Flexible, modular dia line decoration code

[Dave Hoover <spamless4 iasun2 epfl ch>]

  Hi,

I have written some PRELIMINARY modular arrow code for dia (based on 
gaps, of course).  This is just a demonstration of one way of 
implementing modular arrow/line code.  This diff is based on the 
objects/standard/line.c file in CVS.

It is based on a DecorationProp structure that allows one to define a 
kind of transformation:

decorating_segment():
    inputs: DecorationProp structure, line start/end segments
    output: start/end points of decoration

The code demonstrates the relationship between the DecorationProp and 
the drawn decoration.  By providing a list of DecorationProps, it should 
be possible to do just about any type of arrow (the demonstration just 
does one DecorationProp at a time.  It is possible to use more complex 
shapes as decorations. Try it out and give me some feedback.  When the 
code is enabled, every line you draw will have a different decoration 
attached (do not choose an arrow head--they will be placed 
automatically).  Notice that it is possible to specify in 
DecorationProps whether asymmetric decorations are always placed on the 
right or left side of the object endpoint (relative to arrow direction), 
or always EAST of the endpoint or always north-east, and so forth.

It is also possible to specify that decorations be draw a fixed size, or 
proportional to line length, or any combination of the two.

To make this modular, I think we could rewrite the 'GAP_define' stuff so 
that the user can choose the number of decorations, and for each 
decoration specify the angle, shape, and gap properties.  The user could 
specify 4-number gaps if desired, or  select from a list of pre-made GAP 
specifications.  Each specification, like GAP_absolute, 
GAP_proportional, or GAP_relative_alignment_shift_in, could be provided 
with a text description and user-choosable parameters.  For instance, 
for GAP_absolute, the user would have a simple 'length' to choose.  We 
could also compose a huge list of parameterized arrow heads that a user 
could choose directly from.  The arrow heads could be in categories like 
(standard, flat, shaped).  The user would then choose either symmetric 
or asymmetric, or anti-symmetric.  This way, starting from a plain old 
half-head arrow, by selecting asymmetric, symmetric, antisymmetric the 
user would get:

1. asymmetric: half-head
2. symmetric: lines
3. antisymmetric: cross

or from a half-flat-head, a user could select

1. asymmetric: half-flat-head
2. symmetric: flat-head
3. antisymmetric: flat-head

 These are just ideas.  What do you think?

I think it's important to think about how we store gap specifications, 
DecorationProp specifications, and so on.  I think we should have 
data_gap, data_add_gap, data_decorationprop, and data_add_decorationprop 
functions.  Maybe you think I'm getting ahead of myself.  You may be right.

The code also partially implements line start/center/end labels that are 
disabled by default.  When enabled, they print out user-chosen labels 
near the line start/center/end points.  The labels use the 
decorating_segment() code to position the labels so that they never 
intersect the line.  For the moment, only the start label sort of works. 
 The bounding boxes have not been done for the labels, and so you will 
get nasty effects on the screen when you use this.

decorating_segment() is described in more detail in the code.

This code is in no way meant to be 'optimal'.  Many improvements can be 
made (storage of segments, etc., to avoid repeating calculations).  Line 
joins are not done at all, so either this code must be generalized, or 
certain arrows will still have to be coded by hand to obtain optimal 
results.  Without doing joins, we can maybe determine when two different 
segments have a common endpoint, and then draw each of those lines with 
a LINECAPS_ROUND to avoid ugly intersections...??..

As usual, this patch is copyrighted (2002) by David Hoover.  You can 
use, copy, and modify this code according to the conditions of the GNU 
General Public License (GPL) as found in the Dia distribution or found 
at http://www.gnu.org/

decoration_diff.gz