DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Simultaneous contrast ratio is the
ratio of luminances of the lightest
and darkest elements of a scene
(or an image). For details, see
Contrast ratio, on page 197.
Image reproduction is not simply concerned with
physics, mathematics, chemistry, and electronics:
Perceptual considerations play an essential role.
Tone scale alteration
Tone scale alteration is necessary mainly for the two
reasons that I have described: The luminance of a
reproduction is typically dramatically lower than the
luminance of the original scene, and the surround of a
reproduced image is rarely comparable to the surround
of the original scene. Two additional reasons contribute
to the requirement for tone scale alteration: limitation
of contrast ratio, and specular highlights.
An original scene typically has a ratio of luminance
levels – a simultaneous contrast ratio – of 1000:1 or
more. However, contrast ratio in the captured image is
limited by optical flare in the camera. Contrast ratio at
the display is likely to be limited even further – by physical
factors, and by display flare – to perhaps 100:1.
Diffuse white refers to the luminance of a diffusely
reflecting white surface in a scene. Paper reflects
diffusely, and white paper reflects about 90% of incident
light, so a white card approximates diffuse white.
However, most scenes contain shiny objects that reflect
directionally. When viewed in certain directions, these
objects reflect specular highlights having luminances
perhaps ten times that of diffuse white. At the reproduction
device, we can seldom afford to reproduce
diffuse white at merely 10% of the maximum luminance
of the display, solely to exactly reproduce the
luminance levels of the highlights! Nor is there any
need to reproduce highlights exactly: A convincing
image can be formed with highlight luminance greatly
reduced from its true value. To make effective use of
luminance ranges that are typically available in image
display systems, highlights must be compressed.
Incorporation of rendering intent
The correction that I have mentioned is achieved by
subjecting luminance – or, in the case of a color system,
tristimulus values – to an end-to-end power function
having an exponent between about 1.1 and 1.6. The
CHAPTER 9 RENDERING INTENT 83