Assigning black color to zero values of "imagesc" plot

Question

Ronaldo 2013-9-24

1
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/88131-assigning-black-color-to-zero-values-of-imagesc-plot

I want to use "imagesc" to plot the following matrix:

   A=[9 24 6 12 6;
33 24 0 12;
0 13 14 12;
0 0 0 0];

I want to assign black color to 0 values and set the color bar from 6 to 33.

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Answer 1

Image Analyst 2013-9-24

3
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https://ww2.mathworks.cn/matlabcentral/answers/88131-assigning-black-color-to-zero-values-of-imagesc-plot#answer_97720

在 MATLAB Online 中打开

Try this:

A=[9 24 6 12 6;
  0 33 24 0 12;
  0 0 13 14 12;
  0 0 0 0 0];
imagesc(A);
cmap = jet(max(A(:)));
% Make values 0-5 black:
cmap(1:6,:) = zeros(6,3);
colormap(cmap);
colorbar

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Ronaldo 2013-9-24

编辑：Ronaldo 2013-9-24

It is working perfectly. Only to complete the question I ask the following question.

I adjust the color of colorbar by writing the following lines:

caxis([cmin cmax]);

h = colorbar;

set(get(h,'child'),'YData',[cmin cmax])

If the difference between the maximum and the minimum non-zero value is small (for instance 2), then the colorbar does not cover a range of color from blue to red. How can I get a range blue to red for the following matrix

A=[9 9.1 8.3 0 0;

0 9.5 8.4 0 0;

0 0 9.6 8.3 8.1;

0 0 0 9.7 0];

imagesc(A);

cmap = jet(max(A(:)));

% Make values 0-5 black:

cmap(1:6,:) = zeros(6,3);

colormap(cmap);

colorbar

Image Analyst 2013-9-25

在 MATLAB Online 中打开

A=[9 9.1 8.3 0 0;
0 9.5 8.4 0 0;
0 0 9.6 8.3 8.1;
0 0 0 9.7 0];
lowestValue = min(A(A(:)>0))
highestValue = max(A(:))
imagesc(A);
cmap = jet(256);
colormap(cmap);
caxis(gca,[lowestValue-2/256, highestValue]);
% Make less than lowest value black:
cmap(1,:)=[0,0,0];
colormap(cmap)
colorbar

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Answer 2

Kelly Kearney 2013-9-24

1
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https://ww2.mathworks.cn/matlabcentral/answers/88131-assigning-black-color-to-zero-values-of-imagesc-plot#answer_97716

在 MATLAB Online 中打开

It'll be a little tricky to do with imagesc, but you could get a similar result with pcolor:

aplt = nan(size(A)+1);
aplt(1:end-1,1:end-1) = A;
aplt(aplt == 0) = NaN;
pcolor(aplt);
colorbar;
set(gca, 'color', 'k', 'clim', [6 33], 'ydir', 'reverse')

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Ronaldo 2013-9-24

Really appreciated for your quick response. The only problem which remains is the "pcolor" plot is pixelate. I need a plot similar to "imagesc" which is smooth.

Kelly Kearney 2013-9-25

在 MATLAB Online 中打开

Neither function does any smoothing, unless you use interpolated shading with the pcolor plot. If you're referring to the black outlines of each grid cell that is the default under pcolor, you can alter that with a simple

shading flat;

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