[[ Note: This text-only version of the tutorial was produced by hand from
the Impression version. It is therefore likely that there are lots of
infelicities in the formatting; in particular, font changes have been
lost. If a sentence doesn’t seem to make any sense, see whether it’s
any better if you pretend some of the words in it are in italics. :-) ]]
A mkdrawf tutorial
==================
This is a fairly gentle introduction to mkdrawf, a program for creating drawfiles. If you don’t know what a drawfile is, don’t worry: you will soon. If you don’t know what a program is, perhaps you should read some other tutorials first.
In fact, even if that’s the case you can probably get a fair way into this
tutorial, since most of it consists of instructions saying “Put this into a
file and do that with it, and see what happens”.
When you’ve worked through this tutorial, you should find most of the manual pretty easy going. Actually most of the manual is easy going anyway, but it
suffers a bit from being intended as a reference as well as a tutorial; this
document has no such ambitions, and is unashamedly incomplete; it even
contains a few (minor) lies. If it disagrees with the manual, you know which
to trust.
Running mkdrawf
---------------
mkdrawf manufactures drawfiles out of ordinary text files. There are two ways
to use it. Firstly, you can run it from the command line: entering a command
like
mkdrawf textfile drawfile
will process the file textfile and produce an output file drawfile. Secondly,
you can use the Wimp application !Drawf, which allows you to drag a text file
to its icon, whereupon it will run that file through mkdrawf and allow you to
drag the resulting drawfile somewhere to save it. The icon for !Drawf looks
like this: <<sorry...>> .
(As you might guess from the look of the icon, !Drawf will also decode
drawfiles for you, producing output suitable for handing to mkdrawf again.
There is a separate manual for !Drawf which will tell you all about this, and
more besides.)
From now on, I shall assume that you can do one of these two things. When I say “run mkdrawf on this file”, this means: either choose an output filename and
use the command-line, or else drag the file to the !Drawf icon and put the
resulting output somewhere.
Running !Draw
-------------
The easiest way to see the results of using mkdrawf is to view the files it
produces using !Draw. This program comes with every Archimedes or Risc PC, so
you should definitely have a copy; you can probably find it by clicking on an
icon labelled Apps at the left-hand side of your icon bar.
If you haven’t used !Draw before, the first thing you should do is to run it
and play around with it for a while. This should give you an idea of the sort
of thing it can do.
A drawfile is a file which !Draw can understand. A drawfile consists
basically of a number of objects (lines, curves, bits of text, that sort of
thing) strung together. In some cases (have you used the Group option on the
menu?) an object can contain a number of other objects. As this tutorial
proceeds, you will learn rather more about drawfiles than you actually want
to know.
Every time you run mkdrawf you should have a look at the output by either
double-clicking on the file it produces (after dragging it to a directory
display, if you are using !Drawf) or dragging the file to !Draw’s icon on the
icon bar. (Not quite every time; if something goes wrong and you get lots of
error messages, the resulting drawfile may be full of rubbish, or at any rate
not full of what you wanted.)
A very simple example
---------------------
Put the following into a file using your favourite text editor, run it
through mkdrawf, and look at the result using !Draw:
# This is a comment; mkdrawf will ignore it.
Path {
Move 100 100
Line 300 200
}
You can probably guess what the result of this will be before you try it.
Your guess will almost certainly be correct. This simple example actually
demonstrates quite a few things about mkdrawf, though… The first line is
self-explanatory. The remaining lines describe a single object. An object is
introduced by saying what sort of object it is; in this case, it is a path
object. (The typical drawfile consists mostly of path objects.) Further
details about the object are given within those braces {}; in this case, the
path consists of a single line segment from (100,100) to (300,200).
Coordinates are always given, as in this example, as pairs of numbers. The
unit, by the way, is the point; the ambiguity here is unfortunate but seldom
causes trouble in practice. A point is 1/72 of an inch; the spacing between
vertical lines here is about 10 points.
A slightly less simple example
------------------------------
As I already said, a drawfile typically contains several objects, one after
another. This structure is represented in the obvious way in input to
mkdrawf; namely, by putting one object description after another. Here’s an
example, which (as usual) you should try.
