how to manipulate dense 4D data ?

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Kate
Kate 2014-1-3
评论： Kate 2014-1-3
hi,
I have a 4-D array. I plot it as using the code below(You need the function 'plotgrid' below as well). Based on the plot the spacing of the cube is very dense, I want to make the inter-spacing in the cube plot sparser than it currently is. But I dont know how to resample 4D data (or skip rows). Can anyone help ?
clc;clear all;close all;
%%%%%Test script
[x,y,z] = meshgrid(-2:.2:2,-2:.25:2,-2:.16:2);
v = x.*exp(-x.^2-y.^2-z.^2);
xslice = [-1.2,.8,2]; yslice = 2; zslice = [-2,0];
slice(x,y,z,v,xslice,yslice,zslice)
colormap hsv
% I work with NDGRID rather than MESHGRID
[x,y,z] = ndgrid(-2:0.1:2,-2:.1:2,-2:.1:2);
xyz = cat(4, x, y, z);
plotgrid(gca, xyz, 'color', [0 0.5 0]);
%----------------------------------------%
function h = plotgrid(ax, xyz, varargin)
% function h = plotgrid(ax, xyz, varargin)
% Plot the 4D position array xyz(nx,ny,nz,3) in a grid-like plot
if isempty(ax)
ax = gca();
end
hold(ax, 'on');
h = [];
for dim=1:3
p = 1:4;
p([1 dim]) = [dim 1];
a = permute(xyz, p);
m = size(a,1);
a = reshape(a, m, [], 3);
if m > 1
hd = plot3(ax, a(:,:,1), a(:,:,2), a(:,:,3), '.-', varargin{:});
else
hd = plot3(ax, a(:,:,1), a(:,:,2), a(:,:,3), '.', varargin{:});
end
h = [h; hd];
end
hold(ax, 'off');
end % plotgrid

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Walter Roberson
Walter Roberson 2014-1-3
You can resample using interpn
You can also use every [p, q, r]'th point along the first three dimensions using reducevolume(). It is not documented clearly but the code for reducevolume is set up so that any higher dimensions are kept intact.
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Walter Roberson
Walter Roberson 2014-1-3
If you have an xyzv matrix and you want every P'th point in the x, y, z dimensions, leaving v (because it is only 3 wide), then
newmatrix = xyzv(1:P:end, 1:P:end, 1:P:end, :);
This simply drops points. If your matrix varies quickly enough in value then you might prefer to interpolate instead of simply dropping points. If you want to be left with N points along each of x, y, z, keeping 3 along v, then:
SZ = size(xyzv);
[xq,yq,zq,tq] = ndgrid(linspace(1,SZ(1),N), linspace(1,SZ(2),N), linspace(1,SZ(3),N), 1:SZ(4));
newmatrix = interpn(xyzv, xq, yq, zq, tq);

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