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Understanding Color Space in Display

The color space is a very powerful tool that comes in handy when capturing and transmitting color back to the human eye. All systems like cameras, GPUs, transmission cables (HDMI/DP), monitors, etc. use color space metrics to preserve and transform color.  

An image is a collection of pixels in horizontal and vertical direction, where each pixel holds only one color. The pixel, ( a word invented from the terms “picture element”,) is the basic unit of programmable color on a computer display or in a computer image. Think of it as a logical – rather than a physical – unit. The physical size of a pixel depends on how you’ve set the resolution for the display screen. 

Each pixel contains one to three components depending on color space and sub-sampling being used. For example, each RGB or YCbCr pixel has three components, whereas YCbCr 4:2:2 has two components. The specific color that a pixel describes is a blend of components of the color space. A color space is the method of mapping real colors to the color model’s discrete values. 

What are the different Color Space types? 

The CIE 1931 color spaces are the first defined quantitative links between physical pure colors (i.e. wave lengths) in the electromagnetic visible spectrum, and physiological perceived colors in human color vision.

An RGB color space is any additive color space based on the RGB color model. The most commonly used RGB color spaces are sRGB, Adobe RGB and Adobe Wide Gamut RGB.

YUV, YCbCr, YPbPr – Historically, the term YUV was used for a specific analog encoding of color information in television systems, while YCbCr was used for digital encoding of color information suited for video, still-image compression and transmission such as MPEG and JPEG. Today, the term YUV is commonly used in the computer industry to describe file-formats that are encoded using YCbCr. Y stands for the luma component (the brightness) and U/cb and V/cr are the chrominance (color) components.

HSV (hue, saturation, value), also known as HSB (hue, saturation, brightness), is often used by artists because it is perceived to be more natural to think about a color in terms of hue and saturation than in terms of additive or subtractive color components. HSV is a transformation of an RGB color space, and its components and colorimetry are relative to the RGB color space from which it was derived.

CMYK is used in the printing process, because it describes what kind of inks need to be applied so the light reflected from the substrate and through the inks produces a given color.

Color Space Use Case: 

A video captured on camera is copied to a Blue  ray player and which is in turn connected to the TV with an HDMI 1.4 cable. Let’s review the Color Space steps the video goes through in order to be seen on the TV screen…

  1. Camera captured a video in 1080p@120 Adobe RGB.
  2. Data (video) is collected from camera and provided to Blue-ray using storage disk.
  3. Blue-ray player needs to play this on a TV connected over HDMI 1.4. HDMI 1.4 supports maximum pixel frequency of 300MHz whereas video data is in 1080p@120 Adobe RGB whose pixel frequency is 495MHz.
  4. Blue-ray player can transmit this content by converting into YCbCr 4:2:0.
  5. Blue-ray player applies ITU-R BT.709 [7] RGB to YCbCr conversion.
  6. Blue-ray player down samples YCbCr 4:4:4 to 4:2:0.
  7. TV receives 1080p@120 ITU-R BT.709 [7] YCbCr 4:2:0 with pixel frequency of 247.5 MHz. TV converts it back to the original format if needed.

Stay tuned for our next blog about color space conversions.

For more information on Synopsys’ Display and other VIP, please visit www.synopsys.com/vip

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