This makes colourist one of the best LUT Building techniques available!

So, the only thing a colourist needs to know is the process necessary to make an accurate LUT. And here it is.

The first step is to know exactly what you delivery requirements are, and as celluloid film is still the most common 'film' delivery format we will use this as the basis for this LUT Building description.

The next step is to obtain a selection of .dpx and/or .cin reference frames, such as these:

Add to these reference frames a selection of 'real images' to provide a varied range of scenes and looks, and the final LUT can be very accurate indeed.

These frames are then output via the film recorder, processed and printed, all the time making sure each step in the chain is accurate to AIM. To check this the Status-M and Status-A values of the 445 patch for the negative and print can be measured and checked with a densitometer. The following is for Kodak stocks:

Digital LAD Print AIM Density - 445 patch

With these accurate prints you now have the target 'look' that the digital image needs to match...

The next step is to use a light-box, matched to standard 'film' colour temperature and brightness, to view the reference frames with a Loupe and match the digitally displayed images to their film versions using normal colour correction techniques. This colour correction then becomes the data for the LUT to be built.

It really is that simple!

The following images show a light box with a selection of on-AIM reference film prints, with filters to change the colour temp and brightness of the light box, and with a viewing slit to isolate a single frame at a time, with no light spill.

Each print frame is then viewed in turn, and the same digital image graded using normal colour correction techniques - even using a package such as Photoshop - until all the digital images match at best possible the print frames, with the same overall colour correction applied, while being viewed on the monitor/projector to be used for DI grading, within the actual grading environment.

It is even possible to start with an initial LUT to get in the ball-park, and adjust the look to fine tune as required.

Some initial LUTs can be downloaded here

The combination of the initial LUT and the colour correction settings then form the data for the new LUT...

Using LightSpace CMS the automatically generated reference frame, with a user image inserted if desired, is then run through the same image path, including initial LUT and colour correction to burn the LUT data into the image.

This 'corrected' image is loaded back into the LightSpace CMS software where new LUTs can then be exported and used as required.

For more info on this approach the see LightSpace CMS operation manual and the actual LightSpace software.

You can also see that the 'test frame' has all the cubes colour values encoded into the edges of the frame, which means the generated LUT is very accurate to the colour correction performed.

To be this accurate with the automated way of using 'flat' colour frames would require nearly 5000 separate colour patches to be read in turn... Eeek!

Obviously, what the approach to LUT building defined here relies on is that the colourist is able to see and grade out image differences, and that the light box and prints are accurate. And most colourists I know are more than capable of this.

But, even with an automated approach to LUT building, the various images used, and the equipment used to measure them, have to be accurate. And as stated earlier, the human eye is better than any electronic device at seeing differences in colour and brightness - especially in the dark areas of an image, where electronic systems struggle.

As a result, this manual approach to LUT Building can be very accurate.

Next Page - Automated LUT Building