DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

IEC FDIS 61966-2-1, Multimedia
systems and equipment – Colour
measurement and management –
Part 2-1: Colour management –
Default RGB colour space – sRGB.
Stokes, Michael, and Matthew
Anderson, Srinivasan Chandrasekar,
and Ricardo Motta, A Standard
Default Color Space for the Internet –
sRGB. Internet: www.color.org.
See Rendering intent, on page 81.
1
γ ≈ 0.
45 ≈
E 222 .
γ ≈2.
5
D
0. 45 · 2. 5 ≈1.
125
would occupy the range [-40 … 184.3]. If subsequently
offset +48 at an 8-bit interface, they would lie
in the range [8 … 232.3]. However, Rec. 1361 is
intended for use with 10-bit components, at minimum.
As in Rec. 709, the reference black and white levels are
multiplied by 2 k-8, where k is the number of bits at the
interface. When scaled and offset for 10-bit 4:4:4 interface,
Rec. 1361 R’G’B’ ranges 192 through 832
(compared to a range of 64 through 940 for 10-bit
Rec. 709).
sRGB transfer function
The notation sRGB refers to a specification for color
image coding for personal computers, and for image
exchange on the Internet. The FlashPix file format for
digital still cameras incorporates sRGB coding (there
called NIFRGB). The sRGB specification calls for
a transfer function very similar to – but regrettably not
identical to – Rec. 709. The encoding is this:
⎧12.
92L; 0 ≤ L≤
0. 0031308
⎪
V'sRGB
= ⎨ 1
⎩
⎪ ( 24 . ) 1. 055L − 0. 055; 0. 0031308 < L≤1
Eq 23.8
Although the equation contains the exponent 1 ⁄2.4, the
the scale factor and the offset cause the overall function
to approximate a pure 0.45-power function
(γ E ≈0.45). It is misleading to describe sRGB as having
“gamma of 0.42.”
sRGB encoding assumes that conversion of the encoded
R’G’B’ signals will be accomplished at a CRT with
a nominal 2.5-power function, as in Rec. 709 and
SMPTE 240M coding. However, the sRGB specification
anticipates a higher ambient light level for viewing than
Rec. 709 and SMPTE 240M: sRGB’s effective
0.45-power function, displayed on a monitor with
a 2.5-power, results in an end-to-end power of 1.125.
This is considerably lower than the 1.25 value produced
by Rec. 709 encoding.
It is standard to code sRGB components in 8-bit form
from 0 to 255, with no footroom and no headroom.
CHAPTER 23 GAMMA 267