mfpic-doc.pdf.

4.5 RENDERING FIGURES. 38
The macros of this section can all be used to fill (or unfill) the interior of closed paths, even if
the paths cross themselves. Filling an open curve is technically an error, but the METAFONT code
responds by drawing the path and not doing any filling. Note that these macros override the default
rendering, so if you want some sort of fill pattern and an outline drawn, you need an explicit prefix
for both.
\gfill[〈color〉]...
This rendering macro fills in the subsequent closed path. Under METAPOST it fills with 〈color〉,
which defaults to fillcolor. Under METAFONT it approximates the color with a shade of gray,
clears the interior, and then fills with a pattern of black and white pixels simulating gray.
\gclear...
This rendering macro erases everything inside the subsequent closed path (except text labels
under some circumstances, see section 2.2 and 2.3). Under METAPOST it actually fills with the
predefined color named background. Since background is normally white, and so are most actual
backgrounds, this is usually indistinguishable from clearing. However, if an mfpic environment
utilzes background text (see subsection 4.7.1), part of the background text may appear to be ‘erased’.
Unfortunately, there is little that can be done about this.
\gclip...
This rendering macro erases everything outside the subsequent closed path from the picture
(except text labels under some circumstances, see section 2.2 and 2.3). Note that this is a true erasing,
even in METAPOST.
\shade[〈shadesp〉]...
This rendering macro shades the interior of the subsequent closed path with dots. The diameter
of the dots is the METAFONT variable shadewd, set by the macro \shadewd{〈size〉}. Normally
this is 0.5bp. The optional argument specifies the spacing between (the centers of) the dots, which
defaults to the TEX dimension \shadespace, initially 1pt. If shadewd is larger than \shadespace,
the closed path is filled with black, as if with \gfill. Under METAPOST this macro actually fills
the path’s interior with a shade of gray. The shade to use is computed based on \shadespace and
shadewd. The default values of these parameters correspond to a gray level of about 78% of white. 14
The METAFONT version attempts to optimize the dots to the pixel grid corresponding to the printers
resolution (to avoid generating dither lines). Because this involves rounding, it will happen that
values of \shadespace that are relatively close and at the same time close to shadewd produce
exactly the same shade. Most of the time, however, values of \shadespace that differ by at least
20% will produce different patterns. The actual behavior for particular values of the parameters and
particular printer resolutions cannot be predicted, and we even make no guarantee it will not change
from one version of MFPIC to another.
\polkadot[〈space〉]...
This rendering macro fills the interior of a closed path with large dots. This is almost what
\shade does, but there are several differences. \shade is intended solely to simulate a gray fill in
METAFONT where the only color is black. So it is optimized for small dots aligned to the pixel grid
(in METAFONT). In METAPOST \shade only fills with gray and is intended merely for compatibility.
The macro \polkadot is intended for large dots in any color, and so it optimizes spacing (a nice
hexagonal array) and makes no attempt to align at the pixel level. The 〈space〉 defaults to the TEX
14 If \shadewd is w and \shadespace is s, then the level of gray is 1 − (.88w/s) 2 , where 0 denotes black and 1 white.