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Re: summing mulit-D array along 1 dimension, at an angle to the rows [message #73393 is a reply to message #73386]

Thu, 04 November 2010 12:41

Kenneth P. Bowman
Messages: 585
Registered: May 2000

Senior Member

In article
<9481d9a7-07ab-4042-aa2f-35ac15d2425d@w38g2000pri.googlegroups.com>,
rrs <rrstrickler@gmail.com> wrote:

> Suppose you have a 2d or 3d array that contains some information (in
> my case, density) with a relatively complex geometry, and you want to
> collapse this to 1d or 2d by summing (to obtain a column density).
> This is simple if you want to sum along an axis, but I'm having
> problems finding an efficient way to sum along an angle (i.e. if the
> observer is at a 30 degree angle to the x-axis). The best I've been
> able to do is rotate the array by the desired angle, and then perform
> the sum. Example:
> density = transform_volume(TEMPORARY(density), ROTATION=[0,angle,0]);
> column_density = REVERSE(TOTAL(REVERSE(density*voxel_size), 1, /
> CUMULATIVE));
> All the reversing is so that I get a column density from each voxel to
> the observer along the row.
>
> The problem is that I'm working with large arrays, and I need to do
> this for several angles, which makes the whole process both memory-
> intensive and slow.
>
> Am I missing some more sensible way to do this?

I suggest that you interpolate the 2-D or 3-D grid to the line
that you want to average along. Then average the results.

That should be faster than rotating the entire grid.

Assuming your grids are reasonably regular (they can be
stretched, but should be separable), INTERPOLATE is the tool
for this.

Ken Bowman

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[Message index]

		Re: summing mulit-D array along 1 dimension, at an angle to the rows By: rogass on Thu, 04 November 2010 16:14
		Re: summing mulit-D array along 1 dimension, at an angle to the rows By: rrs on Thu, 04 November 2010 14:26
		Re: summing mulit-D array along 1 dimension, at an angle to the rows By: Kenneth P. Bowman on Thu, 04 November 2010 12:41

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