[Octave-bug-tracker] [bug #43782] single precision is insufficient for plotting

[anonymous]

URL:
  <http://savannah.gnu.org/bugs/?43782>

                 Summary: single precision is insufficient for plotting
                 Project: GNU Octave
            Submitted by: None
            Submitted on: Mon 08 Dec 2014 08:58:47 PM UTC
                Category: Plotting with OpenGL
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Inaccurate Result
                  Status: None
             Assigned to: None
         Originator Name: Chad Aeschliman
        Originator Email: address@hidden
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 3.8.2
        Operating System: GNU/Linux

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Details:

The new plotting system apparently uses single precision for plotting
(https://www.gnu.org/software/octave/doc/interpreter/Introduction-to-Plotting.html).
 This is not sufficient for scientific applications.  The website claims that
it is a factor when plotting large values (10^38) but really it becomes a
precision problem at much smaller values.  For example, The following two
figures should look the same, just the x axis is shifted in the second:


x = (0:0.01:2*pi);
y = sin(x);
figure(1);
plot(x, y);
figure(2);
plot(x+1e7,y);


The resulting plots are attached as images.  In this example, the second plot
shows significant quanitzation effects.  The effect gets worse as the size of
the shift increases.  

Having a large bias on the x axis is not unlikely in scientific applications. 
In my particular use case, I was plotting a data signal with a sampling rate
of 1 sample/s against unix timestamps (which has an offset of about 1.4e9
currently). The plot was useless. I had to revert to using gnuplot and its
slower and clunkier interface to generate useful plots

Even if FLTK doesn't support double precision, it's possible to work around
this by shifting and scaling the data before drawing it (and then "undoing"
this shifting and scaling when labeling the axes).  



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File Attachments:


-------------------------------------------------------
Date: Mon 08 Dec 2014 08:58:47 PM UTC  Name: fig1.png  Size: 22kB   By: None

<http://savannah.gnu.org/bugs/download.php?file_id=32622>
-------------------------------------------------------
Date: Mon 08 Dec 2014 08:58:47 PM UTC  Name: fig2.png  Size: 12kB   By: None

<http://savannah.gnu.org/bugs/download.php?file_id=32623>

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