| Re: PLOTting into a 2-D array [message #46767 is a reply to message #46759] |
Mon, 19 December 2005 11:53  |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Mon, 19 Dec 2005 09:42:29 -0800, M. Katz wrote:
> Gurus,
> Is there a way to use a command like PLOTS (or something related) to put
> "z" data from an arbitrary x-y path into a 2-D (floating-point, double, or
> long) image array?
>
> With
> PLOT, x, y, /nodata
> PLOTS, x, y, color=zarray, /data
> IDL draws a picture that can be captured as a 2D image array using simple,
> direct graphics. And IDL does this so much faster than I could possibly do
> it the long way.
>
> Here's a statement of the problem. I have experimental data gathered while
> a system is "scanning" an arbitrary (x,y) path. The data arrays can
> contain hundreds of MB of data. When I use PLOTS, and BYTSCL() the
> "signal," I can generate a decent visualization of my data fairly quickly.
> But to capture the resultant "image" means converting to whatever the
> device-limited color-scale is--like 8-bit.
>
> The "long way" solution would be to discretize the (x,y) path coordinates
> into pixel-x-y values, and then do some
> summing/histogramming/averaging/etc. on the z values. This is obviously
> very time consuming. What would be ideal is to "draw" the data into the
> array pixels, using the speed of PLOTS or a similar, optimized routine.
>
> I understand that there are some subtleties regarding multiple data points
> assigned to the same pixel and such, but I'm willing to live with a
> slightly coarse rendering if I can make back the speed of PLOTS.
I'm not sure if you are more worried about the "long way" taking too
much of your computer's time, or too much of your time. As far as
computer time, if you have many XY points, I'd suspect that
pre-binning to some reasonable image size (like 1024x1024) for display
would actually be much *faster* than attempting to draw each and every
one of many millions of points. If your XY data are in a regular
grid, just treat it as a large array, and use REBIN to get it into a
reasonable size. Very quick and easy (both for your time, and your
computer's), and it will average for you. Otherwise, HISTOGRAM is
fast, and reasonably easy for this type of a problem: have a look at
HIST_2D.
The problem with the approach you outline, is you are letting your
screen resolution, and the order in which points are drawn determine
your final "image". Obviously, if you have points at 20,000 X
positions, and you are plotting to a window of X-size 512, many points
will fall right on top of each other, and the "value" will be
determined by the last one which fell. You'll get much better control
by taking a bit of time to learn how to use REBIN/HISTOGRAM
effectively.
JD
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