[Octave-bug-tracker] [bug #58956] Empty plot for certain xlim / ylim settings

[Hg200]

Follow-up Comment #23, bug #58956 (project octave):

>> Would it be possible to use a different coordinate system for the final
rendering? E.g. one that has its origin in the center of the scene that is
rendered and the camera position (possibly far out) on the negative z-axis? 
IMHO yes but you have take care of some restrictions. First of all it should
be emphasized that you do not have to use the functions glOrtho () and
gluPerspective () either. Without invoking these functions, the "active
region" of the OpenGl renderer becomes the cuboid [-1,+1]^3 and the projection
plane becomes z=0. Everything you place in this region will appear on the
screen.

For example what we can do is to write a new update_camera() that transforms
the camera target (in Octave this is the mean value out of xlim,ylim,zlim)
into the origin [0,0,0]. glOrtho () must then be removed from gl-render.cc.
The problem with this approach is that we still have to make the
transformation from "world coordinates" into "camera coordinates". This is
done with the command

++
m_glfcns.glMultMatrixd (x_mat1.data ());
--

so the transformation remains in OpenGl and hence will be single precision. My
guess is that the limit of approx 3.4e+38 on the axes will remain. And i don't
know how "clipping" and how "light" is done in this approach. But it sounds
like a nice project for next vacation just to find out what is happening if
this change is implemented.

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