I’ve been wanting to write a tutorial about color correction techniques in Compositing for a while now. But since I’m lazy / busy / mostly lazy. So I just haven’t gotten around to it.
Until today that is!
So sit back and kick up your heels, because today I’m going to demo a quick and easy way to color match images using just a Grade node and the RGB luminance values in Nuke.
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Let’s begin with a real world example of how this is useful.
The first step of the typical shot development pipeline for a VFX studio is Previs (aka Previsualization). That includes story boards, 3D animatics, and tone boards. Storyboards and 3D animatics aren’t really that useful to us in Compositing, other than to get an idea of how the final piece will all fit together. What we’re going to get the most meat from however, is the tone boards.
Tone Boards are color palettes, reference photos, textures, you name it. It’s a big mash up of all the ingredients that will make up the over all look of the individual shots and sequences through out the rest of the VFX pipeline. Thus feeding the look development process in Compositing as well.
A perfect online tool for creating quick and easy color palettes is Adobe’s Kuler. It even lets you import your own images. And then uses those images to generate a color scheme based on it. Here’s an example I made using an image I found on Stock.Xchng.
LINK: Adobe’s Kuler
LINK: Stock.Xchng
Big broad swatches, samples from a wide range of colors in the image. This is the type of thing you’ll looking for in a good tone board.
Now lets get down to the nitty gritty. How do we use this? Well lets say you were just given a shot of a downtown city scene. With a note from the Art Director saying that he/she wants this shot to feel like the Sicily tone board. Now urban New York and the Sicilian Countryside couldn’t be more different, so how are you supposed to make these two opposites mix together easily?
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THE THEORY
Well it’s easy if you use RGB Luminance values.
RGB Luminace values is just a fancy way of saying black and white images that either brighter or darker than the next.
So let’s compare our source Sicily color palette with our target New York color palette using the different luminance value to really get a feel of how each palette is constructed.
Sicily
New York
Pretty huh? But what does this tell us? Well it gives us the finger print of these two images.
Our Sicily image’s Red and Green channels are pretty identical, but our Blue channel is really dark. Giving us both the obvious information that this is a very warm image, but also giving us a visual ratio of how each color mixes with the next to create the final source color palette.
The same goes for our New York image. The Red channel has bright sections in the green and orange, while our Green channel is much duller in those hues. And our Blue channel is less dark than our Sicily image telling us the obvious once again, that it’s a much cooler image. But again the important thing to see here is really the ratio between each color and each palette.
BRIGHTER = MORE , DARKER = LESS
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Now at this point you should be a bit confused. Maybe even a little cross-eyed from staring too long and hard at the differences between the color palattes above. The main thing however you need to remember is that you won’t be getting 1+1=2 answers when you’re deep into look development. These are just visual tools to give you the FEEL of how each color relates to the next. And then how the over all palette comes together. There’s no right answer, it just all comes down to feel.
With that being said, it’s now time to get into Nuke.
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THE PRACTICE
First I’m setting up a viewer with the target Sicily color palette plugged into 1, New York into 2, and then…you guessed it…Sicily into 3. This allows me to quickly switch between each image which will help me Grade the New York image to match the Sicily image closer.
Then I’m going to create a Grade node and just start grading each individual channel separately (RGB), each channel getting it’s own individual grade node. Now you could do it all in one but it’s just much easier to visually show you the process if I give each channel it’s own node.
While I’m grading each color it should be noted that I’m mainly using the Sicily image for reference. Only ocassionally going to the color swatches for when I need more exact colors. Like the yellow on the taxis for example.
After spending a few minutes adjusting the multiply and gamma sliders for each color channel, to match more closely with the Sicily image. This is the before and after result.
Now at this point I would be going back to the Art Director and seeing if we can’t dial it back a bit, ha. But you get the idea. And all of that was driven by the RGB Luminance values of each individual channel.
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The same technique can be applied to color matching. Which is a much more day to day task than Grading a plate to match a tone boards.
So say you have a Sky that needs to match a very specific color Blue. Well just use the R G and B channels of A (the Blue you need to match) and match the brightness of each channel of B (your original Sky) to dial in the source color to match the destination color.
Quick, easy, and precise. The holy trinity for a Compositor.
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Man that was long. I might need to just start doing video screencasts of these techniques to save me from so much typing!
Anyways, I hope this was helpful and that it shined the light on a dark corner of the VFX pipeline for most people. It’s a simple technique, that’s long to describe. But RGB Luminance Color Matching is the lynch pin of all photoreal Compositing.
If you have any questions, comments, really anything. Feel free like always to post a comment in this post’s thread below!
