Re: XForms: downsampling data before plotting via xy_plot

From: Bill Unruh (
Date: Wed Mar 07 2001 - 12:14:29 EST

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    # To subscribers of the xforms list from Bill Unruh <> :

    > Another thing of course is that given a specific screenresolution
    > it does not seem to make any sense to try and plot more points in
    > a plot than there are pixels. This is also a waste of processing time.
    > So what I'm wondering about now is
    > whether someone has (already) come up with some sort of
    > downsampling paradigm to reduce the number of points to plot
    > to the sensible number (i.e. hor. or vert. resolution)
    > (avoiding aliasing of course).

    You cannot avoid aliasing. 2000 data points into 500 won't go. You have to
    decide what it is you would like your displayed data points to actually
    show. One possibility which I use is the max and min values of the actual
    data between the samples. This basically requires two plots for each
    stream-- teh max and the min. However it is up to you to decide which
    aspect of the data is most important to you to display.

    (See where I display in a very rough
    and ready way the max-min of the data for a sound sample). Other displays
    tended to produce severe aliasing on the coarsly sampled data. Note that
    this is also what programs like snd do in displaying the sound data.

    William G. Unruh	Canadian Institute for		Tel: +1(604)822-3273
    Physics&Astronomy	   Advanced Research		Fax: +1(604)822-5324
    UBC, Vancouver,BC	 Program in Cosmology
    Canada V6T 1Z1		     and Gravity
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