MetaFun - Pragma ADE

Because of this implementation, shading may behave somewhat unexpected at times. A rather
normal case is the next one, where we place 5 shaded circles in a row.
path p ; p := fullcircle scaled 1cm ;
for i=0 step 2cm until 8cm :
circular_shade(p shifted (i,0),0,\MPcolor{a},\MPcolor{b}) ;
endfor ;
At first sight, in the next situation, we would expect something similar, because we simply copy
the same circle 5 times. However, due to the way we have implemented shading in CONTEXT, we do
indeed copy the circles, but the shade definition is frozen and the same one is used for all 5 circles.
This means that the center of the shading stays at the first circle.
circular_shade(fullcircle scaled 1cm,0,\MPcolor{a},\MPcolor{b}) ;
picture s ; s := currentpicture ; currentpicture := nullpicture ;
for i=0 step 2cm until 8cm :
addto currentpicture also s shifted (i,0) ;
endfor ;
Unlike TEX, METAPOST does not keep its specials attached to the current path, and flushes them
before the graphic data. Since we use these specials to register shading information, it is rather
hard to tightly connect a specific shade with a certain fill, especially if an already performed fill is
not accessible, which is the case when we copy a picture.
This may seem a disadvantage, but fortunately it also has its positive side. In the next example
we don't copy, but reuse an already defined shade. By storing the reference to this shade, and
referring to it by using withshade, we can use a shade that operates on multiple shapes.
sh := define_circular_shade
(origin,origin,0,8cm,\MPcolor{a},\MPcolor{b}) ;
for i=0 step 2cm until 8cm :
fill fullcircle scaled 1cm shifted (i,0) withshade sh ;
endfor ;
The low level macro define_circular_shade is fed with two pairs (points), two radius, and two
colors. The shade is distributed between the colors according to the radius.
Shading can hardly be called an easy issue. The macros that we provide here are in fact simplifications,
which means that at a lower level, one can do more advanced things. Here we limit
ourselves to the more common cases. In the previous examples, we used an arrow to indicate the
Shading Special effects
187