the problem of shifting and pcolor

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jenka 2012-7-4

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评论： Chad Greene 2015-5-1

Hi all, I am doing this code, and it appears that my text pixels are shifted compared to my pcolor output. Any suggestions. Note, my data contains NaN so image() does not work. Also, x and y are both of matrix types, not vectors. Any suggestions would be really appreciated. THANKS!

m_proj('stereographic','lat',90,'radius',60, 'rotateangle',270)
[lat_new,lon_new] = ndgrid(lat,lon);
[x,y]             = m_ll2xy(lon_new,lat_new);
h                 = pcolor(x,y, cor); hold on;axis xy;
shading flat
sz = size(lat_new);
for i=1:sz(1)
 for j=1:sz(2)
   if prob(i,j)==1
    %THESE APPEAR TO BE SHIFTED:(((
    g = text(x(i,j),y(i,j),'*');
    set(g,'fontsize',[10]);
    set(g,'fontweight','bold');
    hold on;
    %z = text(x(i,j),y(i,j),num2str(cor(i,j)));
   end
 end
end

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Answer 1

Image Analyst 2012-7-4

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Right. We talked about this before. Don't use pcolor. image() and imshow() have no problem handling images with nans in them - I actually tried and verified that. You just have to make sure x and y are in the range of 1 to sz(1) or sz(2)

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Image Analyst 2012-7-4

编辑：Image Analyst 2012-7-4

在 MATLAB Online 中打开

I'm not sure why you still insist on pcolor. Let me illustrate. Run this demo:

fontSize = 22;
m =[...
     2     1     3     1     3
     3     1     3     3     1
     2     2     3     2     1
     3     1     2     1     1
     2     1     1     1     1]
figure(1);
pcolor(m)
shading('Faceted');
title('Faceted', 'FontSize', fontSize);
figure(2);
pcolor(m);
shading('interp');
title('interp', 'FontSize', fontSize);
figure(3);
pcolor(m);
shading('flat');
title('flat', 'FontSize', fontSize);

Do you expect to see a picture of 5 by 5 "pixels" where all the "1" elements have the same color, and all the "2" elements have the same color (but different than the color for 1), and all the "3" elements have the same color (but different than the color for 1 and 2)? That would seem reasonable wouldn't it? Is that how it actually looks? The answer is NO. The 1's DON'T all have the same color and neither do the 2's and 3's. Moreover the picture only has 4 by 4 pixels, not 5 by 5. You're warned about this in the source code for pcolor: " the last row and column of C are not used" Is that what you want? Plus there's the problem you originally asked. So, given all that, plus the fact that I told you image() and imshow() both handle (ignore) nans, can you explain to me why you still want to use pcolor?

jenka 2012-7-4

在 MATLAB Online 中打开

sorry here is the code use:

      m_proj('stereographic','lat',90,'radius',60, 'rotateangle',270)
      [lat_new,lon_new] = ndgrid(lat,lon);
      [x,y]             = m_ll2xy(lon_new,lat_new);
      %lon and lat are vectors
      h                 = image(lon,lat, cor); 
      sz = size(lat_new);
      for i=1:sz(1)
       for j=1:sz(2)
         if prob(i,j)==1
          %THESE APPEAR TO BE SHIFTED:(((
          g = text(x(i,j),y(i,j),'*');
          set(g,'fontsize',[10]);
          set(g,'fontweight','bold');
          hold on;
         end
       end
      end

Image Analyst 2012-7-4

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I don't have those functions. If they're in the mapping toolbox, I don't have that. You can use a colormap to get rid of the blue, like

imshow(yourArray, []);
colormap(gray(256));

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Answer 2

jenka 2012-7-4

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This is exactly the issue with pcolor I am facing. I am just not sure how to solve this small problem: p_color with shading flat will draw a panel between the (i,j),(i+1,j),(i+1,j+1),(i,j+1) coordinates of the X/Y matrices with a color corresponding to the data value at (i,j). Thus everything will appear shifted by one half a pixel spacing.

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Walter Roberson 2012-7-4

Yes, that is how it is designed, and is the reason not to use pcolor()

Chad Greene 2015-5-1

Here's a depiction of some funny effects using pcolor.

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the problem of shifting and pcolor

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