Matching Colour and Grade (Gavdalf's Grade Helper)
Updated: Apr 25, 2021
Matching plates in Nuke is a fundamental comp skill (Pixel Wizardry 101 :D ).
It is, by no means a simple or easy task though. Matching 2 plates by eye can be a real challenge and as I understand, can take a long time to master fully.
This is why I created a gizmo to help with the process. Anyone coming from a still image compositing (Photoshop) background might have seen a video or two on YouTube about creating "helper" or "false colour" layers (if not then check out the links below):
These "helper" (Temporary) adjustments are there to support your eye by allowing you to just look at and pay attention to the 3 fundamental elements needed to convincingly match 2 plates or 2 shots. They break down the image and push it further in some cases into:
Hue - (this is the colour without taking into account saturation).
Saturation (this is how intense or saturated the Hue is *how much of that colour*).
Value (or lightness or luminance or Brightness) - how light is the pixel.
**Here is a good Video giving overview of the HSL (Also known as HSV) colour model** (Credit to Khan Academy)
..If you can correctly match these values closely enough to the plate (or to the overall shot) then you will have the starts of a great comp! :)
So, how do you use my little helper Gizmo when you are Node Noodling in Nuke?
I've created Video which explains the process and what the helper tool does as well as how to use it correctly (or at least how i do) ;)
You can download my Grade Helper Gizmo below (or from my Nukepedia page ):
To use you can either unzip and install in .Nuke directory (This Video explains how) or simply open .gizmo in txt editor, copy all then paste into Nuke Node graph (then you can save as toolset if you like).
How does it work?
The Grade Helper Gizmo works by selecting either Options 1, 2, 3 or 0 :
Option 1 (Colour) - We convert from Linier to HSV colour space (which means that Red now = Hue, Green = Saturation and Blue = Value (or Luminance). Working on the Blue or Value channel we shuffle in 50% Grey so that all brightness is flattened out. this allows for us to focus only on the colours without any changes in brightness or value to distract. We then change back to linear (RBG) colour space. Now, we have a very flat image with all colour very dull and flat. if we then boost the saturation we will see all colours much better so that we can then try to match hue's **the differences between plates should be very easy to see at this point - it should also help to see in which direction a hue needs to be pushed to bring plates in-line together**. Switching on and off the helper will show how much difference there is.
Option 2 (Saturation) - Again, we switch to HSV colour space. This time we focus on the "S" (or green in RGB) - this will give us only the Saturation of the image (how much saturation there is in each part of the image is shown in greyscale (the lighter the pixels, the more saturated they are). The aim then is to try and match the lightness of one element or plate to another - this will ensure that they are evenly saturated.
Option 3 (Value, Luminance or Brightness) - We switch to HSV colour space for the last time and just as we did for option 2 - we focus only on 1 channel (this time the Blue or Value (or Luminance) channel). when we switch to HSV and show only the "V" channel then all we are looking at are levels of luminescence (how bright each pixel is) - just as we did with the saturation channel helper - the goal here is to use this to make an informed judgement about how bright each element or plate (or even masked selection) is. Either match plates or match elements within plates which make sense. we can do this here without worrying about colour distracting. *Note* - Although this helper option seems like a black and white (or monochrome) version of the image - its not - with this option activated, you are looking at luminance values represented in shades of grey rather than a just a black and white conversion. there is a notable difference between the two, with this being a more accurate representation without being impacted by saturation or affect of hue differences.
Option 0 ("Do nothing") - this is a simple passthrough (or more accurately; pass around) - no adjustments are made to the image. I included this so that you dont need to dissable and enable the node to see differences vs original.
***Important note*** It is important to have a grasp of colour theory however. Blindly matching grading and levels between plates is not advisable - we need to consider the elements and how they would physically sit within the scene - something further away would be less saturated and something darker would be more saturated and so on.. You need to use your judgement when matching plates and if possible try to match to something that is physically similar within the scene or same place in space and is similarly lit.