Path {
Move 100 100
Line 150 100
}
Path {
FillColour r220g0b0
Move 200 200
Line 300 200
Line 300 300
Close
}
Text {
Style Font "Trinity.Medium"
StartAt 200 300
Size 10 10
Text "Hello!"
}
This produces a drawfile containing three objects: two path objects and a
text object. You can probably make a pretty good guess as to what this will
look like, if I tell you that:
• colours are given as RGB values, each component going from 0 (dark) to
255 (bright);
• Size 10 10 means “I want a 10-point font”;
• Trinity is the typeface in which this tutorial is printed.
An easy exercise
----------------
Write a mkdrawf file which, when fed into mkdrawf and !Draw, produces the
following picture. <<Sorry, again.>> You are not expected, of course, to
produce the grid lines and the numbers labelling them (though you might be
able to do something approximating to them), and you should not worry about
the thicknesses of the lines. The grey colour inside the square is
r119g119b119, I think; and the text is 20-point Trinity.Medium.Italic. When
you’ve done this, check it (of course you won’t be able to check the scale)
by running it through mkdrawf and !Draw.
More about path objects
-----------------------
You may have guessed from the previous picture (or you may have noticed while
using !Draw) that all sorts of things we haven’t discussed yet are possible
with path objects; for instance, the picture includes dashed lines, lines in
colours other than black, and lines of different thicknesses. And there’s
plenty more. Anyway, here is a mkdrawf file demonstrating some more features
of path objects:
Path {
Width 2 # all dimensions, including this, are in points
OutlineColour r0g0b255 # blue lines
Style { EndCap Triangular } # !Draw will show you what this means
Move 100 100
Line 200 100
Curve 300 100 200 200 200 300 # a Bezier curve, ending at (200,300)
Line 200 400
Move 200 100 # a path can be made up of many subpaths
Line 200 300
}
What’s new here? The Width keyword allows you to say how thick you want a
line to be. The width is, like all dimensions, given in points (1/72"). It’s
a “diameter” rather than a “radius”. The Style keyword should be followed by
some stuff in braces; there are several other things you can set in there.
The OutlineColour keyword (so called in contrast to FillColour) determines
the colour of lines and curves, as distinct from the filled-in space inside
them. By the way, the usual 16-colour desktop palette doesn’t include a
colour accurately matching r0g0b255, but !Draw will happily display the best
approximation it can, and won’t throw away its information about exactly what
colour you really wanted.
The line beginning Curve is more interesting. As well as straight lines, a
path object can contain Bezier curves. A Bezier curve is described by giving
its starting and ending points — in this case (200,100) and (200,300) — and
two control points — in this case (300,100) and (200,200). The curve starts
out from its starting point, heading towards its first control point. It
curves around until eventually it reaches its ending point, from the
direction of its second control point. Subject to these restrictions, the
exact shape of the curve depends on how far away the control points are from
their matching endpoints. Got that?
I think the best way to get some intuition for Bezier curves is: Go into
!Draw, select the “open polygon” tool (the one at the very top of the
toolbox), click somewhere, click somewhere else, and then click with the
Adjust (right-hand) mouse button. At this point you should see a grey line
with a square at each end. Now, click with Adjust on the square at whichever
end of the line you clicked on second; the whole line should go red. Click
with the Menu button; the menu you get should include an option “Change to
curve”. Do this. Now you should see, as well as the two endpoints of the
line, two other points in the middle of it. These are the control points of
the Bezier curve you have just produced. Drag them around with the Adjust
button and watch the shape of the curve change. (If you accidentally click
somewhere you didn’t mean to and the coloured squares disappear, then select
the line again using the “arrow” tool and hit control-E.)
No, really. You should actually do this, not just read about it. Otherwise
you’ll never really understand Bezier curves, and they’re important.
Doing things again and again and again
--------------------------------------
There are several other sorts of object, but I think it’s time to look at
some other features of mkdrawf before getting even further bogged down in the
details of what they are and how to specify them. Here’s one of the problems
that first got me working on writing this program. I wanted to be able to
draw graphs of functions (sine, cosine, that sort of thing) and make them
into drawfiles, so that I could add annotations, print them out and so on.
Now, obviously we could do that by writing an enormously long mkdrawf
“script” looking something like
Path {
Move 100 400
Line 100.1 400.097
Line 100.2 400.193
...blah blah blah...
