by Charles Boileau
You’ve heard of LUTs? It is a safe bet that yes. The Look Up Tables have become an almost indispensable tool in the world of digital grading and visual effects. Here’s a quick summary of the use we can make of it.
Postproduction houses use the LUT for calibration and for previewing film distribution. This process is less used, but it is important to know to deepen our knowledge of LUTs. To calibrate and previewing, it is first necessary to know the characteristics of our screen or projector. We must create a profile … This step is done using a software profile / LUT creation and probe. Essentially, you scroll colors and they are recorded by the probe and interpreted by the software. Once the process is done, it is easier to make the colors represented by X (the profile) and transform Y (calibration or preview). It seems simple enough, but in fact it is a bit more complicated.
This is when the LUT has to do all his work, because this transformation in it. But to achieve our goals, we must consider two things: What color space are my pictures? Where do I need to bring? For example, we shoot a film in Alexa Log-C and this project is intended to be distributed in movie … This film distribution has its own characteristics that must be represented in the calibration step. This process is the preserve of large box post because only they know the profile of chemistry and film. Once the process is, we only have to add this calibration to the projector and we have a fairly accurate picture of the final result in the analog cinema. And, attention, this change is only for the eyes of the calibrator. Images must be made without the LUT preview.
Essentially LUTs work the color space translations. And these can be technical and creative purposes. For example, we have a project originally designed for the TV … Our project is calibrated REC709-encoded RGB. Producers decide to make an exit to the cinema. It will be prepared by DCP (Digital Cinema Package) which is governed by a different color space: DCI-P3 *-encoded XYZ. Essentially, if you take the REC709 version that this on a movie projector, the DCI standards, the colors will be a little décontrastées, desaturated and a few green. It is therefore necessary to convert the color space using a LUT. In sum, this LUT convert the color temperature, the new coordinates of the color space, gamma and encoding colors from RGB to XYZ.
Finally, the creative tool. The sources LOG (such as Alexa or Red) are often quite difficult to calibrate. At least, it is difficult to extract their full potential of color. Basically LOG used to represent all levels of light and colors of the film in digital format. LOG is essentially a gamma curve that was applied to a file format such as DPX (explanation somewhat simplified which may be the subject of a future post). It is therefore necessary to reprint the images into a movie. This process has remained as it proved to be an effective way to make the digital calibration. A kind of absolute base image. But, as you probably know, the images can be quite desaturated and décontrastées. Not easy to get back to life. That’s where the LUT film comes to his work. Applying this translation, we can have a near-perfect representation of a film in digital format. LOG turns and suddenly gets a depth that we can better appreciate. It only remains to make the calibration work. But beware, in this case we should leave the LUT in place when exporting images.
So, we went around a bit faster. This text is an introduction to the world of LUTs. We have a chance to cover the topic in more detail without doubts.
* The DCI (Digital Cinema Initiative) is an organization that governs all digital distribution worldwide. It was created by the major U.S. studios to the advent of digital cinema.