How can the colors of ones choice be added to the legends automatically in an image ?

I have an image with 16 classes. I wish to add legends (Class1, Class2,...,Class16) to this image corresponding to the color generated by imagesc colormap. The sample image can be displayed using the code below:

for i=1:16
  for j=1:20
   x(i,j)=i;
  end
end
imagesc(x)

Answer 1

[jonas]

usually when you have a 3d plot like that you use the colorbar instead of a legend. Try adding this:

colormap(parula(16))
colorbar

it will give you a colorbar with 16 distinct colors.

[Shivam Pande]

I wish to add the legends as Class1, Class2,..., Class16 in corresponding colors.

[jonas]

You can change the labels of the yaxis to anything youd like. You can also make a fake legend with patches but that would require a bit of work.

If you draw a sketch of what you want then I might be able to give you some code.

This question from earlier today seems relevant. Actually, it is probably exactly what you are looking for.

https://se.mathworks.com/matlabcentral/answers/421247-colorbar-for-specified-color

I adapted the code for you

sg=1:16;
for i=1:length(sg)
  for j=1:20
   x(i,j)=i;
  end
end
imagesc(x)
cbp=colorbar('southoutside');
cbp.Visible='off';
p=cbp.Position;
w=p(3)/numel(sg)
cmap=colormap(jet(length(sg)))
str=sprintfc('\n Class %g',1:numel(sg))
for i=1:numel(sg)
annotation('textbox',[p(1)+w*(i-1) p(2) w*0.95 p(4)],'backgroundcolor',cmap(i,:),'string',str{i})
end

Also see my attached example for making the bar vertical. This is more complicated, as a break is automatically inserted when the text reaches the horizontal edges of the textbox. You will need to adapt the position properties to make it look good.

Answer 2

[Image Analyst]

Try this:

for i=1:16
  x(i,1:20) = i;  
  ca{i} = sprintf('Class %d', i);
end
imshow(x, [], 'InitialMagnification', 1600);
axis('on', 'image');
cmap = hsv(16);
colormap(cmap);
colorbar('Ticks', 1:16, 'TickLabels', ca)