| Re: summing mulit-D array along 1 dimension, at an angle to the rows [message #73386 is a reply to message #73384] |
Thu, 04 November 2010 14:26   |
rrs
Messages: 2
Registered: November 2010
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Junior Member |
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If I only needed the column density along a single line, interpolating
as you suggest would work well. However, I need the column density at
each voxel at a specific viewing angle, meaning I really do need to
act on, and get information about, the entire array. Sorry that
wasn't clear in the original post.
TRANSFORM_VOLUME is meant to be fairly efficient, so it may be the
best I can do.
On Nov 4, 12:41 pm, "Kenneth P. Bowman" <k-bow...@null.edu> wrote:
> In article
> < 9481d9a7-07ab-4042-aa2f-35ac15d24...@w38g2000pri.googlegroup s.com >,
>
>
>
> rrs <rrstrick...@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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