- Tom tom@tracksandlayers.com
THE COLOUR SPACE: Our guide to 10 common colour grading terms
Introduction
We hope to ensure that every client can focus completely on what really matters with their project and not get bogged down in the technical details (let us worry about that).
With that said, sometimes it's nice to at least have some idea of what's going on or perhaps you would like to brush up on your colour grading lingo.
In no particular order, here are 10 common terms which you are likely to hear at some point in our colour grading suite.
1 - Node
When working inside Davinci Resolve (our colour grading software), a colour grade is developed in stages. We start with the source image and then we make a creative decision such as adjusting brightness, the levels of colour, perhaps even contrast. The look is made by applying additional adjustments and corrections until the end result appears correct.
A node is like a virtual container of settings adjustments in the software, and they link to the next node in a chain - or sometimes they can branch out and be recombined later - or they can be made to extract certain information about the image, and then piped into another node further down the chain.
If your colourist is working methodically - each node will likely be designed to address a specific thing. A very basic chain of nodes may look like this (each box is a node):
A Colourist can turn off a node, disconnect it or move its position in the chain order.
2 - XML file
A typical workflow for a colour grade would be that an editor creates an "offline" edit of a sequence inside their editing software of choice (such as Adobe Premiere Pro or Final Cut). In order to perform the colour grade, we need to carry every edit decision from the editing software to our colour grading software (Davinci Resolve).
Each editing package has it's own native file format for the project - but these won't open in other apps, so we need to use a universal interchangeable file format.
Step in the XML file. An XML file is actually not exclusive for colour grading - or even just for video work. It's a very flexible text based file format which can be used for many different purposes. In this context it contains every edit decision made by the editor, information about where the video files are located, certain effects which may have been applied, playback speed etc).
You will not that it does not contain any actual video data at all! It only contains a written account of how the source video files have been assembled into an edit. As such the XML file is very small and easy to send via email if necessary.
The XML file is loaded into Davinci Resolve and all being good - the edit will appear as it did inside the original editing app, except this time it's inside the colour grading software.
Some things to keep in mind though are that not everything is able to be retained in an XML file - so it's often not a perfect transfer. Such examples are speed ramps - these will need to be baked in or rebuilt after the grade. A topic for another post for sure.
3 - Colour Space
In it's simplest terms - a colour space is range of possible colours (including bright and dark shades of colour and shades of white to black - and often way more than 50 shades of grey).
Each colour is represented as a number, but what is it that dictates which colour is assigned to which number? A colour space will define such a thing.
A "large" colour space will have an enormous possible range of colours (way more in fact that any screen is capable of displaying). A smaller colour space (Rec709 for example - which is used for almost all broadcast television) is far more restrictive and designed to cover the majority of colours we can see.
Most cameras are capable of capturing bigger (or wider) colour space than you can deliver in. As such each camera or camera brand has it's own native colour space which needs to be converted properly into the correct delivery colour space.
For example:
Arri Alexa (Arri wide gamut 4) > converted to > REC.709 (for tv broadcast)
As a consequence of converting from a bigger colour space to a smaller one, certain creative decisions have to be made (also - as you may have guessed, it's an destructive process). This is a classic example of why it's a good idea to use a colourist for even fairly straightforward colour grades.
If you deliver to or create your grade in the wrong colour space - you are likely to have some series problems with the end result. Colours may be incorrect by a very large margin
This is one of the reasons why it's important to find out what colour space your footage has been filmed in and where the finished project is going to be delivered to.
4 - ACES
Speaking of colour spaces (see number 3 above if you're just skipped here) - ACES is a fantastically useful colour space.
ACES or Academy Color Encoding System - is a colour space encoding system designed to create a non-destructive and adaptable way to cross convert from different starting points and to try unify how different kinds of images are colour graded together.
If your colourist mentions it - it's likely that they're explaining that they intend to use this process to grade your project. You need not worry or change your plans - in fact it's likely going to help the process if the colourist thinks its the right tool for your project.
ACES excels at a few key things:
Helping to adapt multiple different camera source images into one unified starting point. So if you've shot your project on several different cameras (a high end A camera, a mid range B camera and a load of drones or action cams for example)
Helping to adapt to multiple delivery formats. So you need a grade for TV Broadcast and a grade for Cinema distribution? ACES can help to make this easy.
Controlling very strong colour sources (often fluorescent LED lights) to ensure they don't look problematic or harsh when they go out of "gamut" (beyond what will fit in the selected colour space).
Allowing computer generated graphics to retain as much data as possible so that it can be graded like video footage.
5 - Power Window
A power window is simply like a kind of masked area so that you can selectively grade either everything inside the mask or only outside the mask. They can be any shape and can be static, animated or even track in 2D or 3D to follow a surface. A common usage of a power window might be to add a nice gradient effect to a sky or perhaps to track an actors face so that you may brighten it up slightly.
6 - Round Trip
A round trip process basically describes the journey a sequence goes in from the editing software to the grade and then back into the editing software. From the client's perspective - they hand over a prepared edit and get back the same thing, except this time it has the colour grade applied to everything.
