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author | Bryan Newbold <bnewbold@robocracy.org> | 2017-02-20 00:05:29 -0800 |
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committer | Bryan Newbold <bnewbold@robocracy.org> | 2017-02-20 00:05:29 -0800 |
commit | 8466d8cfa486fb30d1755c4261b781135083787b (patch) | |
tree | c8c12c67246f543c3cc4f64d1c07e003cb1d45ae /color.txi | |
parent | 87b82b5822ca54228cfa6df29be3ad9d4bc47d16 (diff) | |
download | slib-8466d8cfa486fb30d1755c4261b781135083787b.tar.gz slib-8466d8cfa486fb30d1755c4261b781135083787b.zip |
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diff --git a/color.txi b/color.txi new file mode 100644 index 0000000..ccbb3de --- /dev/null +++ b/color.txi @@ -0,0 +1,345 @@ +@subsubheading Measurement-based Color Spaces + +@noindent +@cindex tristimulus +The @dfn{tristimulus} color spaces are those whose component values +@cindex tristimulus +are proportional measurements of light intensity. The CIEXYZ(1931) +system provides 3 sets of spectra to convolve with a spectrum of +interest. The result of those convolutions is coordinates in CIEXYZ +space. All tristimuls color spaces are related to CIEXYZ by linear +transforms, namely matrix multiplication. Of the color spaces listed +here, CIEXYZ and RGB709 are tristimulus spaces. + +@deftp {Color Space} CIEXYZ +The CIEXYZ color space covers the full @dfn{gamut}. +@cindex gamut +It is the basis for color-space conversions. + +CIEXYZ is a list of three inexact numbers between 0 and 1.1. +'(0. 0. 0.) is black; '(1. 1. 1.) is white. +@end deftp + + +@defun ciexyz->color xyz + +@var{xyz} must be a list of 3 numbers. If @var{xyz} is valid CIEXYZ coordinates, +then @code{ciexyz->color} returns the color specified by @var{xyz}; otherwise returns #f. +@end defun + +@defun color:ciexyz x y z + +Returns the CIEXYZ color composed of @var{x}, @var{y}, @var{z}. If the +coordinates do not encode a valid CIEXYZ color, then an error is +signaled. +@end defun + +@defun color->ciexyz color +Returns the list of 3 numbers encoding @var{color} in CIEXYZ. +@end defun +@deftp {Color Space} RGB709 +BT.709-4 (03/00) @cite{Parameter values for the HDTV standards for +production and international programme exchange} specifies parameter +values for chromaticity, sampling, signal format, frame rates, etc., of +high definition television signals. + +An RGB709 color is represented by a list of three inexact numbers +between 0 and 1. '(0. 0. 0.) is black '(1. 1. 1.) is white. +@end deftp + + +@defun rgb709->color rgb + +@var{rgb} must be a list of 3 numbers. If @var{rgb} is valid RGB709 coordinates, +then @code{rgb709->color} returns the color specified by @var{rgb}; otherwise returns #f. +@end defun + +@defun color:rgb709 r g b + +Returns the RGB709 color composed of @var{r}, @var{g}, @var{b}. If the +coordinates do not encode a valid RGB709 color, then an error is +signaled. +@end defun + +@defun color->rgb709 color +Returns the list of 3 numbers encoding @var{color} in RGB709. +@end defun +@subsubheading Perceptual Uniformity + +@noindent +Although properly encoding the chromaticity, tristimulus spaces do not +match the logarithmic response of human visual systems to intensity. +Minimum detectable differences between colors correspond to a smaller +range of distances (6:1) in the L*a*b* and L*u*v* spaces than in +tristimulus spaces (80:1). For this reason, color distances are +computed in L*a*b* (or L*C*h). + +@deftp {Color Space} L*a*b* +Is a CIE color space which better matches the human visual system's +perception of color. It is a list of three numbers: + +@itemize @bullet +@item +0 <= L* <= 100 (CIE @dfn{Lightness}) +@cindex Lightness + +@item +-500 <= a* <= 500 +@item +-200 <= b* <= 200 +@end itemize +@end deftp + + +@defun l*a*b*->color L*a*b* white-point + +@var{L*a*b*} must be a list of 3 numbers. If @var{L*a*b*} is valid L*a*b* coordinates, +then @code{l*a*b*->color} returns the color specified by @var{L*a*b*}; otherwise returns #f. +@end defun + +@defun color:l*a*b* L* a* b* white-point + +Returns the L*a*b* color composed of @var{L*}, @var{a*}, @var{b*} with @var{white-point}. + +@defunx color:l*a*b* L* a* b* +Returns the L*a*b* color composed of @var{L*}, @var{a*}, @var{b*}. If the coordinates +do not encode a valid L*a*b* color, then an error is signaled. +@end defun + +@defun color->l*a*b* color white-point + +Returns the list of 3 numbers encoding @var{color} in L*a*b* with @var{white-point}. + +@defunx color->l*a*b* color +Returns the list of 3 numbers encoding @var{color} in L*a*b*. +@end defun +@deftp {Color Space} L*u*v* +Is another CIE encoding designed to better match the human visual +system's perception of color. +@end deftp + + +@defun l*u*v*->color L*u*v* white-point + +@var{L*u*v*} must be a list of 3 numbers. If @var{L*u*v*} is valid L*u*v* coordinates, +then @code{l*u*v*->color} returns the color specified by @var{L*u*v*}; otherwise returns #f. +@end defun + +@defun color:l*u*v* L* u* v* white-point + +Returns the L*u*v* color composed of @var{L*}, @var{u*}, @var{v*} with @var{white-point}. + +@defunx color:l*u*v* L* u* v* +Returns the L*u*v* color composed of @var{L*}, @var{u*}, @var{v*}. If the coordinates +do not encode a valid L*u*v* color, then an error is signaled. +@end defun + +@defun color->l*u*v* color white-point + +Returns the list of 3 numbers encoding @var{color} in L*u*v* with @var{white-point}. + +@defunx color->l*u*v* color +Returns the list of 3 numbers encoding @var{color} in L*u*v*. +@end defun +@subsubheading Cylindrical Coordinates + +@noindent +HSL (Hue Saturation Lightness), HSV (Hue Saturation Value), HSI (Hue +Saturation Intensity) and HCI (Hue Chroma Intensity) are cylindrical +color spaces (with angle hue). But these spaces are all defined in +terms device-dependent RGB spaces. + +@noindent +One might wonder if there is some fundamental reason why intuitive +specification of color must be device-dependent. But take heart! A +cylindrical system can be based on L*a*b* and is used for predicting how +close colors seem to observers. + +@deftp {Color Space} L*C*h +Expresses the *a and b* of L*a*b* in polar coordinates. It is a list of +three numbers: + +@itemize @bullet +@item +0 <= L* <= 100 (CIE @dfn{Lightness}) +@cindex Lightness + +@item +C* (CIE @dfn{Chroma}) is the distance from the neutral (gray) axis. +@cindex Chroma +@item +0 <= h <= 360 (CIE @dfn{Hue}) is the angle. +@cindex Hue +@end itemize + +The colors by quadrant of h are: + +@multitable @columnfractions .20 .60 .20 +@item 0 @tab red, orange, yellow @tab 90 +@item 90 @tab yellow, yellow-green, green @tab 180 +@item 180 @tab green, cyan (blue-green), blue @tab 270 +@item 270 @tab blue, purple, magenta @tab 360 +@end multitable + +@end deftp + + +@defun l*c*h->color L*C*h white-point + +@var{L*C*h} must be a list of 3 numbers. If @var{L*C*h} is valid L*C*h coordinates, +then @code{l*c*h->color} returns the color specified by @var{L*C*h}; otherwise returns #f. +@end defun + +@defun color:l*c*h L* C* h white-point + +Returns the L*C*h color composed of @var{L*}, @var{C*}, @var{h} with @var{white-point}. + +@defunx color:l*c*h L* C* h +Returns the L*C*h color composed of @var{L*}, @var{C*}, @var{h}. If the coordinates +do not encode a valid L*C*h color, then an error is signaled. +@end defun + +@defun color->l*c*h color white-point + +Returns the list of 3 numbers encoding @var{color} in L*C*h with @var{white-point}. + +@defunx color->l*c*h color +Returns the list of 3 numbers encoding @var{color} in L*C*h. +@end defun +@subsubheading Digital Color Spaces + +@noindent +The color spaces discussed so far are impractical for image data because +of numerical precision and computational requirements. In 1998 the IEC +adopted @cite{A Standard Default Color Space for the Internet - sRGB} +(@url{http://www.w3.org/Graphics/Color/sRGB}). sRGB was cleverly +designed to employ the 24-bit (256x256x256) color encoding already in +widespread use; and the 2.2 gamma intrinsic to CRT monitors. + +@noindent +Conversion from CIEXYZ to digital (sRGB) color spaces is accomplished by +conversion first to a RGB709 tristimulus space with D65 white-point; +then each coordinate is individually subjected to the same non-linear +mapping. Inverse operations in the reverse order create the inverse +transform. + +@deftp {Color Space} sRGB +Is "A Standard Default Color Space for the Internet". Most display +monitors will work fairly well with sRGB directly. Systems using ICC +profiles +@ftindex ICC Profile +@footnote{ +@noindent +A comprehensive encoding of transforms between CIEXYZ and device color +spaces is the International Color Consortium profile format, +ICC.1:1998-09: + +@quotation +The intent of this format is to provide a cross-platform device profile +format. Such device profiles can be used to translate color data +created on one device into another device's native color space. +@end quotation +} +should work very well with sRGB. + +@end deftp + + +@defun srgb->color rgb + +@var{rgb} must be a list of 3 numbers. If @var{rgb} is valid sRGB coordinates, +then @code{srgb->color} returns the color specified by @var{rgb}; otherwise returns #f. +@end defun + +@defun color:srgb r g b + +Returns the sRGB color composed of @var{r}, @var{g}, @var{b}. If the +coordinates do not encode a valid sRGB color, then an error is +signaled. +@end defun +@deftp {Color Space} xRGB +Represents the equivalent sRGB color with a single 24-bit integer. The +most significant 8 bits encode red, the middle 8 bits blue, and the +least significant 8 bits green. +@end deftp + + +@defun color->srgb color + +Returns the list of 3 integers encoding @var{color} in sRGB. +@end defun + +@defun color->xrgb color +Returns the 24-bit integer encoding @var{color} in sRGB. +@end defun + +@defun xrgb->color k + +Returns the sRGB color composed of the 24-bit integer @var{k}. +@end defun +@deftp {Color Space} e-sRGB +Is "Photography - Electronic still picture imaging - Extended sRGB color +encoding" (PIMA 7667:2001). It extends the gamut of sRGB; and its +higher precision numbers provide a larger dynamic range. + +A triplet of integers represent e-sRGB colors. Three precisions are +supported: + +@table @r +@item e-sRGB10 +0 to 1023 +@item e-sRGB12 +0 to 4095 +@item e-sRGB16 +0 to 65535 +@end table +@end deftp + + +@defun e-srgb->color precision rgb +@var{precision} must be the integer 10, 12, or 16. @var{rgb} must be a list of 3 +numbers. If @var{rgb} is valid e-sRGB coordinates, then @code{e-srgb->color} returns the color +specified by @var{rgb}; otherwise returns #f. +@end defun + +@defun color:e-srgb 10 r g b + +Returns the e-sRGB10 color composed of integers @var{r}, @var{g}, @var{b}. + +@defunx color:e-srgb 12 r g b +Returns the e-sRGB12 color composed of integers @var{r}, @var{g}, @var{b}. + +@defunx color:e-srgb 16 r g b +Returns the e-sRGB16 color composed of integers @var{r}, @var{g}, @var{b}. +If the coordinates do not encode a valid e-sRGB color, then an error +is signaled. +@end defun + +@defun color->e-srgb precision color +@var{precision} must be the integer 10, 12, or 16. @code{color->e-srgb} returns the list of 3 +integers encoding @var{color} in sRGB10, sRGB12, or sRGB16. +@end defun + + + + + + + + + + + + + + + + + + + + + + + + |