Barco RHDM-1701, Руководство пользователя

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154 Barco - RHDM-1701 - User manual
Video signal processing
• Undo step 2: The video signal needs to be converted to the linear light domain by using an
Opto-Electrical Transfer Function (OETF) . More details about this step will be given in the
following sections. The OETF can be defined by the user:
gamma exponent: from 1.0 to 3.0 in steps 0.05.
gamma shape: Rec.709, xvYCC, SMPTE 240M, Pure gamma, Extended gamma, sRGB.
input footroom and headroom levels.
• Undo step 1: The linear RGB signal is presented on the screen and must be shown in a given
color space (primaries and white point). Because the display typically has a wider gamut than
the intended gamut, a display color space conversion is needed. The color space can also be
defined by the user.
white point selection: D55, D65, D75, D93, custom in x,y.
color gamut selection: EBU 3213, Rec.709, SMPTE RP 145, xvYCC, Native, and Custom - R,
G and B primaries in x,y (Option for P-version).
NOTE
:
The fundamentals of the information presented in this chapter can be found in the following references:
• Charles Poynton, Digital Video and HDTV, Algorithms and Interfaces, Morgan Kaufmann Publishers, 2007
• EBU-Tech 3320, User requirements for Video Monitors in Television Production, Version 1.1, Geneva, May 2008
• Alan Roberts: Methods of measuring and calculating display transfer characteristics (gamma), BBC RD 1991/6.
5.2 General definition OETF
In a video camera, linear light (L) can be encoded into video levels (V) in different ways. In most
cases, light is encoded using a power function (something like V’~L
a
). The exponent “a” (0<a<1) is
chosen to guarantee a conversion to perceptual uniform space. The signal is “gamma encoded”, and
this is made clear by writing V’ instead of V. In reality, a pure power function, as presented in the
form above, is not used because analog electronics can not cope with an infinite slope at L=0. A
more detailed encoding function is used which includes a linear “tail” at low luminance levels,
followed by a scaled and offset power function for higher luminance levels.
There are several L-V’ encoding standards, such as ITU-R BT.709 (Rec.709), SMPTE 240M, sRGB,
xvYCC (based on ITU 1361). They all have the form of the transfer function described below.
To characterize the encoding function, the following parameters are used:
• b
r
(the voltage up to which the transfer curve is linear)
• b
s
(the slope of the linear part: b
s
=V/L)
• g
s
(the gamma correction or exponent of the power function)