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;

      0 33 24 0 12;

      0 0 13 14 12;

      0 0 0 0 0];

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

Answer 1

[Image Analyst]

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

[Ronaldo]

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]

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

Answer 2

[Kelly Kearney]

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')

[Ronaldo]

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]

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;