Chroma Subsampling

With YUV color space it is possible to use the chroma subsampling as a method to spatially reduce the color information in a signal while retaining the luminance data. This technique leverages the human visual system's characteristic of being more sensitive to variations in brightness (luminance) than to color (chrominance). By reducing the amount of color information, chroma subsampling significantly lowers the bandwidth and storage requirements for video data without substantially impacting perceived image quality. Video signals consist of luminance information, which represents the brightness levels, and chrominance information, which represents the color. Chrominance is further divided into two components, U and V Chroma subsampling is expressed in a notation like 4:2:2, 4:2:0, 4:4:4, etc. where the first number (4 in these examples) refers to the reference number of luminance samples in the first row of a 2x2 block of pixels.

The second number indicates the number of chrominance samples (U and V) in the first row of pixels, showing how many color samples are taken compared to the luminance samples. The third number indicates the number of chrominance samples in the second row of pixels.

4:4:4 subsampling means that no chroma subsampling is applied. Every pixel has its own color information, resulting in the highest quality but also the largest bitrate.

4:2:2 subsampling reduces the color information by half horizontally but keeps full-color information in the vertical direction. It's a common compromise between quality and bandwidth used in professional video environments.

4:2:0 subsampling reduces the color information by half both horizontally and vertically, which is widely used in consumer video formats (e.g., Blu-ray, DVD, streaming media PTZ and Prosumer Cameras) because it significantly reduces the bitrate with minimal visible loss of quality.

4:1:1 subsampling reduces the color information by a quarter of the horizontally resolution and keeps full color information in the vertical direction.

Alpha Channel:

The YUVA color space is an extension of the YUV color space that includes an Alpha channel. In color spaces, the Alpha channel represents the opacity of a color, allowing for varying levels of transparency and compositing images over one another.

The alpha channel sampling is usually the same as the luminance and is expressed in a notation like 4:2:2:4, 4:2:0:4, 4:4:4:4, etc.