bugGNU Octave - Bugs: bug #56983, Plotting constant values with...


bug #56983: Plotting constant values with small noise gives erratic results

Submitter:  Roland Baudin <roland65>
Submitted:  Wed 02 Oct 2019 02:15:15 PM UTC
Category:  Plotting with OpenGL Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Incorrect Result
Status:  Duplicate Assigned to:  None
Originator Name:  Open/Closed:  * Closed
Release:  * dev Operating System:  * Any
Fixed Release:  None Planned Release:  None
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Thu 03 Oct 2019 07:46:03 AM UTC, comment #6: 

OK, I didn't know about fltk and OpenGL.

I checked there is no issue with gnuplot, so the bug is clearly related to OpenGL.

Roland Baudin <roland65>
Thu 03 Oct 2019 07:36:14 AM UTC, comment #5: 

Yes indeed, this is a duplicate of bug #32980. Closing report as a duplicate.

Pantxo Diribarne <pantxo>
Group Member
Wed 02 Oct 2019 11:03:12 PM UTC, comment #4: 

The fltk toolkit does use OpenGL for rendering.

John W. Eaton <jwe>
Group administrator
Wed 02 Oct 2019 08:50:06 PM UTC, comment #3: 

> Looks like the known issue that OpenGL only works in single precision.

The fltk toolkit doesn't use OpenGL, as far as I know...

Roland Baudin <roland65>
Wed 02 Oct 2019 05:40:01 PM UTC, comment #2: 

Looks like the known issue that OpenGL only works in single precision. There are several open bug reports about his (can't find them ATM).

A way around is to, yes, try the gnuplot backend:
graphics_toolkit ("gnuplot")

Please report back if that works better for you.

Philip Nienhuis <philipnienhuis>
Group Member
Wed 02 Oct 2019 02:17:05 PM UTC, comment #1: 

Here are the screenshots.

Roland Baudin <roland65>
Wed 02 Oct 2019 02:15:15 PM UTC, original submission:  

Here is the problematic code:

close all;
clear all;
s = 1e-3;
x = 100 ones(1, 1000) + s randn(1, 1000);

When s is higher than 1e-6, this plots (as expected) a noise around the mean value 100.

When s = 1e-6, this plots a line that is sometimes at the top of the figure, or at the bottom, or there is no line at all. The figure frame is corrupted, etc. (see screenshot1).

When s = 1e-8, there is sometimes no frame in the figure. The line is often not displayed (see screenshot2).

When s <= 1e-10, the plot correctly displays a line in the middle of the figure and the frame is also displayed, as expected.

I was able to reproduce this issue in Octave 4.2.1 (windows), 4.4.1 (windows), 5.1.0 (windows), 5.1.0 (Ubuntu Linux 18.04).

The problem is the same with qt or fltk toolkit. I didn't test with Gnuplot.

Roland Baudin <roland65>


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Attach Files:

Attached Files
file #47610:  screenshot1.png added by roland65 (10KiB - image/png)
file #47611:  screenshot2.png added by roland65 (8KiB - image/png)


   bug dependencies.

Items that depend on this one: None found


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    Follow 10 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2019-10-03 pantxo Open/ClosedOpen Closed
    2019-10-03 pantxo CategoryPlotting Plotting with OpenGL
        Item GroupUnexpected Error or Warning Incorrect Result
        StatusNeed Info Duplicate
        Release4.4.1 dev
        Dependencies- Depends on bugs #32980
    2019-10-02 philipnienhuis Item GroupIncorrect Result Unexpected Error or Warning
        StatusNone Need Info
    2019-10-02 roland65 Attached File- Added screenshot1.png, #47610
        Attached File- Added screenshot2.png, #47611

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