If a pictorial color image is stored as red, green and blue (RGB) channels, it must use lossless compression to avoid smearing of primary colors.
If the three channels of a pictorial color image use lossy compression, storage as RGB channels is not suitable, because lossy compression spatially affects different channels differently, causing primary colors to smear relative to each other.
To use lossy compression, the three channels must be decorrelated. For effective high definition color decorrelation, the XRH file format uses
The following formulas (Figure 10.1) convert RGB colors to YCbCr:
αr = 0.2126 αg = 0.7152 αb = 0.0722
Y = αr · R + αg · G + αb · B
Cb = 0.5 (B − Y) ⁄ (1 − αb)
Cr = 0.5 (R − Y) ⁄ (1 − αr)
Y is the image intensity (luminance). It is the weighted average of the RGB channels. The weights (αr, αg and αb) are derived from the following chromaticities (Figure 10.2):
xr = 0.6400 yr = 0.3300
xg = 0.3000 yg = 0.6000
xb = 0.1500 yb = 0.0600
xw = 0.3127 yw = 0.3290
Those are the default chromaticities that are automatically used if zero is specified for the chromaciticities in the XRH file header.
To use weights other than the weights shown in
Color transformation using
To use a color transformation other than