@node Shade, Show, Set, Commands

@iftex
@syntax Shade.
@end iftex
@example
Syntax: SHADE INTEGER pexpr pexpr
        SHADE HISTOGRAM INTEGER pexpr pexpr
@end example
@pindex Shade
@findex shading regions

   (`Pexpr' is the name of a vector, or an expression in parentheses,
e.g. @code{SHADE 1000 x (sqrt(y))}).

   Shade `inside' a curve defined by the expressions. The shading is
rotated through the current value of @code{ANGLE}, and lines are spaced
@code{INTEGER} apart (screen coordinates, so the full screen is 32768
across). If @code{INTEGER} is 0, the lines will be drawn as close
together as the device allows, simulating an area fill. This is a very
inefficient way to fill areas, made only slightly better by specifying a
large @code{LWEIGHT} on devices that support such things in hardware
(you'll also get slightly jagged edges).

The meaning of `inside' is that as the shading is done, from
left to right taking the value of @code{ANGLE} into account, lines are drawn
from every odd to every even crossing of the curve. The curve is
considered as being closed by joining the first to the last point. If
a shading line just touches the curve the algorithm may be confused,
change @code{INTEGER} slightly, or adding 180 to ANGLE. Sometimes
joining the ends of the curve may 
not be what you want, try using @code{CONCAT} to add points on the end
yourself. For example,
@findex shade surprises
@example
SET x=0,10 SET y=x**2 LIMITS x y SHADE 1000 x y
@end example
@noindent
looks like a new moon, but
@example
SHADE 1000 (x CONCAT 10) (y concat -1e10)
@end example
@noindent
shades beneath the curve, for @code{ANGLE} 0 that is.
You could also try the macros @code{scribble} and @code{shading} in
demos, e.g. type @code{load demos scribble}.

@code{SHADE HISTOGRAM} is similar, but it shades the histogram that would
be drawn by @code{HISTOGRAM} from the same set of points. In this case the
area to be shaded lies between the histogram and the line y=0. If this
offends you, offset the whole graph and lie about the axes.