never will, I'll only indicate that I see LUT referenced in regard to those cameras, when one wants to induce an S or a Log curve LUT for the camera sensor data.

What this does is map the linear data out of a sensor, into a 'curve' which more closely matches the human eye response, as well as how photographic materials respond. How this is 'saved' for use later on in the work flow, is unknown to me... since I don't work with the types of cameras that allow for this type of processing on the 'raw' data before being encoded in some type of codec.

As an additional note... for 'color' one would have 3 LUTs, one for each color, and so, one could adjust not only the input to output values for 'luminosity', one can adjust for different 'color balances' as well.
 
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