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Binning problem when using datenum with pcolor

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Lukas
Lukas on 6 May 2015
Commented: Larissa Perez on 12 Oct 2016
Hello everyone.
When I plot data using pcolor using ("contemporary") datenum values for the x-axis, pcolor bins the data into one or a few bins together, instead of using the ones given.
An example code which reproduces this problem:
secinday = 24*3600;
t0vec = [2015 05 01 0 0 0];
t0 = datenum(t0vec);
% Create an interval of an hour of timesteps of a second
t = t0 + (0:(1/secinday):(1/24-1/secinday));
y = 1:10; % Just some y-axis values
z = rand(10,3600); % Random z-values
pcolor(t,y,z);
surf has the same problem. Anyone know why this happens and how I can prevent it? Or how I can work around it?
It is funny that the problem disappears when one makes the y-axis logarithmic:
set(gca,'yscale','log');
shading flat; % Otherwise only gridlines visible
But as soon as you make them linear again, the problem reappears.
Note that it only happens for high datenum values. When one takes for example
t0 = [1 05 01 0 0 0];
there is no problem. So my guess is that it has something to do with the small relative difference between a "contemporary" datenum value and the value a second later. So the workaround I currently use is to do t-floor(t0);
  3 Comments
Lukas
Lukas on 19 May 2015
I think
t = (1:3600) + floor(now);
works because here your difference between every time point is a full day, as opposed to the second, where it is seconds. The unit of Matlab datenum is a day...

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Accepted Answer

Jan
Jan on 19 May 2015
Edited: Jan on 19 May 2015
With linear indices Matlab uses the OpenGL renderer, while the logarithmic axis enables another oen (painters or zbuffer depending on teh data and which Matlab version you are using).
The OpenGL renderer converts the values to the type single, or "float" in the underlying C-code. datenum values are large and with the single precision the values might be rounded to the same number.
Try to set the renderer of the figure manually and:
t = double(single(t));
After this line the effects of the renderer should vanish - when the renderer is the problem. If so, TMW cannot fix this.
  2 Comments
Larissa Perez
Larissa Perez on 12 Oct 2016
I am new to Matlab and am going through a very similar problem. However, turning the datenum variable to single is not solving it. Can anyone help me?
time1 = datenum(measure1.time);
time1 = double(single(time1));
t1 = datestr(measure1.time,'HH:MM:SS');
subplot (3,1,1)
h=pcolor(time1,depth,transpose(measure1.data));
colorbar caxis ([20 22])
c = colorbar;
c.Label.String = 'Temperature [°C]';
ylabel ('Depth[m]');
set(h, 'EdgeColor', 'none');
set(gca,'YDir','reverse');
set(gca,'XTicklabel',t1);

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