Join Robbie Carman for an in-depth discussion in this video Decoding technical terminology for color grading, part of Color 1.5 Essential Training.
Like other parts of Post-production, there are several technical concepts involving color grading. The purpose of this movie is to give you the gist of these concepts, so you can be armed within understanding of the concepts and explore and discuss them with colleagues. So, let's get started. The first concept I want to explore is that Luma. You've probably heard someone says were before, and someone's has probably told you that it was short for luminance. Let's stop that idea right in its tracks. Luma, or the black and white or grayscale part of footage, is the part of the signal we use to evaluate contrasts but it's different from luminance.
Luminance is a color science term defined by the CIE, or International Commission on Illumination. And what luminance is is a measurement of the amount of light off of the surface, like the brightness of your video monitor. And its a linear measurement. Luma, meanwhile, is what defines lightness in the clip and its gamma weighted. So, in technical equations, Luma is denoted as Y' prime, where the prime symbol indicates gamma weighted luminance or Luma. So, if Luma is the grayscale part of the signal, then chroma is the color part.
Chroma can be part of this having two parts: Hue and Saturation. And because it's the color portion of the signal, its what we use to evaluate color. We will talk about evaluating both Luma and Chroma using Color's video scopes in a later chapter. You've probably heard the term Contrasts Ratio before, but what does it actually mean? I think the easiest way to think of contrast ratio is the difference between the lightest and darkest portions of a clip. If you take a look at these graphics I have two Waveforms Scopes in Color. One function of the Waveforms Scope is to evaluate the Luma part of the clip and the scale that the waveform scope uses mimics, the Tonal Range.
So, the bottom is black or dark and the top of the scale is white or light. And all this white stuff here in the middle of these graphics is called the Trace. And here, this actually represents the Luma part of the clip. So, in the top graphic, the difference between the lightest and the darkest portions of the clip is small. Meaning that it has a low contrast ratio, while on the lower graphic the difference between the lightest and darkest portions of the clip are much greater, indicating a high contrast ratio. We will explore the scopes in much more detail throughout this title.
Most people will agree that low contrast footage though, tends to look dull and flat, compared to high contrast footage. Well I could do a whole title on talking about the concept of gamma but for our purposes right now, it means two things. First is a non-linear adjustment of the luminance and if you remember from a few slides ago, that's how we get Luma, which is just gamma weighted luminance. Secondly, in color grading, gamma generally refers to the adjustment of midtones. In color grading you will see media encoded using two different color-encoding models. Also, different applications like Color and Final Cut Pro color correct and grade media using two different color-encoding models.
So let's take a look at RGB color encoding. You may or may not see RGB encoding in documentation denoted with the prime symbols. But just like with Luma it means that the RGB components are gamma weighted. In RGB color encoding, Luma and Chroma are linked, adjusting the one affects the other and an easy way to see this is with this graphic, which is the 3-D color space scoop from Color. RGB color encoding is represented by a box and anywhere you move in this box, your hue, saturation, and lightness, all change together.
The cool thing about RGB color encoding is its pretty much how we see the world and new formats like HDCAM SR and codecs like Apple ProRes 4444, support RGB encoding. Color also grades footage internally using the RGB model. The other color-encoding model you'll see is Y'CbCr. And this color encoding model is what most video systems use. Y' is the Luma part of the signal and CbCr represent the Chroma components of the signal.
Applications like Final Cut Pro work in Y'CbCr color space to grade footage. The thing about this type of encoding is that Luma and Chroma are separate. If you take a look at this graphic, which is again the 3D color space scope from Color, set to Y'CbCr. Luma is represented by the pole in the middle of the scoop, while color is the outside boundary. So, it's possible to keep the same Hue and Saturation without making the color lighter or darker. So, let's sum up RGB and Y'CbCr, color encoding.
In RGB encoding, Chroma and Luma are linked. In Y'CbCr they are separate. Computer design applications like Photoshop and, in our case Color, think in RGB. Video applications like Final Cut Pro think in Y'CbCr color space. Neither its right or wrong, just different. Another technical concept that you see a lot in regards to video formats and color grading is Chroma subsampling. Chroma subsampling can occur with Y'CbCr encoded video and it's essentially a way to reduce the bandwidth of the signal by reducing or removing Chroma information.
By reducing or removing this information it's one way modern video systems are able to get such long durations of high- resolution footage on recording mediums like HDV tape or a Sony SxS card. Chroma subsampling works on the principle that our eyes are much more sensitive to Luma than they may are to Color. So, reducing color information is not noticed by most. Chroma subsampling is represented by ratios like 4:2:2 and 4:1:1. Where the first number, which is almost always four, is the Luma part of the signal, and the other numbers are the color components of this signal.
So in a ratio like 4:2:2, color is encoded or sampled at only half the amount of Luma and that 4:1:1 color sample only add a quarter of that amount. A sample ratio of 4:4:4 indicates no Chroma subsampling and it's the best the signals it's going to get. As a general rule of thumb, try to use an acquisition format or transcode to a codec that supports a higher Chroma subsampling ratio. The last technical concept that affects color grading is Bit Depth. A good way to think about Bit Depth is the range of color and lightness available in any given color model.
Most formats you encounter are 8 or 10 Bit. Some formats like DPX can even use 16 or 32 Bit, Bit Depth. For 8-10 Bit formats these values mean that in 8 Bit there are 256 possible values per pixel and in 10 Bit there are 1024. Typically though, because of the way videos encoded, you actually have less values, but some newer formats like HDCAM SR actually work in true 10 Bit or what's called Full Range Bit depth. You can see in the graphics on the right the first image has a very little Bit Depth and seems almost pixilated because there are not a lot of values to describe color and lightness for each pixel.
The other image is 10-bit file and is much more clear. The basic rule of thumb is to work with 10 Bit whenever possible as this will give you the most latitude for correcting your footage. So now you have some of the basics on technical terminology regarding color correction. In the next chapter, will talk about some typical color workflows.
- Round-tripping with Final Cut Pro and Color
- Ingesting RED footage for color grading
- Understanding the Color interface and navigating Color's Finder dialogs
- Performing primary corrections in the Primary In room
- Applying secondary corrections using HSL keys, vignettes, and curves
- Using the Color FX room
- Keyframing corrections in a clip
- Preparing for rendering and output from Color back to Final Cut Pro
Skill Level Beginner
Q: I’m having trouble producing the cage as the author instructs in the "Making luma adjustments to specific colors" video. Double-clicking in the preview only changes the size of the preview. How can I make the cage as the author does?
A: Instead of clicking or double-clicking on the preview, perform a click-and-drag to produce the cage. If you select one of the buttons under the preview (the small swatches), then click and drag, you can also sample or "cage" multiple values (different cages) on the preview, which can help you if you're trying to sample multiple colors or luma values for secondary correction.