What does Rec 709 mean?

The common element of many color spaces is the RGB (Red, Green, Blue) model. This model is widely used in digital systems such as cameras, televisions, monitors and projectors. It is based on the idea that all colors can be created by the appropriate combination of three basic colors: red (R), green (G) and blue (B). Each color in the RGB model can therefore be expressed using three numerical values ​​that represent the intensity of the red, green, and blue components. The range of these values ​​is from 0 to 255 for an 8-bit image, and 1023 for a 10-bit image, which makes it possible to obtain millions of different colors.

The RGB model is an additive model, which means that adding the intensity of the primary colors leads to a brighter color. For example, if we combine the maximum values ​​of red, green and blue, we get white. On the other hand, the absence of light (all RGB values ​​equal to 0) results in a black color. The subtractive model mentioned in the case of film stock (or CMYK prints) behaves exactly the opposite.

When discussing the topic of colors in the digital world, two key concepts often come up: color space and gamut. Although these terms are used interchangeably, they have different meanings and refer to different aspects of color management. A color space is a complete system that describes how colors are organized and presented in a specific digital environment. This is a mathematical model that determines what values ​​are assigned to each color and how they are displayed on the screen. Gamut, on the other hand, refers to the actual range of colors that a particular device, such as a monitor, camera or printer, can reproduce in the context of a given color space.

Rec 709 and sRGB are often compared because they have similar characteristics, but there are a few key differences that result from their different applications. Both color spaces are based on the RGB model and have a very similar color gamut, but they differ in their application and parameters. Rec 709 was developed for television and video production, while sRGB is a color space created for computer graphics, digital photography and displaying images on websites. Rec 709 is optimized for transmitting video signals, while sRGB is better suited for static computer images. The key difference between these color spaces is the gamma value. Rec 709 most often uses a gamma of 2.4, which is standard for video production, while sRGB has a gamma of 2.2, which is better for displaying images on computer monitors.

One of the key elements of any color space is gamma, a curve that describes how brightness values ​​are converted into signals displayed on the screen. In the case of Rec 709, gamma is described as a function, although it is assumed to be 2.4. This is a standard used mainly in professional film and television productions, where the image is displayed in controlled lighting conditions, for example in dark cinema rooms. For bright rooms, the gamma may be 2.2. Another example of gamma curves are logarithmic curves – the so-called logs, more about which in another entry about S-Log.

The Rec 2020 color space is a newer and more advanced standard that replaces Rec 709 in the context of Ultra HD (4K and 8K) television and HDR (High Dynamic Range) technology. Rec 2020 offers a much wider color gamut and a depth of up to 12 bits, which means it can reproduce a greater number of shades, especially in the context of very saturated colors. This standard also offers a brightness range of up to 10,000 nits. Rec 2020 is certainly the future, but its limitation for now is the higher costs of production, post-production, transmission and even the receivers themselves.