ConTeXt: Randomly lighten or darken a colour within a threshold

Question

Overview

Looking to randomize the colours of squares of a checkerboard so that some are slightly lighter and some are slightly darker, relative to another colour.

Problem

The following sets a base colour and a lighter version:

\definecolor[BaseColour][h=66CEF1]
\definespotcolor[BaseColourLighter][BaseColour][a=1,t=.5]

The following code in MetaPost fails:

lightColour := \MPcolor{BaseColourLighter};

Code

Here is the code:

\setupcolors[state=start]

\definecolor[BaseColour][h=66CEF1]
\definespotcolor[BaseColourLighter][BaseColour][a=1,t=.5]

\startuseMPgraphic{ThemeBase}
  fill Page withcolor blue;
\stopuseMPgraphic

\startuseMPgraphic{page:ThemeElement}
  tiles_x := 8;
  tiles_y := 13;
  dim_x := OverlayWidth / tiles_x;
  dim_y := dim_x;
  color baseColour;
  color lightColour;
  baseColour := \MPcolor{BaseColour};
  lightColour := \MPcolor{BaseColourLighter};

  path s;
  s := (0, 0) -- (dim_x, 0) -- (dim_x, dim_y) -- (0, dim_y) -- cycle;

  for x = 0 upto tiles_x:
    for y = 0 upto tiles_y:
      path p;
      p := s shifted( x * dim_x, y * dim_y );

      if (x mod 2) = (y mod 2):
        fill p withcolor baseColour;
        draw p withcolor baseColour; 
      else:
        def lightColour = transparent( 1, .55-.25uniformdeviate(1), baseColour ) enddef;

        fill p withcolor lightColour;
        draw p withcolor lightColour;
      fi
    endfor;
  endfor;

  setbounds currentpicture to
    boundingbox currentpicture shifted( -dim_x/2, dim_y*(tiles_x/tiles_y)/2 );
\stopuseMPgraphic

\defineoverlay[ThemeBase][\uniqueMPgraphic{ThemeBase}]
\defineoverlay[page:ThemeElement][\uniqueMPgraphic{page:ThemeElement}]

\starttext
  \setupbackgrounds[page][background={ThemeBase,page:ThemeElement}]
  \startchapter[title=One]
  \input knuth
  \stopchapter
\stoptext

Error

The error message is:

Equation cannot be performed (color=numeric).
<to be read again> 
                   withprescript

Transparency

The following uses transparency:

fill s shifted( x * dim_x, y * dim_y ) withcolor transparent(1,0.5,baseColour);
draw s shifted( x * dim_x, y * dim_y ) withcolor transparent(1,0.5,baseColour);

However, I really want something like:

transparent( 1, .55-.25uniformdeviate(1), baseColour )

To achieve something like:

Or like:

Or with any shape, really. Hence setting an initial background colour and using MetaPost.

Question

How do you use relatively coloured colours in ConTeXt/MetaPost?

Related

• Define colour transparency in relation to existing colour

Answer 1

Some notes and comments

1) You don't need MetaPost to set the page colour, you can use colours and backgrounds together:

\setupbackgrounds
  [page]
  [background={color, page:ThemeElement},
   backgroundcolor=blue]

2) You can shorten the the checkerboard squares:

%% from
%% s := (0, 0) -- (dim_x, 0) -- (dim_x, dim_y) -- (0, dim_y) -- cycle;

%% to
s := unitsquare xyscaled (dim_x, dim_y);

3) Why are you using fill followed by draw? Consider using filldraw instead.

The code

Here is my solution:

\definecolor
  [BaseColour]
  [h=66CEF1]

\startuseMPgraphic{page:ThemeElement}
  tiles_x := 8;
  tiles_y := 13;
  dim_x   := OverlayWidth / tiles_x;
  dim_y   := dim_x;

  path s; s := unitsquare xyscaled (dim_x, dim_y);

  for x = 0 upto tiles_x:
    for y = 0 upto tiles_y:
      filldraw s shifted( x * dim_x, y * dim_y )
        if (x mod 2) = (y mod 2):
          withcolor \MPcolor{BaseColour};
        else:
          withcolor transparent(1, uniformdeviate .5, \MPcolor{BaseColour});
        fi
    endfor;
  endfor;

  setbounds currentpicture to
    boundingbox currentpicture shifted( -dim_x/2, dim_y*(tiles_x/tiles_y)/2 );
\stopuseMPgraphic

\defineoverlay
  [page:ThemeElement]
  [\uniqueMPgraphic{page:ThemeElement}]

\setupbackgrounds
  [page]
  [background={page:ThemeElement}]

\starttext\null
\stoptext

As you can see, I did not define any MetaPost colours, instead I used the definitions from ConTeXt using \MPcolor.