how to use imshow for this array?

hello, I have attach my image. I want to use imshow. However, it shows as white and nothing appears when I use imshow. I want to see a coloured image. This array is the output of a pixel-based classification. Please advise.

EDIT: I added output example pic that I am looking to get. It will not be the same exact pic but should have the same color scheme and look similar.

thanks Aud

Answer 1

[jonas]

It's not imshow but you could use imagesc instead

imagesc(out_map)

If you really want to use imshow, then

B = mat2gray(out_map)
imshow(B)

in which case you have to apply a colormap, because your matrix is not an RGB image.

[madhan ravi]

+1 learning a lot about image processing from you :)

[audley james]

how to apply colormap after this:

B = mat2gray(out_map) imshow(B)

[jonas]

@madhan, thanks :)

@audlay,

To change colormap

colormap(jet(10))

'jet' is the name of a colormap and the number is the number of segments.

The image youve uploaded appear to be segmented by something other than color?

[audley james]

@jonas my array (uploaded) has numbers (integers) where each number is a class(category). I simply want to assign a color for each number (i.e., each pixel). I am looking for a solution to colourize the image where it is invariant to the number of classes i may have for other images (i.e., a global solution to this problem). -thanks

[Guillaume]

You don't have to apply a colour map or use mat2gray to see the image with imshow if all you want to see the raw grayscale image:

imshow(out_map, [])

The [] tells imshow to map the minimum and maximum intensity in the image to black and white respectively, instead of using the default range of [0 1] = [black white]

You can of course apply a colour map after that:

colormap(parula)

With regards to colour maps, I would avoid jet or similar rainbow maps. A search for 'rainbow color map harmful' in your favorite search engine will find lots of sites explaining why including this paper by Mathworks

[Adam]

You can also create your own colourmap with defined colours or just random as

cmap = rand( 10, 3 )

if you have 10 classes and provided the caxis scaling is appropriate to map 1 per class.

Obviously if choosing your own 10 colours you would just put them into a 10 by 3 array like the one randomly created, e.g.

cmap = [1 0 0; 0 1 0; 0 0 1; 1 0 1;...];

where you put all your colours in instead of the ... of course.

[jonas]

I agree with guilliame that 'jet' is a poor colormap for visualizing intensity and completely useless if you print in grayscale. If the purpose is to show a number of distinct regions in different colors, then it is quite OK because it covers a wide range of colors (still useless in grayscale though).

Just set the number of segments to the number of unique values and pick another colormap. Personally I like the magma and inferno colormaps but they are not part of the default ones.