YUV, Y CB CR and Subsampling

Yuv, YCbCr and Subsampling

YUV

• The Y'UV model defines a color space in terms of one luma (Y') and two chrominance (UV) components.

• • The Y'UV color model is used in the PAL and SECAM

composite color video standards. • • Previous black-and-white systems used only luma (Y')

information. •

YUV

• Color information (U and V) was added separately via a sub-carrier so that a black-and-white receiver would still be able to receive and display a color picture transmission in the receiver's native black-and-white format.

The Digital Video Signal or YUV

• YUV has basically three components: • the luminance or green channel (Y), • the colour value of the luminance deducted from the

colour red (R-Y), • the colour value of the luminance deducted from the

colour blue (B-Y). • When digitised, these three parameters of the component

video signal are assigned a numeric value.• The groupings and Interpolation order of these values

from the sensor determine the resolution and colour density of the camera’s output.

Y Cb Cr

• YCbCr, Y CbCr′ , or Y Pb/Cb Pr/Cr, also written as YCBCR or Y C′ BCR,

• Y is the ′ luma component and CB and CR are the blue-difference and red-difference chroma components. Y (with prime) is distinguished ′from Y which is luminance, meaning that light intensity is non-linearly encoded using gamma correction.

LUMA IN YCbCr

• Luma is carried in the Y ’ channel. • Since the eye is more sensitive to changes in

luma than to changes in colour, the two colour difference channels are usually encoded with less information than the Luma channel, and various image formats are bandwidth limited, employing chroma subsampling to encode more or less colour information ( example 4:2:2, 4:2:0, 4:1:1).

• However, all video formats, regardless of the chroma sampling scheme they employ, encode the full amount of luma.

Example

LUMA IN RGB

• The RGB colour model is followed by Kodak Cineon and DPX sequence formats, which are used from both film scanning and digital cinema camera acquisition. RGB is also used by Sony HDCAM SR video format, as well as by an increasing number of raw colour space formats used by digital cinema camera such as the RED ONE and ARRI Alexa.

• RGB image formats encode discrete colour component channels for red, green, and blue image data. These formats typically encode the full sampled amount of each colour, with no chroma subsampling, although various acquisitions or distribution formats may employ spatial or temporal data compression.

Subsampling

• Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for Luma information. Taking advantage of the human visual system’s lower acuity for colour differences than for Luminance. It is used in many video encoding schemes – both analogue and digital and also in JPEG encoding.

Subsampling Ratio’s

• 4:2:0• 4:1:1• 4:2:2 (2/4)• 4:4:4 (4)

Subsampling Ratio Example 4:2:2:4

• 4:2:2:4• 4: Luma horizontal sampling

reference• 2: Cb and Cr Horizontal factor• 2: Same as second digit; or zero,

indicating Cb and Cr are sub-sampled 2:1 vertically

• 4: If present, same as luma digit; indicates alpha (key) component

XDCAM Subsampling Ratios 4:2:0

• 8 -bit Linear

Sampling Ratio 4:2:0

No colour in the second channel, this means green screen work will be harder to achieve.

Sampling Ratio 4:1:1

• This interpolated HD sampling scan rate is commonly referred to as 4:1:1

• This means there are four luminance or green pixels to every one each red and blue pixel, which hold the colour information for a shot.

•

4:1:1 Sampling Ratio

Sampling Ratio 4:2:2: (2/4)

• This interpolated HD sampling scan rate is commonly referred to as

• This means there are four luminance or green pixels to every two each red and blue pixel, which hold the colour information for a shot.

• • FOR TRUE HD GREEN SCREEN PRODUCTION,

YOU NEED A 4:2:2 CAMERA.

Bayer Pattern 4:2:2 Sampling Ratio

2:1 Green to Red, Blue

Sampling Ratio 4:4:4

• This interpolated HD sampling scan rate is commonly referred to as

• This means there are four luminance or green pixels to every four each red and blue pixel, which hold the colour information for a shot.

• • Perfect for all post production.

4:4:4 Sampling Ratio

Equal Values R,G,B

XDCAM

• 8 -bit Linear

Sampling Ratio 4:2:0

No colour in the second channel, this means green screen work will be harder to achieve.