Colour images RGB to YCbCr Example Example, RGB

Colour images
RGB to YCbCr
When coding images we usually don’t use the RGB colour space. Instead
the image is described in another colour space, where the pixel values are
given using a luminance (or luma) component (called Y), that tells us how
bright the pixel is (ie basically a grayscale signal) and two chrominance (or
chroma) components (called Cb and Cr) that tells the actual colour of the
pixel.
The chrominance components can often be downsampled to a lower
resolution, without a human observer noticing any reduction in image
quality.
There are many variants of luminance-chrominance colour spaces, but
they are rather similar to each other.
Suppose that E R , E G and E B are analog values between 0 and 1 that
describe how much red, green and blue there is in a pixel (given eight bit
quantization we have E R = R/255, E G = G/255 and E B = B/255). A
typical conversion (ITU-R Recommendation 624-4 System B,G) to
luminance-chrominance is then given by
⎧
⎪⎨
⎪⎩
E Y = 0.299 · E R +0.587 · E G +0.114 · E B
E Cb = −0.169 · E R − 0.331 · E G +0.500 · E B
E Cr = 0.500 · E R − 0.419 · E G − 0.081 · E B
where E Y is between 0 and 1 and E Cb and E Cr are between -0.5 and 0.5.
Conversion to 8-bit values are then done by
⎧
⎪⎨
⎪⎩
Y = 219 · E Y +16
Cb = 224 · E Cb + 128
Cr = 224 · E Cr + 128
Example
TSBK06 colour images – p. 1/7
Example, RGB
TSBK06 colour images – p. 2/7
Colour components: R, G and B
As grayscale images
TSBK06 colour images – p. 3/7
TSBK06 colour images – p. 4/7

Example, YCbCr
Example
Colour components: Y, Cb and Cr
As grayscale images
Image where Cb and Cr have been downsampled a factor 2 both
horizontally and vertically, ie half of the image information has been
removed.
Example
TSBK06 colour images – p. 5/7
TSBK06 colour images – p. 6/7
Image where R, G and B have been downsampled a factor √ 2 both
horizontally and vertically, ie half of the image information has been
removed.
TSBK06 colour images – p. 7/7