MATLAB 2012B: The use of pcolor in combination with colorbar

Hi all,

I am trying to make a nice figure with pcolor in combination with colorbar. Running the code shown below results in a figure with way too many tick labels (multiple and double overlapping) on the x-axis and colorbar. Is there a nice solution for this? I have to make quite a number of these graphs (100+). Any idea is welcome!

Best regards, Simon

The code (please try to run it, the result is quite astonishing):

minu = -6;
maxu = 6;

U = 2*(rand(1,1000)+1);
V = 6*rand(1,1000);
Z = 2*(rand(1,1000)-0.5);

Urange = linspace(min(U),max(U),100);
Vrange = linspace(min(V),max(V),100);
[UI,VI] = meshgrid(Urange,Vrange);
ZI = griddata(U,V,Z,UI,VI,'cubic');

h = pcolor(UI,VI,ZI);
set(h, 'EdgeColor', 'none');

hcb = colorbar;
colormap(hsv(256));
caxis([minu, maxu]);
colormap(flipud(colormap));

Answer 1

[Image Analyst]

First of all, I'm not sure why you're using pcolor() instead of imshow() or image(). Do you know that you lose a row and column of your data if you do that? And your data point is actually the place where the lines (which you turned off) cross, not the whole colored pixel/square/tile like you'd think, though it seems that way if you use "flat" shading (if you don't use flat shading, you'll probably be very surprised). So that's why I never use pcolor().

Also, I get 7 colorbar labels and 9 axis marks. You can use set(gca, 'XTick',.... to set up whatever tick marks you want.

Try running my code to see how it looks with imshow(). You'll get all your data (no missing row and column) and you'll get full range of the colormap.

minu = -6; 
maxu = 6;

U = 2*(rand(1,1000)+1);
V = 6*rand(1,1000);
Z = 2*(rand(1,1000)-0.5);

Urange = linspace(min(U),max(U),100);
Vrange = linspace(min(V),max(V),100);
[UI,VI] = meshgrid(Urange,Vrange);
ZI = griddata(U,V,Z,UI,VI,'cubic');

h = pcolor(UI,VI,ZI);
set(h, 'EdgeColor', 'none');
hcb = colorbar;
colormap(hsv(256));
caxis([minu, maxu]);
colormap(flipud(colormap));
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);

% Code added by Image Analyst
figure;
imshow(ZI, [], 'InitialMagnification', 800);
colormap(jet(256));
colorbar;
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);

[Simon]

Thank you for your clear answer. I found it very helpful, I will try to use imshow in future. One important issue I did not figure out yet is they way to add axis to the figure. Could you explain what comments are fit for this purpose?

[Image Analyst]

If you want an axis you can use

axis on;

To turn it off, you'd do

axis off;

[Simon]

Great, thank you!

[Stuart Huston]

The advantage of using pcolor() is that the x and y values don't have to be uniformly spaced.

I used to be able to add colorbars after using pcolor() in previous versions, but it doesn't seem to work in R2012a.

Answer 2

[Andrew Krygier]

The solution has to do with the renderer as discussed here:

http://www.mathworks.com/matlabcentral/answers/53874

If you use the command:

set(gcf, 'renderer', 'zbuffer');

you should get a better result.

There are more suggestions in the link.

Answer 3

[Stuart Huston]

Try the following snippet in the editor:

generate some random x, y, and z values

nx=15;
ny=15;
x=rand(nx,1);
x=sort(x);
y=rand(ny,1);
y=sort(y);
z=rand(length(x),length(y));
% plot using pcolor()
figure
pcolor(x,y,z')
shading flat
hcb = colorbar('location','EastOutside');

now try changing the color axis

caxis([.2 .8])

On my system this gives a proper colorbar. However, now try changing nx to, say 25. In this case changing the color axis produces the spurious labels.

Answer 4

[Nir Dahan]

I have noticed that simply defining colorbar location solves the problem!

h = pcolor(UI,VI,ZI);

set(h, 'EdgeColor', 'none');

hcb = colorbar('location','eastoutside');