Line 500 400
}
with hundreds or thousands of lines, but this would be horrible. But try
feeding the following program into mkdrawf… (This introduces quite a lot of
new ideas, so don’t worry if you can’t see what it does yet. Have a look at
the drawfile it produces anyway.)
Set $2Pi 6.28318530717959
Set $Factor Over 400 $2Pi
Path {
Width 1
Move 100 400
For $x0 0 200 {
Set $x Times $x0 Over $2Pi 200
Set $y Sin $x
Set $t Plus 100 Times $x $Factor
Set $u Plus 400 Times $y $Factor
Line $t $u
}
Line 500 400
}
OK. Let’s take this slowly, since there are lots of tricky things in it.
Anything starting with a dollar sign is a variable. You can set the value of
a variable to be anything at all (including colours like r123g234b11, strings
like "Trinity.Medium", and other even stranger things), but most variables
are used to contain numbers. So, the first line of the program says “Until
further notice, any time you see $2Pi you should replace it with the number
6.2831853071959.”.
The second line introduces arithmetic. I’m afraid the syntax for this is
horrible; its motivating principle is that considering the amount of effort
I’ve put into the program as a whole, you’re lucky to get arithmetic at all
and you have no right to complain if it doesn’t look very nice… More
seriously, the principle is that you have to say what to do before saying
what to do it with. So “2+2” violates this rule, because reading from left to
right you see the first “2” before you know that you’re going to have to add
it to something; to make mkdrawf’s life easier, you have to say Plus first.
Sorry. Anyway, the second line divides 400 by $2Pi’s value, and puts the
result into a variable called $Factor.
The next few lines are familiar: they start a path object, indicate the width
of the line to be drawn, and move to the starting position.
What happens next is altogether new. The For line means, approximately, “Do
everything inside the {} once with $x0 having the value 0, then again with it
having the value 1, then … and finally with it having the value 199.” In
other words, we add 1 to $x0’s value every time, and we give up when it
reaches 200. So, that’s the significance of the magic numbers 0 and 200 on
that line: start at 0, give up at 200.
If you have a look inside those braces, you’ll see that the net result is a
lot of calculation and 200 Lines. I’ll let you work out exactly what the
calculation is doing — it’s not very difficult, especially with the resulting
drawfile in a !Draw window on your screen. (You did run it through mkdrawf,
didn’t you?) Actually, it would be unkind to leave you with no help about the
nasty arithmetic things. The first line inside the {}, for instance, sets $x
to what in any civilised computer language would be called $x0*($2Pi/200).
Finally, we draw a line to the right ending point (we should be very near to
it at the end of the For loop anyway), and that’s the end.
(Incidentally, you may be wondering: Why For rather than Repeat or ManyTimes
or something? Answer: it’s sort-of traditional that this sort of construct is
called a “for loop”. In the first computer languages to use the word “for”
for this sort of thing, the syntax looked more like
For x = 1 to 100 do blah blah blah end.
which you could read as “Do blah blah blah for x=1, then for x=2, and so on”.
This at least makes some sort of sense. This sort of syntax lives on in BASIC
and Pascal.)
Got all that? If not, don’t worry. You can always come back to it.
Another exercise
----------------
Work out what the following does. It is intended to be put in the same
mkdrawf input file as the sine-curve we just drew, but don’t try it until you
think you know what will happen.
Path {
Width 0 # this means: as thin as possible
Move 100 400
Line 500 400
Move 100 Minus 400 $Factor
Line 100 Plus 400 $Factor
# every pi/4 units:
For $n 0 9 {
Set $x Times $n Over $2Pi 8
Set $t Plus 100 Times $x $Factor
Move $t 390
Line $t 410
}
}
And another exercise
--------------------
You should now be able to do this: Produce some squared paper, with lines as
thin as possible. The lines should be spaced at intervals of 10 points, and
occupy the rectangle whose bottom-left and top-right corners are (100,100)
and (500,500).
You will need two loops. I recommend, as a matter of style, having one object
containing the horizontal lines and one containing the vertical lines. Be
very careful about the numbers in your Fors; remember that the second number
is an exclusive rather than an inclusive limit. (Actually you don’t need two
loops. As another exercise, work out a way of doing it with one. Don’t bother
to implement this.)