7 - Handles
Ok so you've locked your edit - happy with everything and you've handed over all your video files and an XML to the colourist for the grade. They do their magic and return to you the exact sequence you gave them except this time it's been graded. But then you watch through your graded sequence and start to feel like some of the edit points need tidying up a bit, extending perhaps.
To save you from having to spend any more money on getting this new slightly more locked edit graded - your colourist can ensure to export all your clips "with handles". This means that on every side of an edit point, a second or two of extra footage will be graded. So if you have a 5 second shot - grading with 1 second handles gives you a 7 second shot.
Thanks to the XML round-trip process - you won't have to manually re-time everything - the extra footage will be hidden unless you decide to alter the in and out points of the graded clips afterwards.
It's fairly standard for handles to be 1 or 2 seconds long - often measured in frames. So a 1 second handle at 25fps is 25 frames long.
Handles are like wiggle room or a buffer for those last minute changes to and edit which was perhaps actually just "on the latch" rather than being locked.
8 - LUT / Look Up Table
A LUT or Look Up Table is a fairly simple thing - but it can be used to achieve powerful results. On a technical level, a look up table is simply a set of corresponding numbers. If a particular pixel in the image has value X then change that to value Y. Every possible Red, Green and Blue combination is measured as a number:
8 bit colour (such as on a phone or dslr) - 1 of 16 million colours
10 bit colour (such as on a mid tier camera) - 1 of over 1 billion colours
16 bit colour (such as on a high end professional camera) - 1 of over 280 trillion colours
So what's the point of changing these values? The primary modern usage of LUT's are to very quickly apply a set look to an image. This is different to their initial historical usage but today a LUT is typically designed to be applied to a particular image profile. LUT's can be designed to work with specific camera image formats, standards such as REC.709, film scan formats such as Cineon etc.
One thing to keep in mind with Look up tables is that they only allow for very basic transformations. You cannot do anything which takes into consideration any neighbouring pixel values.
Amongst other things, LUT's can:
Change the brightness levels
Change the colour saturation
Change the hue
Change white balance
Add or remove contrast
"Clip" or remove data from a particular part of the range (or compress or expand it)
LUT's cannot:
Sharpen the image
Blur the image
Add "structure"
Add grain
Reduce noise
Add a vignette or any other kind of power window
Use spatially aware shadow or highlights adjustments
One final note on LUT's is that they are not reversable. Once applied and exported, they have a "destructive" impact on the image.
LUT's can be loaded into some cameras or camera monitors so that a look can be previewed in real time. They can also be used on a Node inside colour grading software. Some higher end TV's also allow LUTs to be used as a way to calibrate the image.
9 - DCP
A DCP or Digital Cinema Package is essentially a digital file which contains everything needed to properly project something in a movie theatre.
Projectors in large theatres are far from the usual power point type projectors you might find in an office - they're even a far cry from the higher end home cinema models you can buy. They're actually integrated into a kind of computer server system - designed to receive the Digital Cinema Package file and then run the projector and sound system with the correct settings and in the best possible quality.
For the inexperienced, DCP servers can come across as very picky and fussy as they are very intolerant to slight discrepancies format of the data. To this end, a correctly made DCP is created to a very strict standard - even down to how the file name is written and ordered and in some cases the kind of hard drive it's delivered on.
In essence the file itself is a kind of zip file. This file will have quite a long name and it contains a plethora of information such as the frame rate, language, sound mix, aspect ratio, who made the DCP file, subtitle info etc. Inside the file will contain a very high quality still image sequence of the film, subtitle data and the sound mixes.
The video data actually has to be encoded to work in a particular colour space and image profile for it to appear correct so it's important that your colourist knows of the intention to distribute the project as a DCP.
DCP files can be made by anyone (for free) if they have a sufficiently powerful computer. This said however - the possibility for error is high and the consequence would be that the projection system would refuse to even load the presentation. This is why it's a safer bet to have an experienced professional create your DCP.
10 - Broadcast Legal
You've made your film and you've booked your colourist and you can't wait to explore the endless possibilities in the grade. But wait - these endless possibilities may just collide with a very strict rule about what you are able to broadcast.
For a deep set of technical reasons (way beyond the scope of this blog), you cannot simply broadcast any possible colour within the range of values available. The good news is that if you've not noticed these missing colours so far when watching tv - then you probably won't mind working around this limitation now.
As mentioned above, the technical reason for this limitation means that it's not entirely possible to predict exactly when a colour is going to hit the limit of what is permitted (or "legal") and so the best practice is to work with a good margin of error.
All Broadcast TV has to be processed through a "legaliser" at some stage before it hits the air - and if your project still contains illegal colours at this stage, the hard chopping away can result in a very nasty looking image. Far better to stay away from the danger zone entirely.
Colour grading software such as Davinci Resolve contains tools which help to build in this buffer automatically so it doesn't really add too much extra work at all. These tools give the colourist some options as to how much of a buffer to work with. Putting these safety measures aside - the most common place you are going to find things going outside of legal scope would be any colour where the saturation levels are high and the brightness levels are low.