This is a complex colour transform based on analysis of actual filmīut this post is pointless without examples. KODAK FILM EMULATION - After passing the hue circle through a Kodak film emulation lut, see how the colours have changed. So 3D LUTS are usually destructive to the image. Then once you’ve got your purple image how can you go back? All your pixels are now purple and you cannot separate the original colours. If you take every RGB colour and transform that to a shade of purple. The subject of film emulation will be brought up again in another post.Ī 3D LUT is a one way street though, because once you’ve transformed your image you cannot transform it back. Lutists will argue why waste all that time.
Purists will argue that anything that can be done via an emulation LUT can be graded, which is true. Are they all wrong? Not really, they have a use. And indeed there are many commercial products for doing this. Of course the usefulness of this is apparent, you can now use similar LUTs to make your digital video look filmic. When you’re working digitally the film out company could provide LUTs specific to the film stock you will be outputting so when grading you can view your image under an emulation of how it would look projected from film. Indeed the very birthplace of LUTs is to be found in the DI. The LUT can not only change colours but also the tonality of the image to something that looks filmic. Modelling the look of a particular stock of film is a great example. This helps us compare LUTs like for like.ģD LUTS are often sold as ‘looks’ or ‘film emulations’, they ‘bake' in a certain look that would be very difficult to create manually. It has a hue circle and two others at +2 stops and -2 stops. The exposure response appears different in each of these LUTs as well.īASE SRGB IMAGE - This is the control image. The exposure response appears different in each of these LUTs as well.Ĥ examples, from No LUT to three differnet LUTS applied. LUT examples - 4 examples, from No LUT to three differnet LUTS applied. They take a wide colourspace of a camera, transform it to other spaces and allow you to quickly see an image that looks 'correct' on your display. Sony offer a bunch of LUTs to again convert from camera to various display colourspaces.
#DEFINE LUT GENERATOR#
Arri has a LUT generator online which takes Alexa Footage and allows you to generate a LUT to transform it to Rec709 or some post production orientated colourspace. Ben Turley wrote a utility called LUTCalc which offers incredible flexibility in converting between camera colourspaces, tone curves and creating correction LUTs. Some camera manufacturers offer LUTS to help transform their camera output into different colourspaces. A LUT is a simple way of describing any possible colour transform and has a lot of good uses. So all a 3D LUT is doing is taking a set of RGB values and returning different RGB values. Then looks those up in a table and you get a resulting RGB value. What a 3D LUT is doing is similar, but it takes 3 values as input, a R, G and B colour value usually. We’ll come back to this later on.Ī 3D LUT is where the ‘issues’ can start. Of course you can see that a LUT has a well defined start and finishing value, whereas a mathematical curve could be infinite. You can have multiple 1D LUTs, one for each R, G and B but that's less usual.
So if you have a 10 bit number going in (1024 possible values) then that is the size of your table (1024 entries) and you’re good to go. So if you wanted to change a log curve to a gamma curve there is a pure mathematical equation that could be used, but actually it is quicker to work out what all the values are ahead of time and store them in a table. It’s a very quick way of converting between sets of values. All a 1D LUT does is convert one value into another - you have an input value, that is looked up in the table and an output value is found. One is a 1D (one dimensional) LUT and the other is a 3D Cube LUT.Ī 1D LUT is very simple, very useful and not a problem. Lets define a few terms before we dive into what’s going on. This post will attempt to show you what a LUT does and when it doesn’t do it very well.
The truth, as always, is somewhere in between. LUTs have been seen as the holy grail and the very embodiment of the devil itself.
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