Macros
------
Consider the following situation. You want to draw a diagram in which a
number of points are marked with little crosses, thus: . If there are, say,
100 points then this could mean an awful lot of typing. You can save a lot of
effort by using a macro; what this means is best illustrated by an example.
Define Point {
Path {
Width 0.3
Move Minus %x 2 %y RLine 4 0
Move %x Minus %y 2 RLine 0 4
}
}
Point { %x 100 %y 200 }
Point { %x 200 %y 300 }
Point { %x 123 %y 321 }
There are three new features here. The first, trivial, one is the use of the
RLine keyword, which is just a labour-saving device; the same result could
have been produced by
Move Minus %x 2 %y Line Plus %x 2 %y
Move %x Minus %y 2 Line %x Plus %y 2.
There are also RMove and RCurve keywords.
The second feature is the use of a macro. The first 7 lines say: Whenever
Point appears, replace it with the stuff in lines 2–6, making certain
changes. The “certain changes” are the third feature: the first time the
macro is “invoked” (on line 8), “%x” will be replaced everywhere by “100” and
“%y” by “200”. You can probably guess what will happen the other two times.
%x and %y are called “parameters” of the macro. You can find out much more
about all this by reading the manual.
I’ve never worked out why the word “macro” is used for this sort of thing.
Ellipses and rectangles
-----------------------
The “ellipse” and “rectangle” tools of !Draw don’t actually correspond to
special kinds of object. An ellipse or rectangle is just a path which happens
to be the right shape. One consequence of this is that ellipses aren’t really
ellipses, but close approximations by Bezier curves. This is a pain, since
ellipses (especially circles) are often what you want, and finding a sequence
of Bezier curves that approximates a circle well is not entirely trivial.
Fortunately I’ve already done the work for you. In the manual you will find,
as part of one of the example programs, a macro which does just that,
together with a couple of examples of its use. Incidentally, reading through
it and checking you understand what’s going on (apart from the choice of the
magic value for %t, whose justification is half a page of mathematical
scribbling) might be a good idea.
Conditionals
------------
You’re preparing a graph to show your boss, illustrating something terribly
important. The graph consists of lots of points plotted with little crosses,
and you want some of the points — the ones showing unusual results — plotted
in red. To be more precise, let’s suppose that a y-coordinate bigger than 300
indicates something wrong, and you want those points plotted in red. You
could define a RedPoint macro and a BlackPoint macro, and in fact if the
mkdrawf file in question is being produced by a computer program that’s
probably the way to do it. But if you’re doing it by hand, you might find the
extra typing (Red and Black) annoying, and you might not be entirely
confident of your ability to make no mistakes. So, instead…
Define Point {
Path {
Width 0.3
OutlineColour
IfLess Minus 0 y -300 r255g0b0
Else r0g0b0
EndIf
Move Minus %x 2 %y RLine 4 0
Move %x Minus %y 2 RLine 0 4
}
}
Point { %x 100 %y 200 } # this will be in black
Point { %x 200 %y 300 } # so will this
Point { %x 123 %y 321 } # but this will be in red
You can have more complicated conditions than this, of course, provided you
can massage them into the form “so-and-so is less than such-and-such”, or
“so-and-so is equal to such-and-such”, for which you use IfEqual. You should
be warned that arithmetic involving anything other than integers is likely to
be imprecise, so IfEqual may not behave the way you expect it to. For
instance, if you do
Define $Pi 3.141592653589
IfEqual Sin $Pi 0
blah
EndIf
you should not rely on blah being done. There is another kind of If, called
IfExists; see the “arcs of circles” thing in the manual for an example of its
use.
And yet another exercise
------------------------
Write a mkdrawf program that produces 200 random points spread uniformly over
the square whose bottom left and top right corners are at (100,100) and
(200,200), and plots each one with a cross as discussed above, coloured black
if it’s inside the circle that touches all four sides of the square and red
if it’s outside it. (The circle in question has centre (150,150) and radius
50. You may colour points on its circumference either red or black; I don’t
mind.)
That’s all, folks…
------------------
At this point you have seen most of the tricky things about mkdrawf, and you
should now read the manual. If you’ve done all the exercises, this should be
