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

[Hg200]

Follow-up Comment #10, bug #58956 (project octave):

Attached is a demo code that is independent of Octave and that demonstrates
the same behavior in Qt. The code places a unit cube (length = 1) between
"nearVal" and "farVal" and sets "nearVal" and "farVal" in glOrtho() to a large
value, while the difference "farVal - nearVal" is chosen to be small in
comparison to the object-camera distance. The adjustment can be done in
"mainwindow.h":

++
double nearVal = 1e+6 - 2.0;
double farVal = 1e+6 + 2.0;
--

If the values are selected as

++
double nearVal = 1e+11 - 2.0;
double farVal = 1e+11 + 2.0;
--

the cube is not displayed, which is consistent with the observation made in
Octave.

The two relevant commands are:

++
glOrtho(-aspectratio, aspectratio, -1, 1, nearVal, farVal);
--

++
glTranslated(0.0, 0.0, -(farVal - 2.0));
--

The cube is displayed when the difference "farVal-nearVal" becomes larger. For
example, if nearVal is set to zero or set to a negative value (moving the
clipping plane behind the camera is explicitly allowed for glOrtho). And / or
farVal is selected as a factor greater than 1.0 of the object-camera distance.
This observation is also consistent with what i observe in Octave. 

Adding / subtracting a small value to a large value in floating point
arithmetic is dangerous, but 1e+11+2.0 should be fine. In any case, we might
also want to draw axes with 1e+22 rather than 1e+11, and then the problem
becomes more obvious.

What is really happening in Octave depends now on the modelview- and
projection matrix calculated in graphics.cc (update_camera). But according to
GDB it looks like the "farVal-nearVal" difference is not proportional to the
distance between the object and the camera. If no one can provide information
about it, I will dig more into graphics.cc now.


(file #49750)
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Additional Item Attachment:

File name: opengl_test.tar                Size:10 KB
    <https://file.savannah.gnu.org/file/opengl_test.tar?file_id=49750>



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