Understanding Color Space Conversions in Display

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blogs.synopsys.com, Sept. 21, 2020 – 

Color space is a very powerful tool that comes in handy when capturing, transmitting and reproducing color back to the human eye. Systems such as cameras, GPUs, transmission cables (HDMI/DP), and monitors use color space metrics to preserve and transform color. This technology helps map real colors to the color model's discrete values.

Color modelling is a mathematical way of describing colors, independent from physical devices like cameras and displays. There are five major color models that sub-divide into others: CIE, RGB, YUV, HSL/HSV, and CMYK.

In continuation of our earlier blog, let's review color sub sampling and the different color space conversions.

Why YUV is preferred over RGB

YCbCr is a consumer video format and representation of encoded HD, whereas RGB is a traditional computer format. One is not superior to the other because each has its own strengths and weaknesses. YCbCr is generally preferred over RGB because of its native nature. However, many displays (almost all DVI inputs) only accept RGB. YCbCr formats are advanced in terms of transmitting and reproducing HD, 3D, UHD and 8k videos. So, if your display is HDMI it is likely to accept YCbCr if not switched to RGB.

Studies show human eyes are sensitive to luminance (intensity of light) but not so sensitive to chrominance (sensitivity of color). YCbCr color space makes use of this fact to achieve a more appealing representation of scenes and images by separating the luminance and chrominance components of a scene and reducing the chrominance.

Color Sub-sampling

Representation of YCbCr separates luminance and chrominance, making it easy for the computing system to encode the image using less bits. This is enabled through color subsampling, which simply encodes chrominance components with lower resolution. To explain further, let's dive down to cover four commonly used subsampling schemes: 4:4:4, 4:2:2, 4:2:0, and 4:0:0.

4:4:4 Sampling

4:4:4 is a full resolution in both horizontal and vertical directions with no subsampling done. Each pixel has both luminance (Y) and chrominance (cr, cb) components.

Each of the two chroma arrays has the same height and width as the luma array.

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