[Tig] LUTs compared to grading linear

Workflowers-C├ędric Lejeune c at workflowers.net
Mon Jul 24 07:59:50 BST 2006


LUTs are simply tables where you set data, you can use them for 2 reasons:
1: the mathematical function you want to emulate is too complex and 
takes too much processing power, so you calculate every value you need 
and set it in a table so you only ask your machine to use the result 
instead of calculate it.
2: you don't know what the mathematical function is and so you just 
measure the values you can get to emulate the system, i.e. the response 
of a film lab.

The LUTs we are using in our context are mostly translation LUTs to 
convert data from one colourspace to another and viewing LUT that 
emulate the processing of return to film. The first ones are simple 
results of math functions (log to lin for example), the others are 
result of a measurement process where you display some patches on your 
display device, send them to the lab and measure the output, then match 
them. Some tools can help you to generate them (RSR Cinespace, Kodak 
KDM). Those one can only be valid if all the parameters in the chain 
(kind of film stocks used, temperatures of baths, etc.) are constant, so 
if you change any parameter you will have to regenerate a LUT.

In case of return to film you can choose to grade using a LUT that will 
show you how it will look once back on film within a colourspace that 
tries to match as much as possible the film, or grade in a regular HD 
colourspace and then have the conversion done afterward using a colour 
management system tool that will do its best to convert the HD 
colourspace data in a film colourspace. Some boxes give pretty good 
results. If you plan to return to film without one of those correction 
then yes, you're running in a dead end. If you're grading for video 
output and are able to tape out what you see, then WYSIWYG.

So to say unique standard LUT don't exist, it's all about context. Hope 
this helps.

Cedric Lejeune
DI/HD workflows

> To the TIG
> I just want to understand the logic behind logarithmic transfers and 
> figure out how to implement a facility wide workflow working in log 
> space .
> I have had countless discussions about the benefit of log and somehow 
> on the scanning side I am convinced. It does keep most of the colors. 
> (Well if your telecine is calibrated of  course)
> Assuming the telecine gives out calibrated scans in log then I am 
> happy with getting log compared to getting a linear scan. I really get 
> the latitude.
> Here comes the hard and confusing part. 
> Implementing a look up table. 
> Are there standard look up tables that I can just use ... I would 
> think like broadcast standards there are standards LUTs.
> The reason for me looking for standard look up tables eventually I 
> have to tell my clients categorically that what you will see is what 
> most will see. I cannot tell them an arbitrary answer like " because 
> we say so"
> Even if we have done internal testing "if that testing is not 
> standard" it will still be a long argument .
> I would like to adhere to a standard LUT like 6500 is a standard. This 
> is something I can implement facility wide.
> The main problem is that when i turn on the default LUTs that come 
> with our grading system I dont like what I see. The colors are sh at tty 
> and I cannot in my best smile tell the client "thats what they should 
> expect"
> Currently I skip the LUT stage I scan in LOG then grade in linear 
> straight away. I hope this is not a bad idea. If someone knows that I 
> will end up in a dead end I really appreciate your input.
> Thanks
> =================
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