LilyPond Beginnershandleiding

Hoofdstuk 4: Tweaking output 99
Zie ook
Extending: Sectie “Scheme tutorial” in Uitbreidingen.
4.3 Appearance of objects
Let us now put what we have learned into practice with a few examples which show how tweaks
may be used to change the appearance of the printed music.
4.3.1 Visibility and color of objects
In the educational use of music we might wish to print a score with certain elements omitted as
an exercise for the student, who is required to supply them. As a simple example, let us suppose
the exercise is to supply the missing bar lines in a piece of music. But the bar lines are normally
inserted automatically. How do we prevent them printing?
Before we tackle this, let us remember that object properties are grouped in what are called
interfaces – see Sectie 4.2.2 [Properties found in interfaces], pagina 97. This is simply to group
together those properties that may be used together to tweak a graphical object – if one of
them is allowed for an object, so are the others. Some objects then use the properties in some
interfaces, others use them from other interfaces. The interfaces which contain the properties
used by a particular grob are listed in the IR at the bottom of the page describing that grob,
and those properties may be viewed by looking at those interfaces.
We explained how to find information about grobs in Sectie 4.2.1 [Properties of layout objects],
pagina 93. Using the same approach, we go to the IR to find the layout object which
prints bar lines. Going via Backend and All layout objects we find there is a layout object called
BarLine. Its properties include two that control its visibility: break-visibility and stencil.
Barline also supports a number of interfaces, including the grob-interface, where we find the
transparent and the color properties. All of these can affect the visibility of bar lines (and, of
course, by extension, many other layout objects too.) Let’s consider each of these in turn.
The stencil property
This property controls the appearance of the bar lines by specifying the symbol (glyph) which
should be printed. In common with many other properties, it can be set to print nothing by
setting its value to #f. Let’s try it, as before, omitting the implied Context, Voice:
{
\time 12/16
\override BarLine.stencil = ##f
c4 b8 c d16 c d8 |
g,8 a16 b8 c d4 e16 |
e8
}
12
16
The bar lines are still printed. What is wrong? Go back to the IR and look again at the page
giving the properties of BarLine. At the top of the page it says “Barline objects are created
by: Bar engraver”. Go to the Bar_engraver page. At the bottom it gives a list of Contexts in
which the bar engraver operates. All of them are of the type Staff, so the reason the \override
command failed to work as expected is because Barline is not in the default Voice context.
If the context is specified incorrectly, the command simply does not work. No error message
is produced, and nothing is logged in the log file. Let’s try correcting it by adding the correct
context: