Calculate spacing in lilypond

[cab]

Hello everyone,


I have been using lilypond for 2 years now in a project called "psautiers"
(psalm in french) to create database of music : you can see examples at my
website www.psautiers.org. Yet I used the scripts musicxml2ly and lilypond to
create a jpeg from a musicxml file. Now that everything works fine, I would like
to implement a way of changing colors in a few notes (for example to color the
first notes of my score, or to color the predominant note in a melody), but to
do this I have two solutions :

-reexecuting lilypond and musicxml2ly to color a few notes which is very costy
in time and in CPU because I use a system command.
-or finding how you implemented the spacing in lilypond and directly coloring
the corresponding notes in the jpeg file.

I tried to look at the source using apt-get source lilypond and I then went to
the lily directory. However I did not find what I searched for.

Is the algorithm difficult to grasp? You may provide me a better way to
underline of color a few notes.


Thanks very much

Dang Nguyen Bac