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A ''Lab'' color space is a color-opponent space with dimension ''L'' for lightness and ''a'' and ''b'' for the color-opponent dimensions, based on nonlinearly compressed (e.g. CIE XYZ color space) coordinates. The terminology originates from the three dimensions of the Hunter 1948 color space, which are ''L'', ''a'', and ''b''.〔 (Proceedings of the Winter Meeting of the Optical Society of America)〕〔 (Proceedings of the Thirty-Third Annual Meeting of the Optical Society of America)〕 However, ''Lab'' is now more often used as an informal abbreviation for the L-a-b representation of the CIE 1976 color space (or CIELAB, described below). The difference between the original Hunter and CIE color coordinates is that the CIE coordinates are based on a cube root transformation of the color data, while the Hunter coordinates are based on a square root transformation. Other examples of color spaces with Lab representations include the CIE 1994 color space and the CIE 2000 color space. The L *a *b * color space includes all perceivable colors, which means that its gamut exceeds those of the RGB and CMYK color models (for example, ProPhoto RGB includes about 90% all perceivable colors). One of the most important attributes of the L *a *b *-model is device independence. This means that the colors are defined independent of their nature of creation or the device they are displayed on. The L *a *b * color space is used when graphics for print have to be converted from RGB to CMYK, as the L *a *b * gamut includes both the RGB and CMYK gamut. Also it is used as an interchange format between different devices as for its device independency. The space itself is a three-dimensional Real number space, that contains an infinite possible representations of colors. However, in practice, the space is usually mapped onto a three-dimensional integer space for device-independent digital representation, and for these reasons, the ''L *'', ''a *'', and ''b *'' values are usually absolute, with a pre-defined range. The lightness, ''L *'', represents the darkest black at ''L *'' = 0, and the brightest white at ''L *'' = 100. The color channels, ''a *'' and ''b *'', will represent true neutral gray values at ''a *'' = 0 and ''b *'' = 0. The red/green opponent colors are represented along the ''a *'' axis, with green at negative ''a *'' values and red at positive ''a *'' values. The yellow/blue opponent colors are represented along the ''b *'' axis, with blue at negative ''b *'' values and yellow at positive ''b *'' values. The scaling and limits of the ''a *'' and ''b *'' axes will depend on the specific implementation of ''Lab'' color, as described below, but they often run in the range of ±100 or −128 to +127. Both the Hunter and the 1976 CIELAB color spaces were derived from the prior "master" space CIE 1931 XYZ color space, which can predict which spectral power distributions will be perceived as the same color (see metamerism), but which is not particularly perceptually uniform.〔(A discussion and proposed improvement ), Bruce Lindbloom〕 Strongly influenced by the Munsell color system, the intention of both "Lab" color spaces is to create a space that can be computed via simple formulas from the ''XYZ'' space but is more perceptually uniform than ''XYZ''.〔(Explanation of this history ), Bruce MacEvoy〕 ''Perceptually uniform'' means that a change of the same amount in a color value should produce a change of about the same visual importance. When storing colors in limited precision values, this can improve the reproduction of tones. Both Lab spaces are relative to the white point of the ''XYZ'' data they were converted from. Lab values do not define absolute colors unless the white point is also specified. Often, in practice, the white point is assumed to follow a standard and is not explicitly stated (e.g., for "absolute colorimetric" rendering intent, the International Color Consortium ''L *a *b *'' values are relative to CIE standard illuminant D50, while they are relative to the unprinted substrate for other rendering intents).〔 The lightness correlate in CIELAB is calculated using the cube root of the relative luminance. ==Advantages== Unlike the RGB and CMYK color models, ''Lab'' color is designed to approximate human vision. It aspires to perceptual uniformity, and its ''L'' component closely matches human perception of lightness, although it does not take the Helmholtz–Kohlrausch effect into account. Thus, it can be used to make accurate color balance corrections by modifying output curves in the ''a'' and ''b'' components, or to adjust the lightness contrast using the ''L'' component. In RGB or CMYK spaces, which model the output of physical devices rather than human visual perception, these transformations can be done only with the help of appropriate blend modes in the editing application. Because ''Lab'' space is much larger than the gamut of computer displays, printers, or even human vision, a bitmap image represented as Lab requires more data per pixel to obtain the same precision as an RGB or CMYK bitmap. In the 1990s, when computer hardware and software were limited to storing and manipulating mostly 8-bit/channel bitmaps, converting an RGB image to Lab and back was a very lossy operation. With 16-bit/channel and floating-point support now common, the loss due to quantization is negligible. In addition, many of the "colors" within Lab space fall outside the gamut of human vision, and are therefore purely imaginary; these "colors" cannot be reproduced in the physical world. Though color management software, such as that built into image editing applications, will pick the closest in-gamut approximation, changing lightness, chroma, and sometimes hue in the process, author Dan Margulis claims that this access to imaginary colors is useful, going between several steps in the manipulation of a picture. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「lab color space」の詳細全文を読む スポンサード リンク
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