Rec.709

Rec. 709 is a widely used color space standard for high definition television (HDTV) and digital video production. It was released by the International Telecommunication Union (ITU) in 1993 and is also known as ITU-R Recommendation BT.709.

Rec. 709 specifies the parameters for the color primaries and white point for the video. These parameters define the range of colors that can be displayed or captured by a video system.

The color primaries in Rec. 709 are defined using CIE xy chromaticity coordinates, with a white point at D65 (6500K color temperature).
Rec709, being a color space, is transmitted to a display, which in turn applies a luminance distribution curve (for example, a monitor has a luminance curve corresponding to gamma 2.4, broadcast television BT 1886, and mobile phones gamma 2.2).

Rec. 709 is used in many applications, including broadcast television, Blu-ray discs, and digital cinema. It is also used as a reference standard for color grading and color correction in video post-production. It's worth noting that while Rec. 709 is a widely used standard, it has some limitations. For example, it does not cover the full range of colors that can be perceived by the human eye, and can result in limited color accuracy and dynamic range.

As a result, newer standards like Rec. 2020 have been developed to address these limitations and provide a wider range of colors and higher dynamic range.

Rec. 709, while being not as wide as newer spaces is still an industry standard for several reasons:

1. Compatibility: Rec. 709 has been widely adopted and is compatible with a broad range of video equipment and systems, including broadcast television, Blu-ray discs, and digital cinema. This makes it a practical standard for many applications.

2. Ease of use: Rec. 709 is relatively easy to use and implement, making it accessible to a wide range of users. It has a simple gamma curve and color primaries, which makes it easy to calibrate and adjust for color grading and color correction.

3. Cost-effective: Because Rec. 709 is a widely adopted standard, it is often less expensive to work within its parameters than to work with more advanced standards that require specialized equipment and workflows.

4. Legacy compatibility: Rec. 709 is still used extensively in legacy systems and equipment, and many users are familiar with its parameters and workflows. This makes it a practical choice for many applications where compatibility with legacy systems is important.

5. It still provides good quality: While Rec. 709 has some limitations in terms of color accuracy and dynamic range, it still provides good quality for many applications. For example, it is still widely used for broadcast television, where the limited dynamic range is not as critical as it is for high-end digital cinema.

While newer standards like Rec. 2020 offer improved color accuracy and dynamic range, Rec. 709 remains a practical and widely used standard for many applications.

In post-production, the process of converting an image from original camera color space to Rec. 709 involves several steps, including color grading, color correction, and color space conversion.

Scene referred and display referred are two different approaches to color grading in digital imaging.

Display referred color grading is the traditional approach to color grading, where the image is graded based on how it will look on a display screen. In other words, the grading is done with the display's limitations in mind. The color space used for grading is typically a gamma-encoded or a display-referred color space Rec. 709. The image is graded to look good on the display, but this approach can lead to issues such as loss of detail in highlights and shadows, and color banding. All the color conversion happend before the graing in final Rec. 709 space.

Scene referred color grading, on the other hand, is a newer approach that grades the image based on the scene's actual lighting conditions. The color space used for grading is typically a linear color space, or some of the wide camera spaces, or intermediate spaces, such as ACES or DWG with gamma curves that correspond to them, which preserves more information in the image. The grading is done based on how the scene would look in real life, and then mapped to the display's capabilities. This approach can lead to more accurate and realistic results, with fewer issues such as banding or clipping. Once the color grading and color correction are complete, the image is then converted to the Rec. 709 color space. This involves mapping the original colors of the image to the corresponding Rec. 709 colors using a mathematical algorithm. This process may involve some loss of color accuracy and dynamic range, as the Rec. 709 color space is more limited than some other color spaces.

The conversion process may also involve applying a gamma curve to the image to ensure that it is displayed correctly on Rec. 709 compliant displays. The gamma curve used in Rec. 709 is a power function with a gamma value of 2.4, which is designed to match the way that the human eye perceives brightness.

Generally we use scene reffered process allow to have color correction process with less restriction. It invoves, but not excludes: adjusting the color and tone of an image to correct for any errors or inconsistencies that may have occurred during the capture or transfer of the image. This can involve adjusting the white balance, exposure, and other parameters to ensure that the image is accurate and consistent.

Overall, the process of converting an image to Rec. 709 involves a combination of creative and technical decisions to achieve a specific aesthetic and ensure that the image is accurate and consistent with the Rec. 709 standard and final video serves the creative intent of its creators.