TIFF 6.0 Specification
Final—June 3, 1992
Compressibility (Data).
Opponent spaces such as CIELAB are inherently more compressible than
tristimulus spaces such as RGB. The chroma content of an image can be com-
pressed to a greater extent, without objectionable loss, than can the lightness con-
tent. The opponent arrangement of CIELAB allows for spatial subsampling and
efficent compression using JPEG.
Compressibility (Gamut).
Adjusting the color range of an image to match the capabilities of the intended
output device is a critical function within computational color reproduction. Lu-
minance/chrominance separation, especially when provided in a polar form, is
desirable for facilitating gamut compression. Accurate gamut compression in a
tri-linear color space is difficult.
CIELAB has a polar form (metric
hue angle,
and
metric chroma,
described be-
low) that serves compression needs fairly well. Because CIELAB is not perfectly
uniform, problems can arise when compressing along constant hue lines. Notice-
able hue errors are sometimes introduced. This problem is no less severe with
other contending color spaces.
This polar form also provides advantages for local color editing of images. The
polar form is not proposed as part of the TIFF addition.
Getting the Most from CIELAB
Image Editors
The advantages of image editing within a perceptually uniform polar color space
are tremendous. A detailed description of these advantages is beyond the scope of
this section. As previously mentioned, many common tonal manipulation tasks
are most efficiently performed when only a single channel is affected. Edge en-
hancement, contrast adjustment, and general tone-curve manipulation all ideally
affect only the lightness component of an image.
A perceptual polar space works excellently for specifying a color range for mask-
ing purposes. For example, a red shirt can be quickly changed to a green shirt
without drawing an outline mask. The operation can be performed with a loosely,
quickly-drawn mask region combined with a hue (and perhaps chroma) range that
encompasses the shirt’s colors. The hue component of the shirt can then be ad-
justed, leaving the lightness and chroma detail in place.
Color cast adjustment is easily realized by shifting either or both of the chroma
channels over the entire image or blending them over the region of interest.
Converting from CIELAB to a device specific space
For fast conversion to an RGB display, CIELAB can be decoded using 3x3
matrixing followed by gamma correction. The computational complexity required
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