DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

The importance of rendering intent,
and the consequent requirement
for different exponents for
encoding (γ D ) and decoding (γ D ),
has been poorly recognized and
poorly documented in the development
of video.
Eq 23.1
γ ≈ 05 . ; γ ≈ 25 . ;
E D
γ ⋅γ ≈125
.
E D
As I explained in Rendering intent, on page 81, it is
important for perceptual reasons to alter the tone scale
of an image reproduced at a luminance substantially
lower than that of the original scene, reproduced with
limited contrast ratio, or viewed in a dim surround. The
dim surround condition is characteristic of television
viewing. In video, the alteration is accomplished at the
camera by slightly undercompensating the actual power
function of the CRT, to obtain an end-to-end power
function whose exponent is about 1.25, as indicated in
Equation 23.1 in the margin. This achieves end-to-end
reproduction that is subjectively correct (though not
mathematically linear).
Optoelectronic transfer functions (OETFs)
Unfortunately, several different transfer functions have
been standardized and are in use. In the sections to
follow, I will detail these transfer function standards:
• Rec. 709 is an international standard that specifies the
basic parameters of HDTV. Although the Rec. 709
transfer function is intended for HDTV, it is representative
of current SDTV technology, and it is being retrofitted
into SDTV studio standards.
• SMPTE 240M was the first studio HDTV standard; its
transfer function remains in use in some HDTV equipment
deployed today. Recent revisions of SMPTE standards
call for the Rec. 709 transfer function, but
previous revisions allowed an “interim implementation”
using the transfer function of SMPTE 240M-1988.
• Rec. 1361 extends the Rec. 709 coding to accommodate
a wide color gamut; it is not yet deployed.
• sRGB refers to a transfer function used in PCs.
• The transfer function of the original 1953 NTSC specification,
often written 1 ⁄2.2, has been effectively superseded
by Rec. 709.
• The transfer function of European specifications for
576i, often written 1 ⁄2.8, has been effectively superseded
by Rec. 709.
262 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES