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diff --git a/doc/expert/pre_colo b/doc/expert/pre_colo new file mode 100644 index 0000000..21ae85b --- /dev/null +++ b/doc/expert/pre_colo @@ -0,0 +1,65 @@ +@Section + @Title { "@SetColour" and "@SetColor" } + @Tag { colour } +@Begin +@PP +The @@SetColour and @@SetColor symbols, +setcolour.sym @Index { @@SetColour symbol } +setcolor.sym @Index { @@SetColor symbol } +which have identical effect, return their right parameter in +the colour specified by their left parameter. The form of +the left parameter is implementation-dependent; in Basser Lout +it must be an object whose value is a sequence of words comprising +a PostScript command for setting colour. For example, +@ID @Code "{ 1.0 0.0 0.0 setrgbcolor } @SetColour { hello, world }" +produces the red result +@ID { + @BackEnd @Case { + PostScript @Yield { + { 1.0 0.0 0.0 setrgbcolor } @SetColour { hello, world } + } + PDF @Yield { + { 1.0 0.0 0.0 rg 1.0 0.0 0.0 RG } @SetColour { hello, world } + } + } +} +Of course, a colour output device is needed to see the effect; on +a monochrome device the result will be some shade of grey. The +@@SetColour command will not accept an empty left parameter +(it refuses to believe that this could cause a change of colour), +but otherwise it trusts the user to supply something sensible. +@PP +The @@SetColour command accepts the special value {@Code "nochange"} +for the left parameter. This value causes the right parameter to have +the colour it would have had without the @@SetColour command. +@PP +There is no default colour, so the user must ensure that the root galley +or each of its components is enclosed in a @@SetColour symbol whose +left parameter is not {@Code "nochange"}. +@PP +Lout makes no attempt to understand colour, it simply prints +the PostScript commands when appropriate. This has the advantage +of permitting access to any of PostScript's colour models (some +require initialization which can be supplied using @@PrependGraphic), +but the disadvantage of offering no way to make relative changes +(`as before only redder,' and so on). +@PP +For those who wish to obtain colour without working very hard, +the @Code setrgbcolor command used above is available in every +version of PostScript, requires no initialization, and is simple +to use. The three numbers, which range from 0.0 to 1.0, determine +the intensity of red, green, and blue respectively. Some useful +values for the left parameter are +@ID @Tab + @Fmta { @Col @Code A ! @Col @I B } +{ +@Rowa A { 1.0 0.0 0.0 setrgbcolor } B { red } +@Rowa A { 0.0 1.0 0.0 setrgbcolor } B { green } +@Rowa A { 0.0 0.0 1.0 setrgbcolor } B { blue } +@Rowa A { 1.0 1.0 1.0 setrgbcolor } B { white } +@Rowa A { 0.5 0.5 0.5 setrgbcolor } B { grey } +@Rowa A { 0.0 0.0 0.0 setrgbcolor } B { black } +} +Colouring an object white is useful for producing an empty space +whose size is that of some object. +@End @Section |