pcolor issue with irregular grid
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Amin
am 17 Dez. 2020
Kommentiert: Amin
am 22 Dez. 2020
I have been spending way too much time and would appreciate your insight and comments.
I am using “pcolor” to plot my data on an irregular grid (regularizing it is not an option).
-when I plot the even rows of my X, Y, and data the figure is what I expect (figure 1).
-when I plot the odd rows of my X, Y, and data the figure is what I expect (figure 2).
-when I plot the all rows of my X, Y, and data the figure is NOT what I expect (figure 3).
**figure 3 should be similar to figures 1&2 (basically those 2 superimposed).
the data, X, and Y are attched.
Thanks again.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/464300/image.png)
clear all; close all; clc
load('C:\Desktop\Test\X.mat');
load('C:\Desktop\Test\Y.mat');
load('C:\Desktop\Test\data.mat');
% replaceing zeros with NAN
X(X == 0) = NaN;
Y(Y == 0) = NaN;
data(data == 0) = NaN;
% choosing even rows
X1 = X(2:2:end,:);
Y1 = Y(2:2:end,:);
data1 = data(2:2:end,:);
% choosing odd rows
X2 = X(1:2:end,:);
Y2 = Y(1:2:end,:);
data2 = data(1:2:end,:);
figure(1)
h1 = pcolor(X1,Y1,data1);
set(h1,'edgecolor','none');
figure(2);
h2 = pcolor(X2,Y2,data2);
set(h2,'edgecolor','none');
figure(3);
h3 = pcolor(X,Y,data);
set(h3,'edgecolor','none');
2 Kommentare
Mathieu NOE
am 20 Dez. 2020
hi
have to figure out why pcolor is doing that
with imagesc I don't have the problem
clear all; close all; clc
load('X.mat');
load('Y.mat');
load('data.mat');
% replaceing zeros with NAN
X(X == 0) = NaN;
Y(Y == 0) = NaN;
data(data == 0) = NaN;
% choosing even rows
X1 = X(2:2:end,:);
Y1 = Y(2:2:end,:);
data1 = data(2:2:end,:);
% choosing odd rows
X2 = X(1:2:end,:);
Y2 = Y(1:2:end,:);
data2 = data(1:2:end,:);
figure(1)
h1 = imagesc(data1);
% set(h1,'edgecolor','none');
figure(2);
h2 = imagesc(data2);
% set(h2,'edgecolor','none');
figure(3);
h3 = imagesc(data);
% set(h3,'edgecolor','none');
Akzeptierte Antwort
Walter Roberson
am 20 Dez. 2020
surf(X(1:2:end,:), 'edgecolor', 'none')
and rotate it a bit, and you can see that the bulk of it is a single titled plane.
surf(X(2:2:end,:), 'edgecolor', 'none')
is likewise a single titled plane.
Now
surf(X, 'edgecolor', 'none')
and you will observe a thick wedge.
So although even X and odd X individually form consistent planes, when you take both together they do not form a single plane. When used as coordinates, pcolor would have to try to compensate.
Note: pcolor is surf() followed by view(2), together with some logic that sets the Z coordinates of the surf to all 0.
3 Kommentare
Walter Roberson
am 21 Dez. 2020
load X
load Y
load data
X(X == 0) = NaN;
Y(Y == 0) = NaN;
data(data == 0) = NaN;
mask = ~(isnan(X) | isnan(Y) | isnan(data));
F = scatteredInterpolant(X(mask), Y(mask), data(mask));
%trim down to useful part of data
[~, idx] = max(X(:));
[R, C] = ind2sub(size(X), idx);
lastC = C-2; %some of the last couple of columns dip down
Xsub = X(:,1:lastC);
minx = min(Xsub, [], 'all');
maxx = max(Xsub, [], 'all');
Ysub = Y(:,1:lastC);
miny = min(Ysub, [], 'all');
maxy = max(Ysub, [], 'all');
xv = linspace(minx, maxx, lastC);
yv = linspace(miny, maxy, size(X,1));
[XG, YG] = ndgrid(xv, yv);
Z = F(XG, YG);
h = pcolor(XG, YG, Z);
h.EdgeColor = 'none';
In theory this takes all of the data into account. However, it interpolates data coordinates, and in theory it would work out something like taking the bottom (even-numbered) layer and shifting it to the left, and then merging the two sheets. The results look okay, but before using the [XG, YG, Z] data for serious analysis, you should think more about how the triangulation process works to make sure that you really are getting contributions from both half.
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