Some thoughts about 3d plots

[Jean Bréfort <jean brefort normalesup org>]

Hi,

3d plots are the most important missing feature in the goffice charts
engine. I intend to work on that now, in the hope it will be ready for
goffice-0.4. At first, some questions must be answered before starting
real work.

- axes: in 2d plots, length of the axes are just the width and the
height of the chart. In 3d plots, the situation is quite different, we
must give the axes arbitrary lengths, and then we make a projection so
that we fill most of the available place. The question is how should we
decide the relative lengths of the three axes. There are probably
several available options and we'll have to make them all available to
the user.

- rendering: should we use opengl or another solution? Some times ago, I
was quite reluctant about using opengl because there is no priting
support. However, it is still possible to render the chart to a high
resolution pixbuf (at 300 dpi or so), and print the pixbuf; it works,
but my first tests (in another program) show that even if it does not
use much cpu time, it does use a lot of printer time. Also, the
appearance should be as near as possible to our 2d charts.
If we decide to not use opengl, we can explore what other free software
do, and cheat a bit.

- orientation of the scene: to position a solid in the space, three
angles are needed, and we also need a field of view. Excel, use only two
angles for the orientation, and OOoCalc is fully non intuitive for me,
and I could not find how things work in there. I have some experience
with the use of the so called Euler's angles (and working code), but
they are not very intuitive too. IMHO, we should devise a simple widget
(based on an opengl sphere) which the user might use to change the
orientation, which does not precludes the use of entries for the angles.

Waiting for your great ideas about these points,
Cheers

Jean