Barco k5960012-04 User Manual
158 Barco - RHDM-2301 - User manual
Video signal processing
5.6 Selection of OETFs in the RHDM
OETF means “Opto-Electric Transfer Function”. An OETF defines the way the input signal (electric) is
transferred to luminance at the display side. The video encoding functions explained above are
examples of OETFs.
The following OETFs can be selected by the user:
transferred to luminance at the display side. The video encoding functions explained above are
examples of OETFs.
The following OETFs can be selected by the user:
• Rec.709
• SMPTE 240M
• sRGB
• xvYCC: The xvYCC function is identical to Rec.709 in the legal range of values. However, xvYCC
• SMPTE 240M
• sRGB
• xvYCC: The xvYCC function is identical to Rec.709 in the legal range of values. However, xvYCC
can encode headrooms and footrooms that give rise to out-of-gamut colors. With the wide gamut
of the display unit, these colors are displayed correctly on screen.
of the display unit, these colors are displayed correctly on screen.
• Pure Gamma
• Extended Gamma: This option is an extension of the “pure gamma” OETF to headroom and
• Extended Gamma: This option is an extension of the “pure gamma” OETF to headroom and
footroom, with the same intent as xvYCC.
In addition to these possible OETFs, the user can also select the gamma exponent
γ
(ranging from
1.0 to 3.0, with a default value of 2.35 as per EBU Tech.3320). This gamma exponent can be
understood as the resulting “effective gamma” of the display.
understood as the resulting “effective gamma” of the display.
The simplest function is the “pure gamma”, where L-L
0
~V
γ
. But because of differences between CRT
and LCD technology, using a pure gamma function in LCDs will result in crushed blacks. The users
would need to increase the background offset (black level) in order to set up the monitor according
to the Pluge pattern. But this increases the black level and decreases the available contrast range of
the display.
Therefore the RHDM uses the following OETF concept:
would need to increase the background offset (black level) in order to set up the monitor according
to the Pluge pattern. But this increases the black level and decreases the available contrast range of
the display.
Therefore the RHDM uses the following OETF concept:
• Invert the encoding equations (Rec.709, xvYCC, sRGB or SMPTE240M) exactly, preserving the
“linear toe”.
• Make sure that the “rendering intent” is preserved, which means that the resulting effective
gamma is equal to the gamma selected by the user (default 2.35 as per EBU 3320 recommenda-
tion).
tion).
• Take the limited LCD contrast into account so that the dark levels are perceptually discernable.
This implementation avoids crushing of blacks that would be exhibited by using a pure gamma
function and eliminates the need to set the video offset. The rendering intent used in the display
unit ensures that at a luminance level of 100 cd/m
function and eliminates the need to set the video offset. The rendering intent used in the display
unit ensures that at a luminance level of 100 cd/m
2
the lowest step in the Pluge pattern are just
noticeable.